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EVOLUTION AND HUMAN REPRODUCTION
Saturday
EVOLUTION AND HUMAN REPRODUCTION* M.A.,
M.B.
PHILIP RHODES Cantab., F.R.C.S., M.R.C.O.G.
OBSTETRIC PHYSICIAN TO ST.
THOMAS’S HOSPITAL, LONDON, S.E.1
"We forget that nature itself is one vast miracle ... We forget that each one of us in his personal life repeats that miracle.... This is the way of the man who makes nature ‘natural’. He stands at the point where the miraculous comes into being, and after the event he calls it natural." -LOREN EISELEY, The Firmament of Time, 1961.
WE obstetricians stand quite literally where the miraculous comes into being, and we ought to be staggered that starting from two microscopic cells we end up with Man-all 3000 million of him living on this Earth at the moment, with millions more to come and millions more in limbo. Sex
Unicellular organisms began about five hundred million years ago. When first they multiplied asexually there could have been very little genetic variation. Sexual reproduction is the basis of variation, and it is remarkable that two cells fuse to form one with a different genetic
make-up. How sexual reproduction began is a matter for conjecture, but an intelligent guess is that it was at first a variant of phagocytosis. Under adverse conditions one cell might ingest another. If the engulfed cell refused to die, its nuclear material might join with that of its cannibal brother, and start off a different strain. Willmer (1960) protozoan which may be either amoeboid and slow-moving, or flagellate and swift moving. The change from one form to another may be forced on the organism by an alteration in its electrolyte environment. In this may be seen the beginnings of ovum and sperm. But the matter is deeper than this, as Jost’s (1958) researches have shown, in mammalian embryos. When the primordial germ-cells travel from the base of the yolk-sac to the site of the primitive gonad, it is suggested that they are indifferent. They become ova or spermatozoa because they reach an ovary or a testis-in other words the local environment determines how they shall develop. If a pellet of testosterone is inserted near the primitive gonad of a genetic female, the developing ova are pushed off course, and the genital tract is diverted towards a more male form. Now sex hormones can alter electrolyte balances and cell-membranes, and it may be that altering the electrolytes near developing gametes may in part determine whether they shall be male or female. In the Metazoa the cells which are destined to produce the next generation are early in development pushed to the vegetal pole, away from the head end of the embryo. Here they attain a degree of protection, but they are also close to the nephridia and coelomic ducts-the primitive Such excretory excretory apparatus of these forms. maintain the internal systems environment, and it is mentions
a
* The Bartholomew Mosse memorial lecture delivered at the Rotunda Hospital, Dublin, on Nov. 3, 1961. 7226
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February I962
coincidence that germ-cells are nearly always found close to the excretory system, right up to Man. However sex began, it has been supremely successful in providing genetic variation which interacts with the environment. Medawar (1957) happily named his book The Uniqueness of the Individual, and this individual uniqueness is based on genes. We know of this from fingerprint systems, and from the difficulties of surgical grafting. But the uniqueness extends to our very proteins. Harris (1959) records that using seventeen antisera, 475 Londoners were shown to have 296 distinct blood-groups. With more refined testing it seems likely that there would have been 475 groups. It is a fascinating sidelight on scientific progress that, while statistics try to iron out the differences between individuals in a population, biochemistry more and more shows how different from each other we all are. From the mammalian point of view, the female is the more important sex in evolution. The males only supply material for genetic variation. They have a secondary role as protectors of the female; but, when the female is adequately protected, veterinary science has shown us (by the techniques of artificial insemination) how unimportant are nearly all the males. Nevertheless this technique does restrict genetic variation, which may be desirable in domestic animals, though not in the wild. Smythe (1960) has suggested that roughly equal numbers of males and females are produced, so that predators have an even chance of killing the relatively unimportant male.
probably no
Fertilisation The union of sperm and egg is a remarkable phenomenon, and Nature has gone to great lengths to ensure that they shall meet. As a general rule fish produce millions of eggs and millions of spermatozoa. The female lays her eggs, and probably by chemiotaxis attracts the male to the area. He showers sperm over the eggs, which are thus fertilised in somewhat haphazard fashion. The eggs and young are an easy prey. Only by this prodigious effort may some survivors escape to propagate the species. With many of the amphibia the male clings to the back of the female, and the eggs are fertilised as they emerge from the cloaca (Amoroso 1959). With reptiles, birds, and mammals fertilisation is internal, so increasing the chances of sperm meeting egg. Despite evolutionary advances enormous numbers of gametes are still produced. In Man the male may produce 500 million spermatozoa per ejaculation, and the female has been computed to have 400,000 ova in each ovary at birth. Yet during a reproductive life of forty years she will shed only 480 of these. Apart from such backward glances, there are forward ones too; for even in some fish fertilisation is internal, and the egg may be nourished within the female’s body. Care of the Developing Embryo A great advance is made when the developing embryo receives care and protection during its precarious life. As we have seen fish embryos more often come to grief than
390 emerge to successful
reproductive life; though in
some
species the fertilised the young
are
egg is retained in the oviduct, and born alive. In amphibia eggs may be left
relatively unprotected, but some toads have dorsal pouches into which fertilised eggs are pushed by the male, and where the embryos remain till they are more mature. Smith (1958) cites the toad, Alytes obstetricans, in which the male wraps the spawn round his body until hatching occurs-a very early example of the man-midwife! Reptiles lay eggs in burrows, but a few carry the young internally (Parrington 1950); the eggs rarely receive any special care. It is quite otherwise with birds, which have build a nest and incubate the eggs. This is because birds are warm-blooded-an event of supreme importance. This difference in metabolism frees the species from environments where little changes; but it raises problems of embryo nutrition. All embryos need food, water, electrolytes, and so on; but bird embryos need warmth too. This brings male and female together, not purely for the sexual act, but for the male to protect the female, and for him occasionally to relieve her of the duties of incubation. Here is a biological root of family life, and herd life. Cooperation is forced on the sexes for the benefit of the species. This theme of cooperation was recently expounded by Sir Wilfrid le Gros Clark in his presidential address to the British Association (1961). Cooperation is an aspect of evolution which is rightly getting more emphasis than previously, and it is rooted in reproduction. Mammalian embryos and foetuses receive the best and safest care of all because of carriage within the uterus and the efficient placenta. A mother escaping from trouble, or fighting it, is protecting not only herself but also her foetus directly. Nest building is mainly associated with birds; but even the lowly minnow makes efforts in this direction, and in the burrows for the eggs of reptiles a primitive nest might, by a stretch of the imagination, be seen. But the best nest of all is the uterus.
to
The Uterus
Paired oviducts are found in many fish, but only rarely are they used to retain the embryo during gestation. In mammals the oviducts tend to come together, so that often there is a small median chamber and two lateral horns. Now the oviducts are not simply conveyors of the eggs to the exterior; they are receptacles. With two uterine horns there is room for the development of several embryos at This may be satisfactory for terrestrial animals once. where there is little danger on the ground, and especially when the animals live in herds. But in the trees, where the Primates live or have lived, a number of offspring would be at a serious disadvantage from falling or being dropped when the mother is in movement, and therefore selection seems to have favoured the single median uterus. Moreover selection has gone deeper than this, for it has favoured an endocrine apparatus which allows, as a general rule, only one ovulation at a time. There are advantages in having a single foetus, for it is not in competition with others for its food and respiratory and other needs. We see the effects of intrauterine competition in the reckling of a litter, and in the prematurity of twins. Twins are an atavistic reversion to a mode of reproduction less satisfactory for arboreal creatures, but there is still high wastage in twins, both before and after birth. The lower Primates-the tree shrews and the lemurs-
have a twin-chambered uterus, and they produce two or three young at a gestation. The lemurs and lorises are in the main nocturnal and slow-moving. It may be that these are adaptations to the care of the young. In the Tarsioidea about half the uterus (the lower part) is single and the upper part is divided (Wood Jones 1926). By the time of the appearance of the Anthropoidea-the monkeys, apes, and Man-the uterus is single. Occasionally we see the backward evolutionary glance in the varying degrees of double uteri of women, but these are relatively inefficient, as shown by the high abortion-rate. Internal development of the foetus obviously sets a limit on the possible size of the baby or babies at birth, for the abdomen can only distend to a certain size. We see the problems for the mother of over-distension in some cases of hydramnios and twins. The marsupials, especially the kangaroo, show an unusual anatomy in that the ureters pass between the two horns of the uterus, a condition not found in any other species, according to Wood Jones (1950). This is probably one reason why the foetuses are expelled early from ’the uterus in this animal. They then climb up the ventral fur and drop into the pouch, there to fasten on to the nipples. The epithelium of the mouth grows so intimately with the maternal nipple epithelium that if the baby be forcibly removed its mouth is lacerated. Uterine Contractions
The uterine contractions of labour
seem
almost invari-
ably to be painful. Smythe (1960) says that "every prospective mother, living in her natural environment, prefers, to be left in solitude at the time of her parturition", It seems that only women and elephants have someone in attendance at birth, though Peter Scott, in a recent television broadcast, said that whales too usually have a helper who nuzzles the newborn to the surface so that it may breathe. The pain of labour seems to be a signal for the mother to take herself off to a place of safety for herself and her offspring. The exponents of so-called painless childbirth often forget that the same signal is necessary for women today. If they did not have pain, babies would be born in sometimes embarrassing circumstances. This is not to say that I deprecate methods of painless childbirth, for these methods are basically designed to reduce the psychological tensions of childbirth, which because of our culture are often greater than they need be. With this aim I entirely agree; but it is doubtful whether labour can ever be truly painless; the reasons for its discomfort, to put it at its least, are too biologically profound. In man some pain may also be dependent on the relatively large size of the foetal head, and this is a necessary price for the evolution of a brain of our capacity. Menstruation
Menstruation is a purely Primate characteristic. It is found in the Old World monkeys, but less often among those of the New World, which in many other ways seem well off the line leading to Man (Harrison 1958). It is found too in all the Pongidae (gibbon, orang, chimpanzee, and gorilla). Its significance is far from clear. It may be that the preparation of the endometrium for the implantation of the fertilised ovum is so intense, and must needs be so, that only necrosis and shedding is a possible way of involution; or it has been suggested that superficial necrosis supplies essential nutrients for the early development of the embryo. Ramsey (1959) has suggested that the intense coiling of the arterioles of the endometrium is an
391
adaption which allows them to be stretched at the time of conversion, which she defines (1960) as " the period when uterine enlargement by virtue of growth ... gives place to enlargement by stretching alone ". If these coils are not so used, they may cause necrosis by stagnation of blood-flow. Associated with menstruation is, of course, the conAll other mammals have tinuous breeding season. relatively infrequent sexual cycles (Frazer 1959). A continuous breeding season gives tremendous biological advantage, and in part offsets the slowness for the species of producing only one offspring at each gestation. In lower mammals bleeding from the genital tract occurs at restrus (i.e., about the time of ovulation), and also when the ovum first implants. Both these features are occasionally seen in women, and red cells are always to be found microscopically in the cervical mucus at the time of ovulation. The newer pattern of menstruation has suppressed and overlaid the older ones; but they are still there, at least in some women. The Placenta
A most important organ for the biological success of the Indeed the mammals are divided into the Monotremes, the Marsupials, and the Placentals. The Monotremes, exemplified by the duckbilled platypus and porcupine ant-eater, lay eggs but suckle their young. Echidna incubate a single egg in a pouch (Wood Jones 1950). The Marsupials I have already mentioned. The nourishment of the embryo from the mother was tried in part as an evolutionary experiment before the placenta came on the scene. Although embryos of the ovoviviparous species feed off the yolk, there is a contact of the embryonic and maternal bloodstreams through which respiratory exchange probably occurs. Embryos tucked into the dorsal pouches of toads also have similar gaseous exchange. Perhaps neither of these examples are of placentas proper, but they point in that direction. Amoroso (1952, 1959, 1961) has extensively documented the comparative anatomy of the placenta, and his writings are now classical. It would be simple if we could build an evolutionary series on the basis of Grosser’s classification of how close the foetal and maternal bloodstreams come together. But unfortunately this cannot be done. Grosser classified placenta: as epitheliochorial, syndesmochorial, endotheliochorial, haemochorial, and haemoendothe1ial. This is not the place to define these more fully, but the classification is based on the degree of penetration of the foetal placenta into the maternal tissues. The human placenta is, of course, hsemochorial. Among the rest of the Primates all types of placentx are found. The tree-shrews have an endotheliochorial placenta, and the lemurs an epitheliochorial one. On the other hand, other Insectivora, supposedly lower in the scale than the tree-shrews, have a hxmochorial placenta (le Gros Clark 1959), and the same is true of Tarsius. Our understanding of the significance of these various placental types must await further evidence. But one fact does seem clear, and that is that the number of layers separating the maternal and foetal bloodstreams are not a good guide to functional
Mammalia is the placenta.
efficiency. There
variations too in the gross anatomy of the may be diffuse and cover the whole chorion; sometimes it is a zonary band; sometimes it has several discrete tufts; sometimes there are two discs, as in many monkeys; and sometimes there is one disc, as in Man. are
placenta. It
Rarely human placentae show these evolutionary variations, as in the succenturiate lobe, and occasionally in tufts having no blood-supply from the foetus. During development the human placenta passes through a diffuse phase (seen in abortions before about the fourteenth week), and it only becomes discoidal at about the sixteenth week. Perhaps it is not fanciful to see during implantation the rapid transition through epitheliochorial, syndesmochorial, and endotheliochorial stages before the placenta becomes finally hasmochroial. Romer (1933) gives a diagram of a typically reptilian egg (fig. 1). The reptiles began about two hundred and fifty million years ago, and
already chorion, allantois, and yolk-sac are yet the
be seen. The embryo feeds on the yolk, and the to
allantoiss spreads beneath the
shell, becoming richly vascularised.
Fig. 1-Reptilian 1933.)
egg.
(Redrawn from Romer
Respiratory exchanges take place here. With internal development there is, of course, no necessity for a shell; but all the other structures, often modified, are still found as high up the scale as Man. Moreover, they still have much the same functions. The allantois is a receptacle for waste matter, and it is incorporated in the urinary bladder. The placenta of Man is described as being chorioallantoic. Early in development the allantois is close to the chorion; later it is separated from it, but the umbilical arteries are the allantoic vessels prolonged to the placenta. When they become obliterated shortly after birth, their proximal ends remain as the superior vesical arteries-i.e., they supply the remains of the allantois. Here is a most remarkable demonstration of how evolution takes older patterns and remoulds them. Rarely does an entirely new pattern begin. It just grows out of the old. And there is a lesson here of more general importance. Much has been written about the brain and its evolution, and this is natural, for on this is primarily based Man’s ascendancy; but selective pressures act on whole individuals and not on single adaptations. In any account of evolutionary advance the placenta must be accorded a
high place. It is just possible that biological evolution is still affecting the placenta. I have seen it suggested that there may be a gene for deep penetration of the placenta, perhaps allowing for better nutrition of the foetus at the probable cost of postpartum haemorrhage in the mother. There is the gentlest of gentle hints that this may be so in those women who have recurrent postpartum hoemorrhage, and in those rare instances where this complication of in families. A phase of nutrition of the embryo that is often ignored is that before embedding. At first the zygote is floating in the fluid secretions of the fallopian tubes and the endometrium. All division happens at a great rate, so that The metabolic processes must be proceeding apace.
labour appears
to run
392
zygote is living like estuarine waters.
a
fish
or
The Birth of the
amphibian embryo
in
Baby
The Bony Pelvis
It is
to walk through any natural history and see the changes wrung out of the basic pattern of the three bones of the pelvis-ilium, ischium, and pubis. This pattern began with the amphibia, about three hundred million years ago. Always there are the same relations that we know in Man. The ilium articulates with the vertebral column, and the pubis is ventral, but it does not reach across the midline in all forms. Occasionally epipubic bones project along the abdominal wall, perhaps to increase support for the viscera. The pelvis is a skeletal support, an area for muscle attachment, a protector of its contained viscera, and a birth canal. In trying to be so much it tends to fall short of the ideal in some respects (fig. 2). The area of attachment of the ilium to the vertebrae is relatively small; both the pubes and the ischia are joined in an ischiopubic symphysis; and the axis of the pelvic canal and the abdominal cavity are in a straight line. Man’s upright posFig. 2-Diagram of a typical pelvis of ture was the evolutiona quadrupedal mammal. (Redrawn ary change that freed from Wood Jones 1926.) his forelimbs and began his brain development. When an animal is upright and securely based on the hindlimbs, the forelimbs are able to explore the environment and feed information into the brain. In an arboreal environment the hands and arms need a nice coordination of movement supplied by the brain; but more than this is required, because judgment of distance is essential, and this is supplied by stereoscopic
fascinating
museum
vision. To get the skeleton permanently upright requires great anatomical changes. If the quadruped pelvis and spine were rotated through 90° we should have had the benefit of a birth-canal which is absolutely straight, but abdominal pressure would fall with maximum force on the pelvic floor, probably making prolapse of the pelvic viscera even more prevalent than it is now. In fact the pelvis has been rotated dorsally through only 30-40°, giving an angle of inclination of 50-60°. This evolutionary rotation can be seen in the oblique fibres of the hip-joint capsules. To get the trunk fully erect the lumbar lordosis is introduced, and to get the head erect the cervical lordosis. Much of these two secondary curves depends on the intervertebral discs, whose liability to damage and pathological change is so well known. This is one of the prices paid for the human brain. Failure to rotate the pelvis through 90° may prevent the full transmission of intra-abdominal pressure directly on to the pelvic floor. Abdominal viscera now lie partly on the anterior abdominal wall and partly on the pubis, and mesenteric attachments have shifted too, to hold up the viscera. This evolutionary change possibly explains why so many of us in middle age bulge slightly below the umbilicus. Perhaps it is better here than in the perineum. The lumbar lordosis pushes the axis of the pregnant uterus well forwards, and this gives rise in part to the
curved axis of the birth-canal. If we had retained a long ischiopubic symphysis the birth-canal would be a long bony cylinder, because to fulfil its weight-bearing function the area of attachment of the ilium to the vertebral column has increased, so lengthening the posterior wall of the birth-canal. Incidentally, in women the area of attachment is to 2-21/2 vertebrae and in men to 21/2-3. Here are seen the conflicting claims of the pelvis as a support and as a part of the reproductive system; the result is a high incidence of low backache in women. If the ischiopubic symphysis had remained it might have been impossible to get the large human festal head born at all, and therefore the ischial part of the symphysis has become open, so shortening the anterior wall of the birth-canal (fig. 3). This removes some of the support of the bladder. When the two partially terrestrial great apes-the chimpanzee and the gorilla-are on the ground, they progress in part on all fours. The pelvis is partly rotated because the forelimbs are longer than the hind ones (fig. 4), but there is little lumbar lordosis, and the birth-canal is almost straight (Howells 1960). If you look at a newborn baby Fig. 3-Adaptation of the ischiopubic symphysisin you will see that his hips are Man. (Redrawn from well flexed on his body, and the Wood Jones 1926.) thighs cannot be extended to the adult position until he is about a year old. The evolution of the upright posture can be seen compressed into this first year of life. It seems that the development of the brain through the upright posture has imposed certain limitations on reproductive function, for some difficulties of parturition are due to the sinuous curve of the birth-canal, which in some measure is responsible for the extraordinary convolutions through which a baby must go to be born. When the birth-canal is very curved, as in high inclination of the brim, it may be impossible for the foetus to bend round the corner, and the head remains high. The Pelvic Floor
pelvic floor is represented in fish by some strands of muscle, and the pelvis by a cartilaginous bar (Smout The
Fig. 4-The human pelvis and the pelvis of the partially terrestrial great apes.
393
Jacoby 1953). Only with the advent of lungs and a diaphragm does the pelvic floor increase its activity. During respiration, intra-abdominal pressure is constantly changing, and this must be counteracted at the hind end The tail muscles are of the abdominopelvic cavity. this function-another into example of using pressed what has gone before rather than devising something novel. In quadrupeds the pelvic floor is relieved of some stress by gravity, and we see something similar when women are examined in the knee-chest position. Moreover these muscles are in constant activity because of tail movements. These two factors probably explain why prolapse is so rare in quadrupeds. In women there is greater immediate stress on the pelvic floor because of the and
erect posture, and the muscles do not have the exercise derived from a tail. But the tail muscles are still there-the pubococcygeus as the tail-depressor, and the iliococcygeus and ischio-
The insertion of the slipped forwards to give more direct support to the viscera, and so it has become more prone to injury at birth. The iliococcygeus in man has no contact with the ilium, because during evolution its origin has slipped down from its original attachment at the pelvic brim to be in line with the origins of the other two muscles, but there is still a gap between the iliococcygeus and the coccygeus, and the obturator nerve defines the junction between pubococcygeus and iliococcygeus. (It is a general rule that where a nerve perforates a muscle that muscle is derived from morphococcygeus
as
the
tail-waggers.
pubococcygeus has, of
course,
logically distinct components.) The troubles of the pelvic floor in women need no elaboration. They seem to be part of the evolutionary price to be paid for our essential humanity, which resides in the brain. Only when brains are organised at a sociological level can they return, with the arts of obstetrics and gynaecology, to restore some of the ravages which being human thrusts on reproductive function. It has taken perhaps fifty thousand years for the brain to make this elementary act of courteous reparation. Reduction in the Numbers of Offspring Despite several exceptions, there has been an evolutionary trend to reduce the number of offspring at each gestation. As a general rule, fish reproduce in hundreds, amphibia and reptiles in scores, birds in tens, and mammals in under tens. The most remarkable reduction is to one in man. It has already been suggested that this is probably associated with an arboreal life in our ancestors. Primate babies are usually carried clinging to the ventral fur of the mother, though sometimes they cling to her back (Sanderson 1957); gibbons may hold their babies in their laps as they brachiate through the trees. But an important feature of the single offspring is that greater care can be and is lavished on the baby both before and after birth. Here again obstetrics and paediatrics are continuing a biological trend of evolution. Care of the offspring after birth is almost non-existent in fish, amphibia, and reptiles, though there are exceptions. With birds much care is given, but relatively longcontinued care after birth is thrust upon mammals by
lactation. Breasts
Breasts are the distinguishing feature of mammals. Where litters are large the mammary glands extend in a bilateral line from the axillary to the inguinal regions. With a reduction in litter size breasts tend to remain
either
the others being supthe bilateral milk pressed. Nevertheless, ridge appears transiently during embryonic life, and remnants of it are not at all uncommon in accessory nipples. Occasionally too there are accessory breasts in the mammary line; these are usually very near the normal breasts, but they have been described as far down as the groin. A few women have breasts extending from the 3rd to the 7th ribs instead of from the 2nd to the 6th. Inguinal mammary glands are favoured in quadrupeds, as seen in many domestic animals. This may be to leave the mother free at the head end for feeding herself and for keeping on the alert for danger. Pectoral breasts are at
the caudal
or
cephalic end, even
in
man
characteristic of most primates, though lemurs have a non-functional teat in the inguinal region to which the baby clings with its mouth whilst being transported (Wood Jones 1926). These breasts are an adaptation to arboreal life, for the mother cradles the baby in her forelimbs to prevent it falling. Pectoral breasts are similarly developed in the seal, where the young are held during suckling. This may be responsible for the myth of the mermaid. Permanent breasts are almost peculiar to women. In most mammals the mammary glands wax and wane with the reproductive cycle, and a remnant of this in women is the obvious variation in size during pregnancy and lactation, and during the menstrual cycle. It may be that permanency of the breasts is of sexual significance, and it may even influence sexual selection. As the brain develops more emphasis is placed on sight than on smell, and the rhinencephalon diminishes, though the importance of scent is not entirely lost, for the world is combed for perfumes for men and women. That the breast is a sexual symbol of today is apparent from almost every advertisement, and this adulation of the female form is in part responsible for the decline in breast-feeding. Our culture has made the breast more of a symbol than a functioning organ. The Fretalisation of the
Baby
All babies tend to be helpless, but none is so helpless for so long as the baby of Man. For about a quarter of our whole life-span we are physically maturing, and perhaps mental maturing takes even longer. Our physical attributes too are remarkably primitive. We may think of our hands doing wonderful things, but a frog would recognise our limbs as very like his own. The reason we can be so dextrous is not the intrinsic anatomy of the hand and arm, but the brain which informs the activity. This and many other primitive features of our anatomy, which are very unspecialised, are often referred to as foetalisation—i.e., we stay anatomically in a foetal stage. It is this very unspecialisation of anatomy, and immaturity of the baby, that is our greatest evolutionary strength. If in a species a forelimb becomes specialised, as in the horse, it cannot go back to the primitive evolutionarily plastic pentadactyl limb. The species is doomed for ever to be limited to the ground, and to running and walking. It will never grasp, feel, understand texture, and manipulate things, and its brain can never learn from these activities, because its limbs can be used only for support. The anatomically specialised can fill only a narrow environmental niche. With a relatively unspecialised anatomy man seems able to fill any evolutionary niche. Because of hostile environments, the young of most species have to be born mature enough to be able in large measure to look after themselves with little help
394
from their parents. This means that they must be born with some instincts. Among the mammals, seals can swim and bats can fly at birth; fawns can run fast within hours of birth (Harrison 1958). Such adaptations are important in larger mammals, whereas the smaller ones can be hidden away in a nest of straw or a burrow safe from harm for a while. Primate infants have to be able to cling to fur so that they may be transported safely through the trees, and the remarkable grasp of the young human infant may be of this nature. But no baby of any species is born so immature as the human one, and yet the brain is one-quarter of its adult size. The immaturity of the baby and its large brain are to do with the long process of learning. By learning we inherit our culture which " consists of everything which has ever been accepted as a way of doing or thinking, and so taught by one person to another " (Howells 1956). By learning we inherit the acquired characteristics of culture. By this we form a complete break with biological evolution, in which acquired characters cannot be inherited. Biological evolution progresses by the interaction of the environment on the genetical inheritance of the species, which comes at fertilisation and is changed little at each generation. Genes may be likened to a large pack of cards which undergoes constant reshuffling, though the cards themselves remain unchanged. The muscular development of the blacksmith’s brawny arm cannot be impressed on his genes, and so it cannot be handed on to his son; but his acquired skill at the forge can be handed on because his son can learn. In just such fashion is knowledge handed on, though it is not only handed on but added to, selected, and refined. This is why social evolution so completely outstrips biological. Life began five hundred million years ago. Social man probably began only about fifty thousand years ago, and yet the scene has changed beyond recognition. In his early phases Man had to adapt his body to the environment ; now he adapts the environment to his body. The world is turned upside down. There are many biological adaptations of the foetus, so that he can emerge into social man. Intrauterine development seems largely given up to pushing the brain to the maximum size compatible with getting it safely through the birth-canal. This tremendous growth is seen postnatally too, for by the age of seven the brain has attained its adult weight. Of course, other systems have to be started on their way so that the brain may use them when appropriate, but no intra-uterine nourishment is wasted on the legs, and arms, and face. The neonate is little more than brain and viscera. The fcetal circulation is a fantastic adaptation. The liver is bypassed by the ductus venosus, and the lungs by the ductus arteriosus and foramen ovale, because the placenta is carrying out hepatic and pulmonary functions. Yet both lungs and liver are ready to start functioning at birth. Barcroft’s researches (1946) into the developing physiology of respiration revealed an astounding situation: first come the total body movements from which later the respiratory movements are abstracted; then all goes quiescent until after birth, when the whole set of reflexes previously laid down is suddenly called into being, and the infant breathes. I often pause in wonder when palpating a maternal abdomen and I feel those rhythmic tappings of the foetus which I am sure are the same as those Barcroft described in the sheep-the foetus learning to breathe before it has ever sniffed the air. Such wonder
is not sentimental; for, whether you believe this is God’s work or the work of blind evolutionary forces with desoxyribonucleic acid calling the basic tune, it is astounding. Convinced evolutionist though I am, at moments like these I see the yawning chasms dividing the outline of a theory from the full explanation of human life-indeed all life-in terms of that theory.
Psychological and Social Evolution From this point we move into the world of social evolution. Now we are banded together in groups-
family, local, school, professional, technical, political, national, and international. We have the old biological nature, but there is now a vast superstructure built on it. The environment of Man is Man himself and his varying cultures. It is this phase of evolution that we all fervently hope we can control. So far we seem to have little inkling of how to do it. Obstetrics has arisen in response to a need for the social ordering of childbirth. It is a part of the social system, acted upon by it and acting on it. The most recent impact upon it has been the public demand for the better psychological care of mothers. To be fair, the movement arose from within the ranks of obstetrics, but is now carried far beyond its bounds. Society has cast up many more problems for obstetrics -for instance the early maturing of girls and the high illegitimacy-rate, recently estimated as 5% in England and 10 % in London, which by its vast anonymity attracts many unmarried mothers away from their homes and social ostracism. Because better nutrition has increased the size of the bony pelvis, disproportion is of relatively small importance now, and perhaps this may help to lower the frequency of prematurity. At the same time, I think it may well be extending the gestation period and offering problems in postmaturity. Late age at marriage, especially in social group i, or the postponement of childbearing because of financial and housing insecurity, hands us the problems of the elderly primigravida. These are factors imposed by our culture. Similarly imposed are the myths and half-truths which many unfortunate patients bring with them to childbed, making themselves unhappy and apprehensive. This is a fault in the education of girls, inculcated by our own attitudes based on a long history of Puritanism. Attempts are being made to alter this by a steady stream of booklets advising pregnant mothers, antenatal classes, and so on; but to change the cultural attitudes, gained in twenty years, during the last six months of pregnancy is simply not possible. Education about childbirth must reach back much further into the home and school. Obstetrics has thrown up problems for society too. The population explosion, so called, is in some measure due to the saving of more infant lives. Only about fifty years ago 1 in 5 infants and neonates died. Now it is fewer than 1 in 25. Reports from Africa suggest that a third to a half of babies die. But obstetrics, together with many other social improvements, is altering this. The world population is rising and fast. This year there will be 3000 million on the Earth, and it is worth noting that 1 in every 3 of us will be Chinese or Russian (Times 1960). There is beginning reaction to this rising birth-rate, and it is to be seen in increasing use of contraceptives, research into contraception, the legalisation of abortion in many countries, and the vast rate of criminal abortion. It can be computed that in England and Wales there must be about 200 induced abortions every day, and this
395 is probably a conservative estimate. There are pressure groups for the practice of eugenics, and sterilisation for women with large families is being more readily practised in many places. At the opposite end of this scale, never has research into infertility been more fervently pursued. These, and no doubt many others, are the growing edges of patterns in human reproduction. They arise because of tremendous biological and obstetrical success. I am not a sociologist and therefore will not offer any opinion on how some of these problems should be faced. For that matter I am not a biologist in any professional sense; but I think it right that, however, distorted the view may be, we should lift our eyes occasionally from our immediate preoccupations to look at what goes on
around us. REFERENCES
Amoroso, E. C. (1952) in Marshall’s Physiology of Reproduction; vol.II. London. (1959) in Gestation; Vth Conference of the Josiah Macy Foundation.
In the Rabbit It has been known since 1833 that the rabbit’s circle of Willis is a functional anastomosis between the carotid and vertebral systems: Mayer (1833), who injected mercury into the carotid arteries of rabbits, a ram, and a goat, noted post mortem that globules of mercury appeared in the carotid vessels of the opposite side, and in one rabbit in the basilar artery. Marckwald (1890) made similar observations in rabbits by injecting dyed paraffin and olive oil. McDonald and Potter (1951), in very precise experiments, showed that in the normal rabbit there was no flow along the communicating arteries because pressure was equal at both ends; but immediately after one carotid or vertebral artery was occluded, flow occurred through the appropriate communicating channel. Chungcharoen et al. (1952) also demonstrated the efficacy of the normal communicating arteries in maintaining perfusion pressure in the carotid sinus immediately after common-carotid occlusion.
—
New York. (1961) Brit. med. Bull. 17, 81. Barcroft, J. (1946) Researches on Pre-natal Life. Oxford. Eiseley, L. (1961) The Firmament of Time. London. Frazer, J. F. D. (1959) The Sexual Cycles of Vertebrates. London. Harris, H. (1959) Human Biochemical Genetics. London. Harrison, R. J. (1958) Man the Peculiar Animal. London. Howells, W. (1956) Man in the Beginning. London. (1960) Mankind in the Making. London. Jost, A. (1958) in Hermaphrcditism, Genital Anomalies and Related Endocrine Disorders (edited by H. W. Jones and W. W. Scott). London. le Gros Clark, W. E. (1959) The Antecedents of Man. Edinburgh. (1961) in the Times, Aug. 31. Medawar, P. B. (1957) The Uniqueness of the Individual. London. Parrington, F. R. (1950) in Chambers’s Encyclopædia (Reptiles). London. Ramsey, E. M. (1959) in Gestation; Vth Conference of the Josiah Macy Foundation. New York. (1960) in The Placenta and Foetal Membranes (edited by C. A. Villee). Baltimore. Romer, A. S. (1933) Man and the Vertebrates. London. Sanderson, I. T. (1957) The Monkey Kingdom. London. Smith, J. M. (1958) The Theory of Evolution. London. Smout, C. F. V., Jacoby, F. (1953) Gynæcological and Obstetrical Anatomy. London. Smythe, R. H. (1960) The Female of the Species. London. Times (1960) Report on Population Reference Bureau Statistics, Washington. Willmer, E. N. (1960) Cytology and Evolution. London. Wood Jones, F. (1926) Arboreal Man. London. (1950) in Chambers’s Encyclopædia (Marsupials; Monotremata). London.
METHODS
—
—
—
—
—
Rabbits weighing over 2-2 kg. were subjected to artery under pentobarbitone and ether anxsthesia. The carotid preparation was a bilateral ligation of the internal and external carotid arteries which preserved the central artery of the ear for recording blood-pressure (Lowe 1962a). All arteries were cut between silk ligatures. The development of a collateral supply was investigated by measurement of retinal-artery pressure in vivo and by making resin casts of the cerebral vessels in vitro. Systolic retinal-artery pressure was measured in anxsthetised rabbits by ophthalmodynamometry, using the technique described for man by Thomas and Petrohelos (1953). The results are expressed as grammes of applied pressure, which estimates, in arbitrary units, how much the retinal-artery pressure exceeds the resting intraocular pressure (Lowe 1962b). At various stages rabbits were killed by an intravenous injection of pentobarbitone 240 mg. containing heparin 1000 units, and resin casts of the arteries of head and neck were made with’Marco’ resin mixture. t
ligation
RESULTS
ADAPTATION OF THE CIRCLE OF WILLIS TO OCCLUSION OF THE CAROTID OR VERTEBRAL ARTERY: ITS IMPLICATION IN CAROTICOVERTEBRAL STENOSIS R. D. LOWE Cantab., M.R.C.P.
In Vivo
The sequence of
retinal-artery pressure changes after carotid-artery ligation is shown in fig. 1: Normally there is little difference between the two eyes. t Supplied by Scott Bader & Co., Wollaston, Wellingborough, Northants.
M.B.
LATELY RESEARCH ASSISTANT, MEDICAL UNIT, UNIVERSITY COLLEGE HOSPITAL, LONDON, w.C.1*
THERE is no doubt that occlusion or stenosis of carotid vertebral arteries is a common finding in cases of stroke, and it is natural to suppose that the stroke is a direct consequence of obstruction to the flow in the artery, producing a lowered perfusion pressure and cerebral ischaemia in its area of supply. Such an explanation would imply that the anterior and posterior communicating arteries do not provide an adequate collateral flow and are unable to compensate for ischsemia in one quadrant of the circle of Willis. This paper reports experiments designed to study adaptation in the circle of Willis following carotid occlusion. The rabbit was selected as a model because its circle of Willis is remarkably like that of man, and retinal-artery pressure measurements in vivo were used as an indirect estimate of pressures in the circle of Willis both in man and in the rabbit. In the rabbit the findings with this technique were confirmed in vitro by preparing resin casts of the arteries. or
* Present address: St. George’s Hospital,
London,
S.W.1.
Fig. 1-Retinal-artery systolic carotid-artery ligations.
pressure in rabbits
during successive
Each point shows the mean and standard error of duplicate ophthalmodynamometer readings. (Twenty-six animals, reducing to twelve by week 18.) R = right carotid ligation. L left carotid ligation. v vertebral artery ligated or stenosed (six animals). =
=
EVOLUTION AND HUMAN REPRODUCTION* M.A.,
M.B.
PHILIP RHODES Cantab., F.R.C.S., M.R.C.O.G.
OBSTETRIC PHYSICIAN TO ST.
THOMAS’S HOSPITAL, LONDON, S.E.1
"We forget that nature itself is one vast miracle ... We forget that each one of us in his personal life repeats that miracle.... This is the way of the man who makes nature ‘natural’. He stands at the point where the miraculous comes into being, and after the event he calls it natural." -LOREN EISELEY, The Firmament of Time, 1961.
WE obstetricians stand quite literally where the miraculous comes into being, and we ought to be staggered that starting from two microscopic cells we end up with Man-all 3000 million of him living on this Earth at the moment, with millions more to come and millions more in limbo. Sex
Unicellular organisms began about five hundred million years ago. When first they multiplied asexually there could have been very little genetic variation. Sexual reproduction is the basis of variation, and it is remarkable that two cells fuse to form one with a different genetic
make-up. How sexual reproduction began is a matter for conjecture, but an intelligent guess is that it was at first a variant of phagocytosis. Under adverse conditions one cell might ingest another. If the engulfed cell refused to die, its nuclear material might join with that of its cannibal brother, and start off a different strain. Willmer (1960) protozoan which may be either amoeboid and slow-moving, or flagellate and swift moving. The change from one form to another may be forced on the organism by an alteration in its electrolyte environment. In this may be seen the beginnings of ovum and sperm. But the matter is deeper than this, as Jost’s (1958) researches have shown, in mammalian embryos. When the primordial germ-cells travel from the base of the yolk-sac to the site of the primitive gonad, it is suggested that they are indifferent. They become ova or spermatozoa because they reach an ovary or a testis-in other words the local environment determines how they shall develop. If a pellet of testosterone is inserted near the primitive gonad of a genetic female, the developing ova are pushed off course, and the genital tract is diverted towards a more male form. Now sex hormones can alter electrolyte balances and cell-membranes, and it may be that altering the electrolytes near developing gametes may in part determine whether they shall be male or female. In the Metazoa the cells which are destined to produce the next generation are early in development pushed to the vegetal pole, away from the head end of the embryo. Here they attain a degree of protection, but they are also close to the nephridia and coelomic ducts-the primitive Such excretory excretory apparatus of these forms. maintain the internal systems environment, and it is mentions
a
* The Bartholomew Mosse memorial lecture delivered at the Rotunda Hospital, Dublin, on Nov. 3, 1961. 7226
24
February I962
coincidence that germ-cells are nearly always found close to the excretory system, right up to Man. However sex began, it has been supremely successful in providing genetic variation which interacts with the environment. Medawar (1957) happily named his book The Uniqueness of the Individual, and this individual uniqueness is based on genes. We know of this from fingerprint systems, and from the difficulties of surgical grafting. But the uniqueness extends to our very proteins. Harris (1959) records that using seventeen antisera, 475 Londoners were shown to have 296 distinct blood-groups. With more refined testing it seems likely that there would have been 475 groups. It is a fascinating sidelight on scientific progress that, while statistics try to iron out the differences between individuals in a population, biochemistry more and more shows how different from each other we all are. From the mammalian point of view, the female is the more important sex in evolution. The males only supply material for genetic variation. They have a secondary role as protectors of the female; but, when the female is adequately protected, veterinary science has shown us (by the techniques of artificial insemination) how unimportant are nearly all the males. Nevertheless this technique does restrict genetic variation, which may be desirable in domestic animals, though not in the wild. Smythe (1960) has suggested that roughly equal numbers of males and females are produced, so that predators have an even chance of killing the relatively unimportant male.
probably no
Fertilisation The union of sperm and egg is a remarkable phenomenon, and Nature has gone to great lengths to ensure that they shall meet. As a general rule fish produce millions of eggs and millions of spermatozoa. The female lays her eggs, and probably by chemiotaxis attracts the male to the area. He showers sperm over the eggs, which are thus fertilised in somewhat haphazard fashion. The eggs and young are an easy prey. Only by this prodigious effort may some survivors escape to propagate the species. With many of the amphibia the male clings to the back of the female, and the eggs are fertilised as they emerge from the cloaca (Amoroso 1959). With reptiles, birds, and mammals fertilisation is internal, so increasing the chances of sperm meeting egg. Despite evolutionary advances enormous numbers of gametes are still produced. In Man the male may produce 500 million spermatozoa per ejaculation, and the female has been computed to have 400,000 ova in each ovary at birth. Yet during a reproductive life of forty years she will shed only 480 of these. Apart from such backward glances, there are forward ones too; for even in some fish fertilisation is internal, and the egg may be nourished within the female’s body. Care of the Developing Embryo A great advance is made when the developing embryo receives care and protection during its precarious life. As we have seen fish embryos more often come to grief than
390 emerge to successful
reproductive life; though in
some
species the fertilised the young
are
egg is retained in the oviduct, and born alive. In amphibia eggs may be left
relatively unprotected, but some toads have dorsal pouches into which fertilised eggs are pushed by the male, and where the embryos remain till they are more mature. Smith (1958) cites the toad, Alytes obstetricans, in which the male wraps the spawn round his body until hatching occurs-a very early example of the man-midwife! Reptiles lay eggs in burrows, but a few carry the young internally (Parrington 1950); the eggs rarely receive any special care. It is quite otherwise with birds, which have build a nest and incubate the eggs. This is because birds are warm-blooded-an event of supreme importance. This difference in metabolism frees the species from environments where little changes; but it raises problems of embryo nutrition. All embryos need food, water, electrolytes, and so on; but bird embryos need warmth too. This brings male and female together, not purely for the sexual act, but for the male to protect the female, and for him occasionally to relieve her of the duties of incubation. Here is a biological root of family life, and herd life. Cooperation is forced on the sexes for the benefit of the species. This theme of cooperation was recently expounded by Sir Wilfrid le Gros Clark in his presidential address to the British Association (1961). Cooperation is an aspect of evolution which is rightly getting more emphasis than previously, and it is rooted in reproduction. Mammalian embryos and foetuses receive the best and safest care of all because of carriage within the uterus and the efficient placenta. A mother escaping from trouble, or fighting it, is protecting not only herself but also her foetus directly. Nest building is mainly associated with birds; but even the lowly minnow makes efforts in this direction, and in the burrows for the eggs of reptiles a primitive nest might, by a stretch of the imagination, be seen. But the best nest of all is the uterus.
to
The Uterus
Paired oviducts are found in many fish, but only rarely are they used to retain the embryo during gestation. In mammals the oviducts tend to come together, so that often there is a small median chamber and two lateral horns. Now the oviducts are not simply conveyors of the eggs to the exterior; they are receptacles. With two uterine horns there is room for the development of several embryos at This may be satisfactory for terrestrial animals once. where there is little danger on the ground, and especially when the animals live in herds. But in the trees, where the Primates live or have lived, a number of offspring would be at a serious disadvantage from falling or being dropped when the mother is in movement, and therefore selection seems to have favoured the single median uterus. Moreover selection has gone deeper than this, for it has favoured an endocrine apparatus which allows, as a general rule, only one ovulation at a time. There are advantages in having a single foetus, for it is not in competition with others for its food and respiratory and other needs. We see the effects of intrauterine competition in the reckling of a litter, and in the prematurity of twins. Twins are an atavistic reversion to a mode of reproduction less satisfactory for arboreal creatures, but there is still high wastage in twins, both before and after birth. The lower Primates-the tree shrews and the lemurs-
have a twin-chambered uterus, and they produce two or three young at a gestation. The lemurs and lorises are in the main nocturnal and slow-moving. It may be that these are adaptations to the care of the young. In the Tarsioidea about half the uterus (the lower part) is single and the upper part is divided (Wood Jones 1926). By the time of the appearance of the Anthropoidea-the monkeys, apes, and Man-the uterus is single. Occasionally we see the backward evolutionary glance in the varying degrees of double uteri of women, but these are relatively inefficient, as shown by the high abortion-rate. Internal development of the foetus obviously sets a limit on the possible size of the baby or babies at birth, for the abdomen can only distend to a certain size. We see the problems for the mother of over-distension in some cases of hydramnios and twins. The marsupials, especially the kangaroo, show an unusual anatomy in that the ureters pass between the two horns of the uterus, a condition not found in any other species, according to Wood Jones (1950). This is probably one reason why the foetuses are expelled early from ’the uterus in this animal. They then climb up the ventral fur and drop into the pouch, there to fasten on to the nipples. The epithelium of the mouth grows so intimately with the maternal nipple epithelium that if the baby be forcibly removed its mouth is lacerated. Uterine Contractions
The uterine contractions of labour
seem
almost invari-
ably to be painful. Smythe (1960) says that "every prospective mother, living in her natural environment, prefers, to be left in solitude at the time of her parturition", It seems that only women and elephants have someone in attendance at birth, though Peter Scott, in a recent television broadcast, said that whales too usually have a helper who nuzzles the newborn to the surface so that it may breathe. The pain of labour seems to be a signal for the mother to take herself off to a place of safety for herself and her offspring. The exponents of so-called painless childbirth often forget that the same signal is necessary for women today. If they did not have pain, babies would be born in sometimes embarrassing circumstances. This is not to say that I deprecate methods of painless childbirth, for these methods are basically designed to reduce the psychological tensions of childbirth, which because of our culture are often greater than they need be. With this aim I entirely agree; but it is doubtful whether labour can ever be truly painless; the reasons for its discomfort, to put it at its least, are too biologically profound. In man some pain may also be dependent on the relatively large size of the foetal head, and this is a necessary price for the evolution of a brain of our capacity. Menstruation
Menstruation is a purely Primate characteristic. It is found in the Old World monkeys, but less often among those of the New World, which in many other ways seem well off the line leading to Man (Harrison 1958). It is found too in all the Pongidae (gibbon, orang, chimpanzee, and gorilla). Its significance is far from clear. It may be that the preparation of the endometrium for the implantation of the fertilised ovum is so intense, and must needs be so, that only necrosis and shedding is a possible way of involution; or it has been suggested that superficial necrosis supplies essential nutrients for the early development of the embryo. Ramsey (1959) has suggested that the intense coiling of the arterioles of the endometrium is an
391
adaption which allows them to be stretched at the time of conversion, which she defines (1960) as " the period when uterine enlargement by virtue of growth ... gives place to enlargement by stretching alone ". If these coils are not so used, they may cause necrosis by stagnation of blood-flow. Associated with menstruation is, of course, the conAll other mammals have tinuous breeding season. relatively infrequent sexual cycles (Frazer 1959). A continuous breeding season gives tremendous biological advantage, and in part offsets the slowness for the species of producing only one offspring at each gestation. In lower mammals bleeding from the genital tract occurs at restrus (i.e., about the time of ovulation), and also when the ovum first implants. Both these features are occasionally seen in women, and red cells are always to be found microscopically in the cervical mucus at the time of ovulation. The newer pattern of menstruation has suppressed and overlaid the older ones; but they are still there, at least in some women. The Placenta
A most important organ for the biological success of the Indeed the mammals are divided into the Monotremes, the Marsupials, and the Placentals. The Monotremes, exemplified by the duckbilled platypus and porcupine ant-eater, lay eggs but suckle their young. Echidna incubate a single egg in a pouch (Wood Jones 1950). The Marsupials I have already mentioned. The nourishment of the embryo from the mother was tried in part as an evolutionary experiment before the placenta came on the scene. Although embryos of the ovoviviparous species feed off the yolk, there is a contact of the embryonic and maternal bloodstreams through which respiratory exchange probably occurs. Embryos tucked into the dorsal pouches of toads also have similar gaseous exchange. Perhaps neither of these examples are of placentas proper, but they point in that direction. Amoroso (1952, 1959, 1961) has extensively documented the comparative anatomy of the placenta, and his writings are now classical. It would be simple if we could build an evolutionary series on the basis of Grosser’s classification of how close the foetal and maternal bloodstreams come together. But unfortunately this cannot be done. Grosser classified placenta: as epitheliochorial, syndesmochorial, endotheliochorial, haemochorial, and haemoendothe1ial. This is not the place to define these more fully, but the classification is based on the degree of penetration of the foetal placenta into the maternal tissues. The human placenta is, of course, hsemochorial. Among the rest of the Primates all types of placentx are found. The tree-shrews have an endotheliochorial placenta, and the lemurs an epitheliochorial one. On the other hand, other Insectivora, supposedly lower in the scale than the tree-shrews, have a hxmochorial placenta (le Gros Clark 1959), and the same is true of Tarsius. Our understanding of the significance of these various placental types must await further evidence. But one fact does seem clear, and that is that the number of layers separating the maternal and foetal bloodstreams are not a good guide to functional
Mammalia is the placenta.
efficiency. There
variations too in the gross anatomy of the may be diffuse and cover the whole chorion; sometimes it is a zonary band; sometimes it has several discrete tufts; sometimes there are two discs, as in many monkeys; and sometimes there is one disc, as in Man. are
placenta. It
Rarely human placentae show these evolutionary variations, as in the succenturiate lobe, and occasionally in tufts having no blood-supply from the foetus. During development the human placenta passes through a diffuse phase (seen in abortions before about the fourteenth week), and it only becomes discoidal at about the sixteenth week. Perhaps it is not fanciful to see during implantation the rapid transition through epitheliochorial, syndesmochorial, and endotheliochorial stages before the placenta becomes finally hasmochroial. Romer (1933) gives a diagram of a typically reptilian egg (fig. 1). The reptiles began about two hundred and fifty million years ago, and
already chorion, allantois, and yolk-sac are yet the
be seen. The embryo feeds on the yolk, and the to
allantoiss spreads beneath the
shell, becoming richly vascularised.
Fig. 1-Reptilian 1933.)
egg.
(Redrawn from Romer
Respiratory exchanges take place here. With internal development there is, of course, no necessity for a shell; but all the other structures, often modified, are still found as high up the scale as Man. Moreover, they still have much the same functions. The allantois is a receptacle for waste matter, and it is incorporated in the urinary bladder. The placenta of Man is described as being chorioallantoic. Early in development the allantois is close to the chorion; later it is separated from it, but the umbilical arteries are the allantoic vessels prolonged to the placenta. When they become obliterated shortly after birth, their proximal ends remain as the superior vesical arteries-i.e., they supply the remains of the allantois. Here is a most remarkable demonstration of how evolution takes older patterns and remoulds them. Rarely does an entirely new pattern begin. It just grows out of the old. And there is a lesson here of more general importance. Much has been written about the brain and its evolution, and this is natural, for on this is primarily based Man’s ascendancy; but selective pressures act on whole individuals and not on single adaptations. In any account of evolutionary advance the placenta must be accorded a
high place. It is just possible that biological evolution is still affecting the placenta. I have seen it suggested that there may be a gene for deep penetration of the placenta, perhaps allowing for better nutrition of the foetus at the probable cost of postpartum haemorrhage in the mother. There is the gentlest of gentle hints that this may be so in those women who have recurrent postpartum hoemorrhage, and in those rare instances where this complication of in families. A phase of nutrition of the embryo that is often ignored is that before embedding. At first the zygote is floating in the fluid secretions of the fallopian tubes and the endometrium. All division happens at a great rate, so that The metabolic processes must be proceeding apace.
labour appears
to run
392
zygote is living like estuarine waters.
a
fish
or
The Birth of the
amphibian embryo
in
Baby
The Bony Pelvis
It is
to walk through any natural history and see the changes wrung out of the basic pattern of the three bones of the pelvis-ilium, ischium, and pubis. This pattern began with the amphibia, about three hundred million years ago. Always there are the same relations that we know in Man. The ilium articulates with the vertebral column, and the pubis is ventral, but it does not reach across the midline in all forms. Occasionally epipubic bones project along the abdominal wall, perhaps to increase support for the viscera. The pelvis is a skeletal support, an area for muscle attachment, a protector of its contained viscera, and a birth canal. In trying to be so much it tends to fall short of the ideal in some respects (fig. 2). The area of attachment of the ilium to the vertebrae is relatively small; both the pubes and the ischia are joined in an ischiopubic symphysis; and the axis of the pelvic canal and the abdominal cavity are in a straight line. Man’s upright posFig. 2-Diagram of a typical pelvis of ture was the evolutiona quadrupedal mammal. (Redrawn ary change that freed from Wood Jones 1926.) his forelimbs and began his brain development. When an animal is upright and securely based on the hindlimbs, the forelimbs are able to explore the environment and feed information into the brain. In an arboreal environment the hands and arms need a nice coordination of movement supplied by the brain; but more than this is required, because judgment of distance is essential, and this is supplied by stereoscopic
fascinating
museum
vision. To get the skeleton permanently upright requires great anatomical changes. If the quadruped pelvis and spine were rotated through 90° we should have had the benefit of a birth-canal which is absolutely straight, but abdominal pressure would fall with maximum force on the pelvic floor, probably making prolapse of the pelvic viscera even more prevalent than it is now. In fact the pelvis has been rotated dorsally through only 30-40°, giving an angle of inclination of 50-60°. This evolutionary rotation can be seen in the oblique fibres of the hip-joint capsules. To get the trunk fully erect the lumbar lordosis is introduced, and to get the head erect the cervical lordosis. Much of these two secondary curves depends on the intervertebral discs, whose liability to damage and pathological change is so well known. This is one of the prices paid for the human brain. Failure to rotate the pelvis through 90° may prevent the full transmission of intra-abdominal pressure directly on to the pelvic floor. Abdominal viscera now lie partly on the anterior abdominal wall and partly on the pubis, and mesenteric attachments have shifted too, to hold up the viscera. This evolutionary change possibly explains why so many of us in middle age bulge slightly below the umbilicus. Perhaps it is better here than in the perineum. The lumbar lordosis pushes the axis of the pregnant uterus well forwards, and this gives rise in part to the
curved axis of the birth-canal. If we had retained a long ischiopubic symphysis the birth-canal would be a long bony cylinder, because to fulfil its weight-bearing function the area of attachment of the ilium to the vertebral column has increased, so lengthening the posterior wall of the birth-canal. Incidentally, in women the area of attachment is to 2-21/2 vertebrae and in men to 21/2-3. Here are seen the conflicting claims of the pelvis as a support and as a part of the reproductive system; the result is a high incidence of low backache in women. If the ischiopubic symphysis had remained it might have been impossible to get the large human festal head born at all, and therefore the ischial part of the symphysis has become open, so shortening the anterior wall of the birth-canal (fig. 3). This removes some of the support of the bladder. When the two partially terrestrial great apes-the chimpanzee and the gorilla-are on the ground, they progress in part on all fours. The pelvis is partly rotated because the forelimbs are longer than the hind ones (fig. 4), but there is little lumbar lordosis, and the birth-canal is almost straight (Howells 1960). If you look at a newborn baby Fig. 3-Adaptation of the ischiopubic symphysisin you will see that his hips are Man. (Redrawn from well flexed on his body, and the Wood Jones 1926.) thighs cannot be extended to the adult position until he is about a year old. The evolution of the upright posture can be seen compressed into this first year of life. It seems that the development of the brain through the upright posture has imposed certain limitations on reproductive function, for some difficulties of parturition are due to the sinuous curve of the birth-canal, which in some measure is responsible for the extraordinary convolutions through which a baby must go to be born. When the birth-canal is very curved, as in high inclination of the brim, it may be impossible for the foetus to bend round the corner, and the head remains high. The Pelvic Floor
pelvic floor is represented in fish by some strands of muscle, and the pelvis by a cartilaginous bar (Smout The
Fig. 4-The human pelvis and the pelvis of the partially terrestrial great apes.
393
Jacoby 1953). Only with the advent of lungs and a diaphragm does the pelvic floor increase its activity. During respiration, intra-abdominal pressure is constantly changing, and this must be counteracted at the hind end The tail muscles are of the abdominopelvic cavity. this function-another into example of using pressed what has gone before rather than devising something novel. In quadrupeds the pelvic floor is relieved of some stress by gravity, and we see something similar when women are examined in the knee-chest position. Moreover these muscles are in constant activity because of tail movements. These two factors probably explain why prolapse is so rare in quadrupeds. In women there is greater immediate stress on the pelvic floor because of the and
erect posture, and the muscles do not have the exercise derived from a tail. But the tail muscles are still there-the pubococcygeus as the tail-depressor, and the iliococcygeus and ischio-
The insertion of the slipped forwards to give more direct support to the viscera, and so it has become more prone to injury at birth. The iliococcygeus in man has no contact with the ilium, because during evolution its origin has slipped down from its original attachment at the pelvic brim to be in line with the origins of the other two muscles, but there is still a gap between the iliococcygeus and the coccygeus, and the obturator nerve defines the junction between pubococcygeus and iliococcygeus. (It is a general rule that where a nerve perforates a muscle that muscle is derived from morphococcygeus
as
the
tail-waggers.
pubococcygeus has, of
course,
logically distinct components.) The troubles of the pelvic floor in women need no elaboration. They seem to be part of the evolutionary price to be paid for our essential humanity, which resides in the brain. Only when brains are organised at a sociological level can they return, with the arts of obstetrics and gynaecology, to restore some of the ravages which being human thrusts on reproductive function. It has taken perhaps fifty thousand years for the brain to make this elementary act of courteous reparation. Reduction in the Numbers of Offspring Despite several exceptions, there has been an evolutionary trend to reduce the number of offspring at each gestation. As a general rule, fish reproduce in hundreds, amphibia and reptiles in scores, birds in tens, and mammals in under tens. The most remarkable reduction is to one in man. It has already been suggested that this is probably associated with an arboreal life in our ancestors. Primate babies are usually carried clinging to the ventral fur of the mother, though sometimes they cling to her back (Sanderson 1957); gibbons may hold their babies in their laps as they brachiate through the trees. But an important feature of the single offspring is that greater care can be and is lavished on the baby both before and after birth. Here again obstetrics and paediatrics are continuing a biological trend of evolution. Care of the offspring after birth is almost non-existent in fish, amphibia, and reptiles, though there are exceptions. With birds much care is given, but relatively longcontinued care after birth is thrust upon mammals by
lactation. Breasts
Breasts are the distinguishing feature of mammals. Where litters are large the mammary glands extend in a bilateral line from the axillary to the inguinal regions. With a reduction in litter size breasts tend to remain
either
the others being supthe bilateral milk pressed. Nevertheless, ridge appears transiently during embryonic life, and remnants of it are not at all uncommon in accessory nipples. Occasionally too there are accessory breasts in the mammary line; these are usually very near the normal breasts, but they have been described as far down as the groin. A few women have breasts extending from the 3rd to the 7th ribs instead of from the 2nd to the 6th. Inguinal mammary glands are favoured in quadrupeds, as seen in many domestic animals. This may be to leave the mother free at the head end for feeding herself and for keeping on the alert for danger. Pectoral breasts are at
the caudal
or
cephalic end, even
in
man
characteristic of most primates, though lemurs have a non-functional teat in the inguinal region to which the baby clings with its mouth whilst being transported (Wood Jones 1926). These breasts are an adaptation to arboreal life, for the mother cradles the baby in her forelimbs to prevent it falling. Pectoral breasts are similarly developed in the seal, where the young are held during suckling. This may be responsible for the myth of the mermaid. Permanent breasts are almost peculiar to women. In most mammals the mammary glands wax and wane with the reproductive cycle, and a remnant of this in women is the obvious variation in size during pregnancy and lactation, and during the menstrual cycle. It may be that permanency of the breasts is of sexual significance, and it may even influence sexual selection. As the brain develops more emphasis is placed on sight than on smell, and the rhinencephalon diminishes, though the importance of scent is not entirely lost, for the world is combed for perfumes for men and women. That the breast is a sexual symbol of today is apparent from almost every advertisement, and this adulation of the female form is in part responsible for the decline in breast-feeding. Our culture has made the breast more of a symbol than a functioning organ. The Fretalisation of the
Baby
All babies tend to be helpless, but none is so helpless for so long as the baby of Man. For about a quarter of our whole life-span we are physically maturing, and perhaps mental maturing takes even longer. Our physical attributes too are remarkably primitive. We may think of our hands doing wonderful things, but a frog would recognise our limbs as very like his own. The reason we can be so dextrous is not the intrinsic anatomy of the hand and arm, but the brain which informs the activity. This and many other primitive features of our anatomy, which are very unspecialised, are often referred to as foetalisation—i.e., we stay anatomically in a foetal stage. It is this very unspecialisation of anatomy, and immaturity of the baby, that is our greatest evolutionary strength. If in a species a forelimb becomes specialised, as in the horse, it cannot go back to the primitive evolutionarily plastic pentadactyl limb. The species is doomed for ever to be limited to the ground, and to running and walking. It will never grasp, feel, understand texture, and manipulate things, and its brain can never learn from these activities, because its limbs can be used only for support. The anatomically specialised can fill only a narrow environmental niche. With a relatively unspecialised anatomy man seems able to fill any evolutionary niche. Because of hostile environments, the young of most species have to be born mature enough to be able in large measure to look after themselves with little help
394
from their parents. This means that they must be born with some instincts. Among the mammals, seals can swim and bats can fly at birth; fawns can run fast within hours of birth (Harrison 1958). Such adaptations are important in larger mammals, whereas the smaller ones can be hidden away in a nest of straw or a burrow safe from harm for a while. Primate infants have to be able to cling to fur so that they may be transported safely through the trees, and the remarkable grasp of the young human infant may be of this nature. But no baby of any species is born so immature as the human one, and yet the brain is one-quarter of its adult size. The immaturity of the baby and its large brain are to do with the long process of learning. By learning we inherit our culture which " consists of everything which has ever been accepted as a way of doing or thinking, and so taught by one person to another " (Howells 1956). By learning we inherit the acquired characteristics of culture. By this we form a complete break with biological evolution, in which acquired characters cannot be inherited. Biological evolution progresses by the interaction of the environment on the genetical inheritance of the species, which comes at fertilisation and is changed little at each generation. Genes may be likened to a large pack of cards which undergoes constant reshuffling, though the cards themselves remain unchanged. The muscular development of the blacksmith’s brawny arm cannot be impressed on his genes, and so it cannot be handed on to his son; but his acquired skill at the forge can be handed on because his son can learn. In just such fashion is knowledge handed on, though it is not only handed on but added to, selected, and refined. This is why social evolution so completely outstrips biological. Life began five hundred million years ago. Social man probably began only about fifty thousand years ago, and yet the scene has changed beyond recognition. In his early phases Man had to adapt his body to the environment ; now he adapts the environment to his body. The world is turned upside down. There are many biological adaptations of the foetus, so that he can emerge into social man. Intrauterine development seems largely given up to pushing the brain to the maximum size compatible with getting it safely through the birth-canal. This tremendous growth is seen postnatally too, for by the age of seven the brain has attained its adult weight. Of course, other systems have to be started on their way so that the brain may use them when appropriate, but no intra-uterine nourishment is wasted on the legs, and arms, and face. The neonate is little more than brain and viscera. The fcetal circulation is a fantastic adaptation. The liver is bypassed by the ductus venosus, and the lungs by the ductus arteriosus and foramen ovale, because the placenta is carrying out hepatic and pulmonary functions. Yet both lungs and liver are ready to start functioning at birth. Barcroft’s researches (1946) into the developing physiology of respiration revealed an astounding situation: first come the total body movements from which later the respiratory movements are abstracted; then all goes quiescent until after birth, when the whole set of reflexes previously laid down is suddenly called into being, and the infant breathes. I often pause in wonder when palpating a maternal abdomen and I feel those rhythmic tappings of the foetus which I am sure are the same as those Barcroft described in the sheep-the foetus learning to breathe before it has ever sniffed the air. Such wonder
is not sentimental; for, whether you believe this is God’s work or the work of blind evolutionary forces with desoxyribonucleic acid calling the basic tune, it is astounding. Convinced evolutionist though I am, at moments like these I see the yawning chasms dividing the outline of a theory from the full explanation of human life-indeed all life-in terms of that theory.
Psychological and Social Evolution From this point we move into the world of social evolution. Now we are banded together in groups-
family, local, school, professional, technical, political, national, and international. We have the old biological nature, but there is now a vast superstructure built on it. The environment of Man is Man himself and his varying cultures. It is this phase of evolution that we all fervently hope we can control. So far we seem to have little inkling of how to do it. Obstetrics has arisen in response to a need for the social ordering of childbirth. It is a part of the social system, acted upon by it and acting on it. The most recent impact upon it has been the public demand for the better psychological care of mothers. To be fair, the movement arose from within the ranks of obstetrics, but is now carried far beyond its bounds. Society has cast up many more problems for obstetrics -for instance the early maturing of girls and the high illegitimacy-rate, recently estimated as 5% in England and 10 % in London, which by its vast anonymity attracts many unmarried mothers away from their homes and social ostracism. Because better nutrition has increased the size of the bony pelvis, disproportion is of relatively small importance now, and perhaps this may help to lower the frequency of prematurity. At the same time, I think it may well be extending the gestation period and offering problems in postmaturity. Late age at marriage, especially in social group i, or the postponement of childbearing because of financial and housing insecurity, hands us the problems of the elderly primigravida. These are factors imposed by our culture. Similarly imposed are the myths and half-truths which many unfortunate patients bring with them to childbed, making themselves unhappy and apprehensive. This is a fault in the education of girls, inculcated by our own attitudes based on a long history of Puritanism. Attempts are being made to alter this by a steady stream of booklets advising pregnant mothers, antenatal classes, and so on; but to change the cultural attitudes, gained in twenty years, during the last six months of pregnancy is simply not possible. Education about childbirth must reach back much further into the home and school. Obstetrics has thrown up problems for society too. The population explosion, so called, is in some measure due to the saving of more infant lives. Only about fifty years ago 1 in 5 infants and neonates died. Now it is fewer than 1 in 25. Reports from Africa suggest that a third to a half of babies die. But obstetrics, together with many other social improvements, is altering this. The world population is rising and fast. This year there will be 3000 million on the Earth, and it is worth noting that 1 in every 3 of us will be Chinese or Russian (Times 1960). There is beginning reaction to this rising birth-rate, and it is to be seen in increasing use of contraceptives, research into contraception, the legalisation of abortion in many countries, and the vast rate of criminal abortion. It can be computed that in England and Wales there must be about 200 induced abortions every day, and this
395 is probably a conservative estimate. There are pressure groups for the practice of eugenics, and sterilisation for women with large families is being more readily practised in many places. At the opposite end of this scale, never has research into infertility been more fervently pursued. These, and no doubt many others, are the growing edges of patterns in human reproduction. They arise because of tremendous biological and obstetrical success. I am not a sociologist and therefore will not offer any opinion on how some of these problems should be faced. For that matter I am not a biologist in any professional sense; but I think it right that, however, distorted the view may be, we should lift our eyes occasionally from our immediate preoccupations to look at what goes on
around us. REFERENCES
Amoroso, E. C. (1952) in Marshall’s Physiology of Reproduction; vol.II. London. (1959) in Gestation; Vth Conference of the Josiah Macy Foundation.
In the Rabbit It has been known since 1833 that the rabbit’s circle of Willis is a functional anastomosis between the carotid and vertebral systems: Mayer (1833), who injected mercury into the carotid arteries of rabbits, a ram, and a goat, noted post mortem that globules of mercury appeared in the carotid vessels of the opposite side, and in one rabbit in the basilar artery. Marckwald (1890) made similar observations in rabbits by injecting dyed paraffin and olive oil. McDonald and Potter (1951), in very precise experiments, showed that in the normal rabbit there was no flow along the communicating arteries because pressure was equal at both ends; but immediately after one carotid or vertebral artery was occluded, flow occurred through the appropriate communicating channel. Chungcharoen et al. (1952) also demonstrated the efficacy of the normal communicating arteries in maintaining perfusion pressure in the carotid sinus immediately after common-carotid occlusion.
—
New York. (1961) Brit. med. Bull. 17, 81. Barcroft, J. (1946) Researches on Pre-natal Life. Oxford. Eiseley, L. (1961) The Firmament of Time. London. Frazer, J. F. D. (1959) The Sexual Cycles of Vertebrates. London. Harris, H. (1959) Human Biochemical Genetics. London. Harrison, R. J. (1958) Man the Peculiar Animal. London. Howells, W. (1956) Man in the Beginning. London. (1960) Mankind in the Making. London. Jost, A. (1958) in Hermaphrcditism, Genital Anomalies and Related Endocrine Disorders (edited by H. W. Jones and W. W. Scott). London. le Gros Clark, W. E. (1959) The Antecedents of Man. Edinburgh. (1961) in the Times, Aug. 31. Medawar, P. B. (1957) The Uniqueness of the Individual. London. Parrington, F. R. (1950) in Chambers’s Encyclopædia (Reptiles). London. Ramsey, E. M. (1959) in Gestation; Vth Conference of the Josiah Macy Foundation. New York. (1960) in The Placenta and Foetal Membranes (edited by C. A. Villee). Baltimore. Romer, A. S. (1933) Man and the Vertebrates. London. Sanderson, I. T. (1957) The Monkey Kingdom. London. Smith, J. M. (1958) The Theory of Evolution. London. Smout, C. F. V., Jacoby, F. (1953) Gynæcological and Obstetrical Anatomy. London. Smythe, R. H. (1960) The Female of the Species. London. Times (1960) Report on Population Reference Bureau Statistics, Washington. Willmer, E. N. (1960) Cytology and Evolution. London. Wood Jones, F. (1926) Arboreal Man. London. (1950) in Chambers’s Encyclopædia (Marsupials; Monotremata). London.
METHODS
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—
—
—
—
Rabbits weighing over 2-2 kg. were subjected to artery under pentobarbitone and ether anxsthesia. The carotid preparation was a bilateral ligation of the internal and external carotid arteries which preserved the central artery of the ear for recording blood-pressure (Lowe 1962a). All arteries were cut between silk ligatures. The development of a collateral supply was investigated by measurement of retinal-artery pressure in vivo and by making resin casts of the cerebral vessels in vitro. Systolic retinal-artery pressure was measured in anxsthetised rabbits by ophthalmodynamometry, using the technique described for man by Thomas and Petrohelos (1953). The results are expressed as grammes of applied pressure, which estimates, in arbitrary units, how much the retinal-artery pressure exceeds the resting intraocular pressure (Lowe 1962b). At various stages rabbits were killed by an intravenous injection of pentobarbitone 240 mg. containing heparin 1000 units, and resin casts of the arteries of head and neck were made with’Marco’ resin mixture. t
ligation
RESULTS
ADAPTATION OF THE CIRCLE OF WILLIS TO OCCLUSION OF THE CAROTID OR VERTEBRAL ARTERY: ITS IMPLICATION IN CAROTICOVERTEBRAL STENOSIS R. D. LOWE Cantab., M.R.C.P.
In Vivo
The sequence of
retinal-artery pressure changes after carotid-artery ligation is shown in fig. 1: Normally there is little difference between the two eyes. t Supplied by Scott Bader & Co., Wollaston, Wellingborough, Northants.
M.B.
LATELY RESEARCH ASSISTANT, MEDICAL UNIT, UNIVERSITY COLLEGE HOSPITAL, LONDON, w.C.1*
THERE is no doubt that occlusion or stenosis of carotid vertebral arteries is a common finding in cases of stroke, and it is natural to suppose that the stroke is a direct consequence of obstruction to the flow in the artery, producing a lowered perfusion pressure and cerebral ischaemia in its area of supply. Such an explanation would imply that the anterior and posterior communicating arteries do not provide an adequate collateral flow and are unable to compensate for ischsemia in one quadrant of the circle of Willis. This paper reports experiments designed to study adaptation in the circle of Willis following carotid occlusion. The rabbit was selected as a model because its circle of Willis is remarkably like that of man, and retinal-artery pressure measurements in vivo were used as an indirect estimate of pressures in the circle of Willis both in man and in the rabbit. In the rabbit the findings with this technique were confirmed in vitro by preparing resin casts of the arteries. or
* Present address: St. George’s Hospital,
London,
S.W.1.
Fig. 1-Retinal-artery systolic carotid-artery ligations.
pressure in rabbits
during successive
Each point shows the mean and standard error of duplicate ophthalmodynamometer readings. (Twenty-six animals, reducing to twelve by week 18.) R = right carotid ligation. L left carotid ligation. v vertebral artery ligated or stenosed (six animals). =
=