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Species recognition and homo erectus
C u r r e n t Events
Journal of Human Evolution (1986) 15, 823 826
Species Recognition and H o m o
erectus
A recent paper by Tattersall (1986), prompting further comment by Lewin (1986), raises questions about the genus Homo which are familiar to paleontologists. How species are to be recognized from the fossil record has been debated before and the extent to which such extinct taxa are real and discrete rather than arbitrarily defined is still unsettled. Tattersall adopts the first view and argues that the number of Homo species has been seriously underestimated. Our own genus is usually regarded as a single lineage, composed o f Homo habilis, Homo erectus and Homo sapiens, but Tattersall would recognize several additional taxa, adding branches to this simple linear progression. In this brief communication, I shall address only the question of geographic variation within Homo erectus. A more detailed monographic discussion is in preparation. One particularly interesting time period is the earlier Pleistocene, between about 1-5 and 0-5 million years ago. According to Tattersall and Lewin, one or more species in addition to Homo erectus may have lived during this interval. The legend accompanying the photograph of one well preserved Koobi Fora cranium illustrated by Lewin asks whether this African fossil represents the same species found in Java and elsewhere in the Far East. This is an important issue, as species (not grades) should indeed be the focus of evolutionary study. Whether Homo erectus is restricted to Asia will have to be determined before human phylogeny can be reconstructed with any certainty. Fortunately, quite a lot of fossils are available for examination. Unlike Homo habilis, Homo erectus is known from numerous specimens, some of which are very complete. I contend that all of this African and Asian material can be allocated to one species, which must be broadly ancestral to later people. The claim that at least two distinct morphs are present in assemblages usually attributed to Homo erectus was widely aired at a conference held at Bad Homburg in 1983 (Stringer, 1984; Wood, 1984). One set of fossils was said to include individuals from Trinil, Sangiran, Ngandong, Zhoukoudian and various other Asian localities, all thought to be latest Early Pleistocene or Middle Pleistocene in age. Another group, represented by specimens from Koobi Fora, is substantially more ancient. It is presumably to this latter collection that other East African material from the Turkana Basin and from Olduvai Gorge should be assigned. Middle Pleistocene hominids from Ternifine (Tighenif) in Algeria and sites in Morocco were not discussed in detail at the conference, so their position with respect to the Asian and East African morphs was not clarified. 0047 2484/86/080823 + 04 $03.00/0
9 1986AcademicPress Inc. (London) Limited
824
CURRENTEVENTS
It was argued that the many anatomical characters shared by these specimens and cited routinely in descriptions of Homo erectus are in fact primitive, being present also in other hominids or in living hominoid primates. Such traits, considered to be retentions from a common ancestor, do not help generally to define Homo erectus relative to other taxa. Species diagnoses should be based instead on characters which are novel or derived, and preferably unique. The importance of autapomorphies is stressed (e.g., by Wood, 1984), despite the difficulties which will inevitably arise as more and more characters are described as unique. If it is insisted that a species be defined in this way, then the problem of recognizing descendants will become increasingly acute. We are likely to be confronted by a paradox. As a species is diagnosed more completely, it will be harder to place it in any ancestor-descendant sequence. Specializations in Homo erectus are few in number but seem to include several anatomical structures associated with the cranium. Examples are midline keeling of the frontal or parietal bones, a torus or bulge at the mastoid angle of the parietal bone, development of a fissure between the mastoid process and the tympanic plate, narrowing of the medial part of the glenoid cavity, and thickening of the bones of the vault. The addition to this list of a prominent brow ridge backed by a flattened expanse of frontal bone, and a strong occipital torus, was supported by some of the conference participants but not by others. A crucial point in the discussions was the assertion that these derived characters are expressed in the Far Eastern fossils but not in the Koobi Fora crania. Andrews (1984) interpreted this anatomical evidence to prove that Homo erectus is a species restricted to Asia. He has suggested that this taxon became extinct later in the Middle Pleistocene, while another (unnamed) lineage accommodating the African assemblages probably continued to evolve toward Homo sapiens. The view that Homo erectus is strictly an Asian branch of the hominid clade is thus based on the distribution of a few key characters, said to be lacking from the East African remains. If this claim is to be evaluated fairly, then not only the most complete Koobi Fora crania but also other Turkana specimens must be examined. Several fossils from the Koobi Fora region have been described recently (Leakey & Walker, 1985), and new discoveries have been made at Nariokotome (Brown et al., 1985). Comparisons should include this material as well as that from Olduvai. When all individuals are checked systematically, it is apparent that differences between the African and Far Eastern assemblages are not clear-cut. The supraorbital torus is especially gracile in ER-3733, although a supratoral shelf is present. It is noteworthy that this Koobi Fora cranium does display a sagittal keel, to either side of which the frontal squama is quite flat. The brow of ER-3883 is thicker, as is that of ER-730. Here frontal form begins to resemble that seen in the very massive hominid from upper Bed I I at Olduvai. The skull from Nariokotome, while still juvenile, already shows substantial torus development and would likely have become more heavily buttressed than ER-3733. Thickness of the brow and overall robusticity of these African crania must surely overlap with the condition found at Trinil and Sangiran. Another important trait is the angular torus. Neither of the more complete Koobi Fora crania has a strong torus, but such a parietal bulge must have been expressed in the small, thick-walled vault of O H 12. This character occurs in Sangiran 4 and (faintly) in Sangiran 12 but not in Sangiran 2 or in Sangiran 10. It is prominent on some but not all of the Zhoukoudian parietals, and it may be recognized in several other archaic skulls usually referred to Homo sapiens.
CURRENTEVENTS
825
ER-3733 exhibits a blunt occipital torus which is most projecting near the midline. The lower margin of this structure is defined by the (superior) nuchal lines, which converge at a central linear tubercle. There is some supratoral hollowing. This morphology is mirrored by other African occipitals, with minor variation. Several of the small Asian hominids are similar, and the transverse torus is much more prominent and shelf-like only in a few individuals, such as Sangiran 4. The glenoid cavity and adjacent structures are well preserved in a number of cases. In the Koobi Fora fossils and in O H 9, this fossa is constricted medially, to produce a deep recess between the entoglenoid pyramid and the tympanic plate. The plate itself is thickened in comparison to that of modern humans and terminates in a blunt process supratubarius. Sangiran 2 is much the same, while in Sangiran 4 the tympanic bone is elongated vertically and is less robust. The larger-brained Indonesian and Chinese individuals present a similar pattern. Perhaps the only feature of the mandibular fossa which can be linked to a particular geographic region is the narrow fissure separating the tympanic plate from the mastoid process behind. This trait seems to be present in (some) Asian hominids but not in Africa. However, such a crevice may easily be distorted through bone crushing or filled with matrix, and its apparent absence in African crania should not be weighted too heavily in any discussion of affinities. I conclude that characters judged to be derived for Homo erectus are variably expressed, both in the Asian and in the East African remains. Hublin (1986) also points out that features which are derived for Homo erectus are present inconsistently, even among individuals making up a single Asian "population" (e.g., Ngandong). There is no indication that the African assemblages should be identified as a different species. Anatomical specializations together with many primitive traits of the braincase, lower jaw, teeth and postcranial skeleton (Howell, 1978; Howells, 1980; Rightmire, 1984) describe Homo erectus and serve to diagnose this taxon relative to Homo sapiens. Apomorphies listed above help also to distinguish Homo erectus from earlier Homo and from other hominids. I share with Tattersall (1986) the view that Homo erectus, like other paleontological species, should be defined anatomically as a discrete entity rather than an amorphous grade. This taxon seems to have originated in Africa. During the Early Pleistocene, Homo erectus spread widely through the Old World tropics. Some later populations were probably ancestral to more modern people, which appear in the European record and elsewhere in the Middle Pleistocene. Whether these later hominids are to be allocated to more than one species as suggested by Tattersall (1986) is a question deserving careful study. G. PHILIP RIGHTMIRE Department o f Anthropology, State University o f New York, Binghamton, New York 13901, U.S.A.
References
Andrews, P. (1984). An alternative interpretation of characters used to define Homo erectus. Cour. Forsch. Inst. Senckenberg 69, 167 175. Brown, F., Harris, J., Leakey, R. & Walker, A. (1985). Early Homo erectusskeleton from west Lake Turkana, Kenya. Nature 316, 78~792. Howell, F. C. (1978). Hominidae. In (V. J. Maglio & H. B. S. Cooke, Eds) Evolution of African Mammals, pp. 154-248. Cambridge: Harvard University Press. Howells, W. W. (1980). Homoerectus- who, when and where? A survey. Yb. phys. Anthrop. 23, 1 23.
~26
CURRENTEVENTS
Hublin, J-J. (1986). Some commences on the diagnostic features of Homo erectus. Anthropos (Brno) 23, 175-185. Leakey, R. & Walker, A. (1985). Further hominids from the Plio-Pleistocene ofKoobi Fora, Kenya. Am.J. phys. Anthrop. 67, 135-163. Lewin, R. (1986). Recognizing ancestors is a species problem. Science 234, 1500. Rightmire, G. P. (1984). Comparisons of Homo erectus from Africa and southeast Asia. Cour. Forsch. inst. Senckenberg 69, 83-98. Stringer, C. B. (1984). The definition of Homo erectus and the existence of the species in Africa and Europe. Cour. Forsch. Inst. Senckenberg 69, 131-143. Tattersall, I. (1986). Species recognition in human paleontology. J. hum. Evol. 15, 165-176. Wood, B. (1984). The origin of Homo erectus. Cour. Forsch. Inst. Senckenberg 69, 99-111.
Journal of Human Evolution (1986) 15, 823 826
Species Recognition and H o m o
erectus
A recent paper by Tattersall (1986), prompting further comment by Lewin (1986), raises questions about the genus Homo which are familiar to paleontologists. How species are to be recognized from the fossil record has been debated before and the extent to which such extinct taxa are real and discrete rather than arbitrarily defined is still unsettled. Tattersall adopts the first view and argues that the number of Homo species has been seriously underestimated. Our own genus is usually regarded as a single lineage, composed o f Homo habilis, Homo erectus and Homo sapiens, but Tattersall would recognize several additional taxa, adding branches to this simple linear progression. In this brief communication, I shall address only the question of geographic variation within Homo erectus. A more detailed monographic discussion is in preparation. One particularly interesting time period is the earlier Pleistocene, between about 1-5 and 0-5 million years ago. According to Tattersall and Lewin, one or more species in addition to Homo erectus may have lived during this interval. The legend accompanying the photograph of one well preserved Koobi Fora cranium illustrated by Lewin asks whether this African fossil represents the same species found in Java and elsewhere in the Far East. This is an important issue, as species (not grades) should indeed be the focus of evolutionary study. Whether Homo erectus is restricted to Asia will have to be determined before human phylogeny can be reconstructed with any certainty. Fortunately, quite a lot of fossils are available for examination. Unlike Homo habilis, Homo erectus is known from numerous specimens, some of which are very complete. I contend that all of this African and Asian material can be allocated to one species, which must be broadly ancestral to later people. The claim that at least two distinct morphs are present in assemblages usually attributed to Homo erectus was widely aired at a conference held at Bad Homburg in 1983 (Stringer, 1984; Wood, 1984). One set of fossils was said to include individuals from Trinil, Sangiran, Ngandong, Zhoukoudian and various other Asian localities, all thought to be latest Early Pleistocene or Middle Pleistocene in age. Another group, represented by specimens from Koobi Fora, is substantially more ancient. It is presumably to this latter collection that other East African material from the Turkana Basin and from Olduvai Gorge should be assigned. Middle Pleistocene hominids from Ternifine (Tighenif) in Algeria and sites in Morocco were not discussed in detail at the conference, so their position with respect to the Asian and East African morphs was not clarified. 0047 2484/86/080823 + 04 $03.00/0
9 1986AcademicPress Inc. (London) Limited
824
CURRENTEVENTS
It was argued that the many anatomical characters shared by these specimens and cited routinely in descriptions of Homo erectus are in fact primitive, being present also in other hominids or in living hominoid primates. Such traits, considered to be retentions from a common ancestor, do not help generally to define Homo erectus relative to other taxa. Species diagnoses should be based instead on characters which are novel or derived, and preferably unique. The importance of autapomorphies is stressed (e.g., by Wood, 1984), despite the difficulties which will inevitably arise as more and more characters are described as unique. If it is insisted that a species be defined in this way, then the problem of recognizing descendants will become increasingly acute. We are likely to be confronted by a paradox. As a species is diagnosed more completely, it will be harder to place it in any ancestor-descendant sequence. Specializations in Homo erectus are few in number but seem to include several anatomical structures associated with the cranium. Examples are midline keeling of the frontal or parietal bones, a torus or bulge at the mastoid angle of the parietal bone, development of a fissure between the mastoid process and the tympanic plate, narrowing of the medial part of the glenoid cavity, and thickening of the bones of the vault. The addition to this list of a prominent brow ridge backed by a flattened expanse of frontal bone, and a strong occipital torus, was supported by some of the conference participants but not by others. A crucial point in the discussions was the assertion that these derived characters are expressed in the Far Eastern fossils but not in the Koobi Fora crania. Andrews (1984) interpreted this anatomical evidence to prove that Homo erectus is a species restricted to Asia. He has suggested that this taxon became extinct later in the Middle Pleistocene, while another (unnamed) lineage accommodating the African assemblages probably continued to evolve toward Homo sapiens. The view that Homo erectus is strictly an Asian branch of the hominid clade is thus based on the distribution of a few key characters, said to be lacking from the East African remains. If this claim is to be evaluated fairly, then not only the most complete Koobi Fora crania but also other Turkana specimens must be examined. Several fossils from the Koobi Fora region have been described recently (Leakey & Walker, 1985), and new discoveries have been made at Nariokotome (Brown et al., 1985). Comparisons should include this material as well as that from Olduvai. When all individuals are checked systematically, it is apparent that differences between the African and Far Eastern assemblages are not clear-cut. The supraorbital torus is especially gracile in ER-3733, although a supratoral shelf is present. It is noteworthy that this Koobi Fora cranium does display a sagittal keel, to either side of which the frontal squama is quite flat. The brow of ER-3883 is thicker, as is that of ER-730. Here frontal form begins to resemble that seen in the very massive hominid from upper Bed I I at Olduvai. The skull from Nariokotome, while still juvenile, already shows substantial torus development and would likely have become more heavily buttressed than ER-3733. Thickness of the brow and overall robusticity of these African crania must surely overlap with the condition found at Trinil and Sangiran. Another important trait is the angular torus. Neither of the more complete Koobi Fora crania has a strong torus, but such a parietal bulge must have been expressed in the small, thick-walled vault of O H 12. This character occurs in Sangiran 4 and (faintly) in Sangiran 12 but not in Sangiran 2 or in Sangiran 10. It is prominent on some but not all of the Zhoukoudian parietals, and it may be recognized in several other archaic skulls usually referred to Homo sapiens.
CURRENTEVENTS
825
ER-3733 exhibits a blunt occipital torus which is most projecting near the midline. The lower margin of this structure is defined by the (superior) nuchal lines, which converge at a central linear tubercle. There is some supratoral hollowing. This morphology is mirrored by other African occipitals, with minor variation. Several of the small Asian hominids are similar, and the transverse torus is much more prominent and shelf-like only in a few individuals, such as Sangiran 4. The glenoid cavity and adjacent structures are well preserved in a number of cases. In the Koobi Fora fossils and in O H 9, this fossa is constricted medially, to produce a deep recess between the entoglenoid pyramid and the tympanic plate. The plate itself is thickened in comparison to that of modern humans and terminates in a blunt process supratubarius. Sangiran 2 is much the same, while in Sangiran 4 the tympanic bone is elongated vertically and is less robust. The larger-brained Indonesian and Chinese individuals present a similar pattern. Perhaps the only feature of the mandibular fossa which can be linked to a particular geographic region is the narrow fissure separating the tympanic plate from the mastoid process behind. This trait seems to be present in (some) Asian hominids but not in Africa. However, such a crevice may easily be distorted through bone crushing or filled with matrix, and its apparent absence in African crania should not be weighted too heavily in any discussion of affinities. I conclude that characters judged to be derived for Homo erectus are variably expressed, both in the Asian and in the East African remains. Hublin (1986) also points out that features which are derived for Homo erectus are present inconsistently, even among individuals making up a single Asian "population" (e.g., Ngandong). There is no indication that the African assemblages should be identified as a different species. Anatomical specializations together with many primitive traits of the braincase, lower jaw, teeth and postcranial skeleton (Howell, 1978; Howells, 1980; Rightmire, 1984) describe Homo erectus and serve to diagnose this taxon relative to Homo sapiens. Apomorphies listed above help also to distinguish Homo erectus from earlier Homo and from other hominids. I share with Tattersall (1986) the view that Homo erectus, like other paleontological species, should be defined anatomically as a discrete entity rather than an amorphous grade. This taxon seems to have originated in Africa. During the Early Pleistocene, Homo erectus spread widely through the Old World tropics. Some later populations were probably ancestral to more modern people, which appear in the European record and elsewhere in the Middle Pleistocene. Whether these later hominids are to be allocated to more than one species as suggested by Tattersall (1986) is a question deserving careful study. G. PHILIP RIGHTMIRE Department o f Anthropology, State University o f New York, Binghamton, New York 13901, U.S.A.
References
Andrews, P. (1984). An alternative interpretation of characters used to define Homo erectus. Cour. Forsch. Inst. Senckenberg 69, 167 175. Brown, F., Harris, J., Leakey, R. & Walker, A. (1985). Early Homo erectusskeleton from west Lake Turkana, Kenya. Nature 316, 78~792. Howell, F. C. (1978). Hominidae. In (V. J. Maglio & H. B. S. Cooke, Eds) Evolution of African Mammals, pp. 154-248. Cambridge: Harvard University Press. Howells, W. W. (1980). Homoerectus- who, when and where? A survey. Yb. phys. Anthrop. 23, 1 23.
~26
CURRENTEVENTS
Hublin, J-J. (1986). Some commences on the diagnostic features of Homo erectus. Anthropos (Brno) 23, 175-185. Leakey, R. & Walker, A. (1985). Further hominids from the Plio-Pleistocene ofKoobi Fora, Kenya. Am.J. phys. Anthrop. 67, 135-163. Lewin, R. (1986). Recognizing ancestors is a species problem. Science 234, 1500. Rightmire, G. P. (1984). Comparisons of Homo erectus from Africa and southeast Asia. Cour. Forsch. inst. Senckenberg 69, 83-98. Stringer, C. B. (1984). The definition of Homo erectus and the existence of the species in Africa and Europe. Cour. Forsch. Inst. Senckenberg 69, 131-143. Tattersall, I. (1986). Species recognition in human paleontology. J. hum. Evol. 15, 165-176. Wood, B. (1984). The origin of Homo erectus. Cour. Forsch. Inst. Senckenberg 69, 99-111.