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Sports and the human brain: an evolutionary perspective
Handbook of Clinical Neurology, Vol. 158 (3rd series) Sports Neurology B. Hainline and R.A. Stern, Editors https://doi.org/10.1016/B978-0-444-63954-7.00001-X Copyright © 2018 Elsevier B.V. All rights reserved
Chapter 1
Sports and the human brain: an evolutionary perspective 1
IAN J. WALLACE1*, CLOTILDE HAINLINE2, AND DANIEL E. LIEBERMAN1 Department of Human Evolutionary Biology, Harvard University, Cambridge, MA, United States 2
Department of Neurology, Boston University School of Medicine, Boston, MA, United States
Abstract An evolutionary perspective helps explain a conundrum faced by sports neurologists: why is the human brain dependent on physical activity to function optimally, yet simultaneously susceptible to harm from particular types of athletics? For millions of years, human bodies and brains co-evolved to meet the physical and cognitive demands of the uniquely human subsistence strategy of hunting and gathering. Natural selection favored bodies with adaptations for endurance-based physical activity patterns, whereas brains were selected to be big and powerful to navigate the complex cultural and ecologic landscapes of huntergatherers. Human brains require physical activity to function optimally because their physiology evolved among individuals who were rarely able to avoid regular physical activity. Moreover, because energy from food was limited, human brains, like most energetically costly physiologic systems, evolved to require stimuli from physical activity to adjust capacity to demand. Consequently, human brains are poorly adapted to excessive physical inactivity. In addition, while brain enlargement during human evolution was vital to successful hunting and gathering, it came at the cost of a decreased ability to withstand brain accelerations and decelerations, which commonly occur during contact/collision sports.
INTRODUCTION Numerous dimensions of human health are influenced beneficially by physical activity, including neurologic health. Multiple compelling lines of evidence indicate that physical activity, especially aerobic exercise, is a potent stimulus for neurogenesis, protects new neurons, and bolsters cognition and brain performance (Hillman et al., 2008; van Praag, 2008; Raichlen and Polk, 2013; Chieffi et al., 2017). Moreover, physical activity is recognized as a powerful strategy for preventing and ameliorating neurologic disorders such as Alzheimer disease, Parkinson disease, and multiple sclerosis (Rovio et al., 2005; Goodwin et al., 2008; Motl and Gosney, 2008; Stephen et al., 2017). Nonetheless, certain types of physical activity can also result in acute brain damage and possible neurodegenerative sequelae. The most notable examples are sports that increase risk of concussion and repetitive head impact exposure, injuries to which
humans appear to be especially predisposed compared to other animals (Gibson, 2006; Lieberman, 2011). Sports neurologists thus face an interesting conundrum: why is the human brain dependent on physical activity to function optimally, yet simultaneously susceptible to harm from particular forms of athletics? As the renowned geneticist Theodosius Dobzhansky (1973) observed, “nothing in biology makes sense except in the light of evolution.” He meant that while knowledge of the mechanisms immediately responsible for a biologic phenomenon (usually cellular and molecular) is necessary for explaining how that phenomenon exists, only evolutionary theory and data can explain why it exists. It therefore follows that a satisfactory scientific understanding of why athletic activities are both necessary and potentially dangerous for human brain health requires an evolutionary perspective. Here, we aim to provide answers to three questions. First, for what types of physical activity are humans
*Correspondence to: Ian J. Wallace, Ph.D., Department of Human Evolutionary Biology, Peabody Museum, Harvard University, 11 Divinity Ave., Cambridge MA 02138, United States. Tel: +1-617-496-1193, Email: [email protected]
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adapted? Second, how have human brains co-evolved with physical activity patterns? And third, for what types of physical activity are human brains poorly or inadequately adapted?
WHAT TYPES OF PHYSICAL ACTIVITY ARE HUMANS ADAPTED FOR? To show that humans are adapted for particular types of physical activity, it is useful to recall Darwin’s (1859) theory of natural selection. Natural selection is the emergent outcome of three phenomena: (1) all organisms have traits that vary; (2) some of these traits are inherited; and (3) organisms compete for resources. Darwin’s profound insight was that, over time, heritable traits that enhance or hinder an organism’s ability to compete and produce offspring become either more or less common across generations. Traits that arise and become more widespread because they improve an organism’s ability to survive and reproduce are referred to as adaptations, whereas traits that are unfavorable are maladaptive and typically selected against (Rose and Lauder, 1996). However, traits with certain negative qualities can be favored by natural selection if their benefits outweigh their costs (Wallace and Garland, 2016). Many human adaptations relate to our ability to be physically active. Survival and reproductive success among humans, as in all animals, hinge on the capacity to move to obtain resources, find mates, and avoid predators. Natural selection has given rise to diverse physical activity patterns among animals (Biewener, 2003), but activities for which humans are adapted are unusual, even compared to our closest primate relatives (Fleagle and Lieberman, 2015). Specifically, humans are well adapted for activities that require endurance rather than power (Lieberman et al., 2009; Lieberman, 2013) and for activities that are rare or absent in other primates and mammals, such as the ability to walk and run long distances at relatively fast speeds in hot, arid conditions (Carrier, 1984; Bramble and Lieberman, 2004). Humans’ unique physical activity patterns began once the human and chimpanzee lineages diverged from our last common ancestor (LCA) between 8 and 5 million years ago (Ma). Fossils of the LCA have yet to be discovered, but this species was some form of quadrupedal ape that lived in the tropical forests of Africa (Fleagle and Lieberman, 2015). It was almost certainly well adapted for tree climbing, fighting, and other activities requiring power, but less capable of endurance activities such as long-distance travel. In all likelihood, the LCA spent most of the day resting, with only infrequent, short bouts of strenuous activity. The closest living analog for the LCA are probably chimpanzees (Pilbeam and Lieberman, 2017), who are about 50% stronger than
humans (O’Neill et al., 2017) but typically travel only 2–3 km/day (Pontzer and Wrangham, 2006). Chimpanzees have generally massive muscles dominated by fast-twitch fibers (Myatt et al., 2011), an indication of their capacity for power, but they spend four times more energy per unit body mass per unit distance when walking than humans (Sockol et al., 2007). Fragmentary fossils of human ancestors dated between 7 and 4 Ma exhibit early signs of the shift to bipedalism, and there is abundant fossil evidence of habitual bipedality among Australopithecus species that lived in Africa between 4 and 1 Ma. The general picture of Australopithecus is that their bodies were adapted for a mixture of activities on both the ground and in trees, with their lower extremities displaying key adaptations for bipedal walking, but their upper extremities retaining many features useful for climbing (Fig. 1.1) (Stern, 2000; Ward and Hammond, 2016). Although Australopithecus bipedalism was probably not entirely human-like, natural selection apparently favored adaptations that allowed them to travel and obtain food more efficiently in open, nonforested habitats, which were increasingly common in Africa during the Pliocene epoch (Lieberman, 2013). To what extent Australopithecus walking was slower, less stable, and less energetically efficient than modern human walking is debated (Stern, 2000; Thompson et al., 2015; Hatala et al., 2016), but their form of bipedalism was effective enough to persist for several million years. Multiple species of early Homo appeared in Africa between approximately 3 and 2 Ma, of which the best known is Homo erectus. H. erectus was the earliest known human ancestor with a body that was essentially human-like (Fig. 1.1) (Walker and Leakey, 1993; Bramble and Lieberman, 2004). Unlike Australopithecus, who retained adaptations for life in the trees, H. erectus was a fully committed biped. H. erectus was probably the first species to disperse outside of Africa, with fossil evidence from the Caucasus Mountains by 1.8 Ma and eastern Asia by 1.6 Ma. In addition, H. erectus was the first species to practice a hunting and gathering way of life (Lieberman, 2013). Hunting and gathering is a uniquely human strategy for acquiring food that combines the pursuit of animals and the extraction of wild plants with a heavy reliance on food sharing and cooperation among group members (Marlowe, 2005; Kelly, 2013). From the time of H. erectus up until farming first began roughly 10 thousand years ago (kya), all humans were hunter-gatherers. The emergence of hunting and gathering meant a heavy dependence on activities requiring endurance, most notably long-distance walking. Today, few human groups continue to live by hunting and gathering, but those who do and who inhabit hot, arid African
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Fig. 1.1. Important locomotor adaptations in chimpanzees, Australopithecus, and Homo erectus. Highlighted are adaptations for climbing in chimpanzees, adaptations for bipedal walking in Australopithecus, and adaptations for running in Homo erectus.
Fig. 1.2. San hunter-gatherers walking across the Kalahari Desert in southern Africa. (Courtesy of L.K. Marshall and L.J. Marshall. Copyright President and Fellows of Harvard College, Peabody Museum #2001.29.390.)
environments like those occupied by H. erectus must travel, on average, 9–15 km/day, typically while carrying children, food, and tools (Fig. 1.2) (Marlowe, 2005). Given its similar body size, H. erectus likely walked similar distances. In addition, H. erectus probably engaged in long bouts of digging up edible tubers with sticks, an activity to which modern hunter-gatherers devote 2–3 hours/day (Marlowe, 2010). Modern huntergatherers expend, on average, 22 kcal per unit body mass daily on physical activity (Pontzer et al., 2015), approximately twice what chimpanzees spend (Pontzer et al., 2016). It is thus not surprising that natural selection favored adaptations in H. erectus associated with greater locomotor energy efficiency, particularly its long legs (Pontzer et al., 2010). Another pivotal adaptation of H. erectus was its capacity for endurance running, most likely for hunting
(Bramble and Lieberman, 2004). Today, huntergatherers hunt with weapons like bows and arrows, but these were not invented until after 100 kya (Shea, 2016), and even simple stone spear points appear only 500 kya (Wilkins et al., 2012). Instead, H. erectus probably dispatched their prey using a strategy known as persistence hunting (Carrier, 1984; Bramble and Lieberman, 2004; Liebenberg, 2006). Persistence hunting is made possible by humans’ ability to run long distances at speeds that require quadrupedal mammals to gallop. One advantage humans have at this speed is the uniquely evolved ability to cool the body by sweating. All other mammals must cool by panting, which they cannot do while galloping (Bramble and Jenkins, 1993). Thus, when chased for lengthy periods, especially in hot conditions, animals will overheat and hide to cool down. Human hunters intermittently chase their prey while running and then track them while walking; eventually the animals collapse from hyperthermia, at which point they become easy targets. By chasing animals over long distances at speeds that force their prey to gallop, persistence hunters also can exhaust their prey or drive them into either natural or manufactured traps where the animals can be safely killed. Although now rare, persistence hunting is still occasionally practiced by modern hunter-gatherers in Africa and elsewhere (Lieberman et al., 2009). A study of persistence hunts by San hunter-gatherers in the Kalahari documented that the average temperature typically was 39°C and total distance was 27.8 km. Hunters ran only about half the time, resulting in a total average speed of 6.2 km/hour (a fast walk below the walk/run transition speed) (Liebenberg, 2006). The fossil record of H. erectus suggests that it was probably the first species
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capable of persistence hunting; anatomic adaptations specifically for endurance running that do not benefit walking include short toes and an expanded gluteus maximus (Fig. 1.1) (Bramble and Lieberman, 2004).
HOW HAVE HUMAN BRAINS CO-EVOLVED WITH PHYSICAL ACTIVITY PATTERNS? Human brains are approximately three times bigger than expected for an ape of our body size, and approximately five times bigger relative to body size than all mammals. Although Australopithecus had slightly larger brains than chimpanzees, brain enlargement was pronounced in the genus Homo, especially in H. erectus (Fig. 1.3) (Lieberman, 2011). On average, brain size was at least 60% larger in H. erectus than in chimpanzees after controlling for body size. Over the nearly 2 million years of this species’ existence, brain size increased from about 600–700 to approximately 1200 cm3. Brains grew even larger in the descendants of H. erectus, including H. neanderthalensis (1170–1740 cm3) and modern humans, H. sapiens (1100–1900 cm3). The coincidence between the initial spike in brain size in H. erectus and the emergence of adaptations for endurance and the hunter-gatherer lifestyle strongly suggests these phenomena are evolutionarily linked (Bortz, 1985; Lieberman, 2013; Raichlen and Polk, 2013). Hunter-gatherer subsistence depends critically on our uniquely complex cognitive skills. Among the most vital skills is an enhanced ability to cooperate, which must have been enabled in H. erectus and more recent ancestors by brain expansion (Lieberman, 2013; Henrich, 2016), as well as changes in neurochemistry that do not readily fossilize (Raghanti et al., 2018). Effective hunting and gathering are impossible without intense and regular cooperation among members of a group. In hunter-gatherer societies, everyone helps everybody regardless of whether they are kin (Hill et al., 2011),
Fig. 1.3. Expansion of brain size (endocranial volume) throughout human evolution.
and almost all activities are collective efforts, including foraging and food processing, tool production and camp maintenance, and caring for children and the sick (Hill, 2002; Gurven, 2004; Ivey et al., 2005). To achieve this requires a brain equipped for cultural learning, in which a person’s social behavior is shaped by information gained from other group members (Henrich, 2016). Cultural learning, in turn, requires a fully developed theory of mind (the ability to understand the minds of others), the power to reason, the faculty to communicate through symbolic behavior and language, the means to keep track of complex social interactions, and the wherewithal to curb selfish and aggressive impulses. All of these human cognitive abilities are absent or poorly developed in other primates (Whiten and Erdal, 2012). Apes occasionally cooperate, but only in limited contexts. For example, male chimpanzees will share meat with other males after a hunt, but primarily to appease beggars and thus avoid having to defend their kills (Gilby, 2006). Sharing among chimpanzees contrasts starkly to sharing within huntergatherer societies in which the yields of foraging are generally redistributed according to the needs of each group member, young or old, female or male, healthy or sick (Gurven, 2004). No other primate relies on prosocial food sharing the way that humans do (Jaeggi and Gurven, 2013). Another key cognitive skill essential to hunting and gathering is the ability to acquire and mentally process complex ecologic information in different habitats and conditions. The hunter-gatherer subsistence strategy permitted humans to exploit animal and plant resources across diverse and changing environments, and thus eventually to disperse to nearly every corner of the world. Brains had to be capable of storing and organizing massive amounts of culturally acquired ecologic knowledge accumulated over generations, and extending this knowledge via inference to solve unanticipated challenges (Henrich, 2016). Even in arid African environments, hunter-gatherer diets consist of as many as 100 different animal and plant species (Marshall, 1976; Marlowe, 2010). Exploitation of these resources requires learning and knowing where and when to find them, as well as how to make them edible by processing them with tools and cooking (Wrangham, 2009; Shea, 2016). Hunting poses the additional and demanding cognitive challenge of having to anticipate the movement patterns of evasive and often cryptic prey, not just to locate the animal in the first place but also to then track it. Accomplishing these feats requires both inductive and deductive thinking: inductive logic to find and follow the animal based on clues from footprints, spoor, and other sights and smells, and deductive logic to formulate hypotheses about how the animal is likely to behave, and to use clues to test these predictions. The cognitive elements employed in
SPORTS AND THE HUMAN BRAIN: AN EVOLUTIONARY PERSPECTIVE animal tracking may represent the roots of scientific thinking (Liebenberg, 1990; Carruthers, 2002). Although human brains comprise only about 2% of body weight, they expend at least 20% of resting metabolism. In absolute numbers, an average human adult brain requires approximately 300–400 kcal/day, more than three times as much as a chimpanzee’s brain (Lieberman, 2011). A typical H. erectus brain would have required at least 200 kcal/day more than a chimpanzee’s brain. Bigger brains presumably paid for themselves by giving our ancestors the cognitive means to exploit a broader range of high-quality foods. The initial increase in brain size among H. erectus, coupled with its bodily adaptations for endurance, likely set in motion a positive-feedback loop in which the emergence of hunting and gathering enabled additional brain enlargement, which in turn further enhanced subsistence skills, which permitted more brain growth, and so on (Lieberman, 2013). Over time, as cognition advanced in parallel with brain size, culturally acquired knowledge became progressively more complex and sophisticated, and cultural skills and practices became increasingly elaborate and vital to survival (Henrich, 2016). Based on evolutionary theory and data, any major evolutionary change in animal behavior requires natural selection for adaptations that not only allow but also compel individuals to engage in the new behavior (Rhodes and Kawecki, 2009; Wallace and Garland, 2016). Thus, the evolution of hunting and gathering presumably selected for brain changes that enhanced thinking abilities as well as increased motivation to participate in activities essential to hunter-gatherer survival. Neurologic adaptations that encouraged endurance-based physical activities would have been especially important, given the strong pressure to avoid such activities imposed by their energetic costs (Raichlen et al., 2012). Although the neurophysiologic underpinnings of humans’ motivation and propensity for particular kinds of physical activity are not fully understood, one crucial component is likely the brain’s natural reward circuit (Rhodes and Kawecki, 2009). This highly sensitive system is responsive to stimuli generated by physical activity, especially aerobic exercise, and appears to play a major role in motivating individuals to be physically active and improve performance (Sparling et al., 2003; Dietrich and McDaniel, 2004; Boecker et al., 2008; Raichlen et al., 2012). Neurobiologic rewards could thus be an important target for natural selection favoring particular physical activity patterns, especially endurancebased activities (Rhodes and Kawecki, 2009; Raichlen et al., 2012). Consistent with this hypothesis, in an experiment in which mice were selectively bred across several generations for high levels of voluntary wheel running, many of
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the most consequential traits that were favored by artificial selection were in the brain and primarily related to natural rewards (Rhodes et al., 2005; Wallace and Garland, 2016). Although much remains unknown about how the human brain’s neurobiologic reward system was shaped by the evolution of hunting and gathering, recent research has identified one feature that appears to partly explain at least our species’ propensity for endurance running. Specifically, compelling experimental evidence indicates that humans exhibit markedly greater upregulation of endocannabinoid signaling in response to longdistance running than animals trained to run the same proportional distance that do not normally use strenuous aerobic activity in their foraging behaviors (Raichlen et al., 2012), suggesting that human brains are adapted to better encourage running with natural rewards. In all likelihood, alterations to the endocannabinoid system were just one of many neurophysiologic factors affecting behavioral motivation that were required for hunting and gathering to evolve.
FOR WHAT TYPES OF PHYSICAL ACTIVITY ARE HUMAN BRAINS NOT WELL ADAPTED? Although clinicians may think of exercise as a strategy for preventing or helping to treat illness, evolutionary biologists think of physical activity as an ancient adaptation. The human propensity for physical activity almost certainly did not evolve because it promoted health, but because it increased individuals’ ability to gain resources and produce offspring (Lieberman, 2013). This brings us back to one of the two questions posed by the sports neurology conundrum: why is it that human brains depend on physical activity to function optimally? From an evolutionary perspective, until very recently, humans never could avoid physical activity; thus, human brains require physical activity simply because we evolved to be physically active. The more interesting and perhaps less obvious question is why human brains evolved to function so poorly without physical activity. Evolutionary mechanisms will always be favored that limit energy waste and allocate as much energy as possible to producing offspring (Raichlen and Alexander, 2017). As a result, many energetically costly physiologic systems require a functional stimulus to adjust capacity to demand to reserve energy for reproduction. A familiar example is the relationship between physical activity and muscle size. Because muscles consume approximately 20% of the body’s resting energy budget, muscles grow bigger and stronger primarily when they are used, and waste away under conditions of disuse. Similar physiologic responses to varying levels of demand exist in almost every system in the body. In the brain, increased
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levels of physical activity, especially aerobic exercise, result in increased production of neurotrophic factors, which, in turn, increase neurogenesis, neuroprotection, and cognitive function (Hillman et al., 2008; van Praag, 2008; Raichlen and Polk, 2013; Chieffi et al., 2017). Conversely, in individuals who avoid physical activity, brains are weaker and more susceptible to degeneration. The argument that human brains are poorly adapted to too much physical inactivity because evolution favored physically active but energy-limited individuals leads to the concept of mismatch diseases. Mismatches are diseases that are more prevalent or severe today because our bodies and brains are poorly or inadequately adapted to modern environments (Lieberman, 2013; Stearns and Medzhitov, 2015). Just as cavities were once rare but are now common because our teeth are poorly adapted to diets rich in sugar and starch, numerous chronic noninfectious diseases have become more prevalent and severe because humans did not evolve to be as physically inactive as many people are today. The two principal criteria for identifying mismatch diseases are that: (1) the disease is actually more prevalent or severe today than among past human populations; and (2) preventable determinants of the disease have become more common in modern environments (Lieberman, 2013). Several neurologic and psychiatric diseases are good candidates for mismatch diseases, especially those for which physical inactivity is a risk factor or may hasten disease progression, including Alzheimer disease, Parkinson disease, multiple sclerosis, anxiety, and depression (Dunn et al., 2001; Rovio et al., 2005; Goodwin et al., 2008; Motl and Gosney, 2008). Since the prevalence of some of these diseases increases with age, longer life expectancy today might be expected to be responsible for the current burden of particular conditions on the elderly. However, although this may be partly true, hunter-gatherer longevity is greater than commonly assumed, with the modal age of adult death in modern hunter-gatherers being 68–78 years old (Gurven and Kaplan, 2007). That said, there is little good evidence regarding neurologic and mental health among past populations, so hypotheses about mismatch diseases of the brain must be considered cautiously. Achieving a better understanding of the neurologic and mental health of living hunter-gatherers and other contemporary traditionalliving, nonindustrialized populations should be a goal of future research. An evolutionary perspective is also relevant to addressing why human brains are so susceptible to harm from particular activities that are common in some sports. Throughout evolutionary history, human skulls became increasingly fragile and prone to fracture. Compared to earlier species of Homo, modern humans have skulls with neurocrania that are markedly thinner (Lieberman, 1996)
as well as facial skeletons that lack protective bony buttresses such as browridges and flaring zygomatics (Carrier and Morgan, 2015). Although the mechanisms responsible for this increase in skull fragility are not well understood, one important factor might be the reduction in androgen reactivity in modern humans, which may have been favored by natural selection because it facilitated hunting and gathering by promoting social tolerance (Cieri et al., 2014). Regardless of causes, however, the consequence of this evolutionary change is that human brains are especially vulnerable to hemorrhages from skull fractures. This vulnerability was no more apparent than during the first half of the twentieth century when fracture-induced brain death claimed the lives of many American football players, largely due to direct trauma to the skull coupled with the lack or inadequacy of protective headgear. By the mid twentieth century, hard plastic helmets began to be used on a more regular basis, which had the positive effect of dramatically decreasing skull fractures. However, improvements in helmet design allowed individuals to repeatedly hit their heads against their opponent with less painful consequences, which gave way to another maladaptive neurologic health concern, namely, enhanced risk of concussion and repetitive head impact exposure. Although humans are not the only animals susceptible to concussion or repetitive head impact exposure, the extraordinary expansion of the human brain throughout evolution has made it prone to damage from acceleration and deceleration forces (Gibson, 2006; Lieberman, 2011). Such forces produce a combination of rotational and linear movements of the brain; the potential for either type of motion to damage brain tissue depends on brain size. Forces generated by a given rotational movement, which can cause shearing of axons and neuronal cell bodies, increase as a function of the brain’s mass times the square of its distance from the center of rotation (Wainwright et al., 1976). Forces generated by a given linear movement increase as a function of the ratio of brain mass to surface area (Gibson, 2006). As a result, while brain enlargement was a vital adaptation for hunting and gathering, it came at the cost of a severely decreased ability to withstand brain accelerations and decelerations, which commonly occur among individuals participating in contact/collision sports.
CONCLUSION Human brains require physical activity to function optimally because they evolved among our huntergatherer ancestors who were almost never able to avoid regular, endurance-based physical activity. Moreover, because energy from food was limited among our ancestors, human brains – like most energetically costly
SPORTS AND THE HUMAN BRAIN: AN EVOLUTIONARY PERSPECTIVE physiologic systems – evolved to require stimuli from physical activity, to adjust capacity to demand. As a result, human brains are poorly adapted to the extreme physical inactivity typical of many people today, which in all likelihood contributes to the current high prevalence and severity of many neurologic and mental health disorders. Thanks to our evolutionary history as physically active hunter-gatherers, human brains are adapted to reap major health benefits from sports and other athletic activities, especially those involving aerobic exercise. Unfortunately, however, humans’ exceptionally large brains are also vulnerable to being harmed by certain sports, especially contact/collision sports, which increase risk of concussion and repetitive head impact exposure.
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Rhodes JS, Gammie SC, Garland Jr T (2005). Neurobiology of mice selected for high voluntary wheel-running activity. Integr Comp Biol 45: 438–455. Rose MG, Lauder GV (1996). Adaptation, Academic Press, San Diego, CA. Rovio S, Ka˚reholt I, Helkala EL et al. (2005). Leisure-time physical activity at midlife and the risk of dementia and Alzheimer’s disease. Lancet Neurol 4: 705–711. Shea JJ (2016). Stone tools in human evolution: behavioral differences among technological primates, Cambridge University Press, Cambridge. Sockol MD, Raichlen D, Pontzer HD (2007). Chimpanzee locomotor energetics and the origin of human bipedalism. Proc Natl Acad Sci USA 104: 12265–12269. Sparling PB, Giuffrida A, Piomelli D et al. (2003). Exercise activates the endocannabinoid system. Neuroreport 14: 2209–2211. Stearns SC, Medzhitov R (2015). Evolutionary medicine, Sinauer Associates, Sunderland, MA. Stephen R, Hongisto K, Solomon A et al. (2017). Physical activity and Alzheimer’s disease: a systematic review. J Gerontol A Biol Sci Med Sci 72: 733–739. Stern Jr JT (2000). Climbing to the top: a personal memoir of Australopithecus afarensis. Evol Anthropol 9: 113–133. Thompson NE, Demes B, O’Neill MC et al. (2015). Surprising trunk rotational capabilities in chimpanzees and implications for bipedal walking proficiency in early hominins. Nat Commun 6: 8416. van Praag H (2008). Neurogenesis and exercise: past and future directions. Neuromol Med 10: 128–140. Wainwright SA, Biggs WD, Currey JD et al. (1976). Mechanical design in organisms, Halsted Press, New York. Walker A, Leakey R (1993). The Nariokotome Homo erectus skeleton, Springer, Netherlands. Wallace IJ, Garland T (2016). Mobility as an emergent property of biological organization: insights from experimental evolution. Evol Anthropol 25: 98–104. Ward CV, Hammond AS (2016). Australopithecus and kin. Nature Education Knowledge 7: 1. Whiten A, Erdal D (2012). The human socio-cognitive niche and its evolutionary origins. Phil Trans Roy Soc B 367: 2119–2129. Wilkins J, Schoville BJ, Brown KS et al. (2012). Evidence for early hafted hunting technology. Science 338: 942–946. Wrangham R (2009). Catching fire: how cooking made us human, Basic Books, New York.
Chapter 1
Sports and the human brain: an evolutionary perspective 1
IAN J. WALLACE1*, CLOTILDE HAINLINE2, AND DANIEL E. LIEBERMAN1 Department of Human Evolutionary Biology, Harvard University, Cambridge, MA, United States 2
Department of Neurology, Boston University School of Medicine, Boston, MA, United States
Abstract An evolutionary perspective helps explain a conundrum faced by sports neurologists: why is the human brain dependent on physical activity to function optimally, yet simultaneously susceptible to harm from particular types of athletics? For millions of years, human bodies and brains co-evolved to meet the physical and cognitive demands of the uniquely human subsistence strategy of hunting and gathering. Natural selection favored bodies with adaptations for endurance-based physical activity patterns, whereas brains were selected to be big and powerful to navigate the complex cultural and ecologic landscapes of huntergatherers. Human brains require physical activity to function optimally because their physiology evolved among individuals who were rarely able to avoid regular physical activity. Moreover, because energy from food was limited, human brains, like most energetically costly physiologic systems, evolved to require stimuli from physical activity to adjust capacity to demand. Consequently, human brains are poorly adapted to excessive physical inactivity. In addition, while brain enlargement during human evolution was vital to successful hunting and gathering, it came at the cost of a decreased ability to withstand brain accelerations and decelerations, which commonly occur during contact/collision sports.
INTRODUCTION Numerous dimensions of human health are influenced beneficially by physical activity, including neurologic health. Multiple compelling lines of evidence indicate that physical activity, especially aerobic exercise, is a potent stimulus for neurogenesis, protects new neurons, and bolsters cognition and brain performance (Hillman et al., 2008; van Praag, 2008; Raichlen and Polk, 2013; Chieffi et al., 2017). Moreover, physical activity is recognized as a powerful strategy for preventing and ameliorating neurologic disorders such as Alzheimer disease, Parkinson disease, and multiple sclerosis (Rovio et al., 2005; Goodwin et al., 2008; Motl and Gosney, 2008; Stephen et al., 2017). Nonetheless, certain types of physical activity can also result in acute brain damage and possible neurodegenerative sequelae. The most notable examples are sports that increase risk of concussion and repetitive head impact exposure, injuries to which
humans appear to be especially predisposed compared to other animals (Gibson, 2006; Lieberman, 2011). Sports neurologists thus face an interesting conundrum: why is the human brain dependent on physical activity to function optimally, yet simultaneously susceptible to harm from particular forms of athletics? As the renowned geneticist Theodosius Dobzhansky (1973) observed, “nothing in biology makes sense except in the light of evolution.” He meant that while knowledge of the mechanisms immediately responsible for a biologic phenomenon (usually cellular and molecular) is necessary for explaining how that phenomenon exists, only evolutionary theory and data can explain why it exists. It therefore follows that a satisfactory scientific understanding of why athletic activities are both necessary and potentially dangerous for human brain health requires an evolutionary perspective. Here, we aim to provide answers to three questions. First, for what types of physical activity are humans
*Correspondence to: Ian J. Wallace, Ph.D., Department of Human Evolutionary Biology, Peabody Museum, Harvard University, 11 Divinity Ave., Cambridge MA 02138, United States. Tel: +1-617-496-1193, Email: [email protected]
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adapted? Second, how have human brains co-evolved with physical activity patterns? And third, for what types of physical activity are human brains poorly or inadequately adapted?
WHAT TYPES OF PHYSICAL ACTIVITY ARE HUMANS ADAPTED FOR? To show that humans are adapted for particular types of physical activity, it is useful to recall Darwin’s (1859) theory of natural selection. Natural selection is the emergent outcome of three phenomena: (1) all organisms have traits that vary; (2) some of these traits are inherited; and (3) organisms compete for resources. Darwin’s profound insight was that, over time, heritable traits that enhance or hinder an organism’s ability to compete and produce offspring become either more or less common across generations. Traits that arise and become more widespread because they improve an organism’s ability to survive and reproduce are referred to as adaptations, whereas traits that are unfavorable are maladaptive and typically selected against (Rose and Lauder, 1996). However, traits with certain negative qualities can be favored by natural selection if their benefits outweigh their costs (Wallace and Garland, 2016). Many human adaptations relate to our ability to be physically active. Survival and reproductive success among humans, as in all animals, hinge on the capacity to move to obtain resources, find mates, and avoid predators. Natural selection has given rise to diverse physical activity patterns among animals (Biewener, 2003), but activities for which humans are adapted are unusual, even compared to our closest primate relatives (Fleagle and Lieberman, 2015). Specifically, humans are well adapted for activities that require endurance rather than power (Lieberman et al., 2009; Lieberman, 2013) and for activities that are rare or absent in other primates and mammals, such as the ability to walk and run long distances at relatively fast speeds in hot, arid conditions (Carrier, 1984; Bramble and Lieberman, 2004). Humans’ unique physical activity patterns began once the human and chimpanzee lineages diverged from our last common ancestor (LCA) between 8 and 5 million years ago (Ma). Fossils of the LCA have yet to be discovered, but this species was some form of quadrupedal ape that lived in the tropical forests of Africa (Fleagle and Lieberman, 2015). It was almost certainly well adapted for tree climbing, fighting, and other activities requiring power, but less capable of endurance activities such as long-distance travel. In all likelihood, the LCA spent most of the day resting, with only infrequent, short bouts of strenuous activity. The closest living analog for the LCA are probably chimpanzees (Pilbeam and Lieberman, 2017), who are about 50% stronger than
humans (O’Neill et al., 2017) but typically travel only 2–3 km/day (Pontzer and Wrangham, 2006). Chimpanzees have generally massive muscles dominated by fast-twitch fibers (Myatt et al., 2011), an indication of their capacity for power, but they spend four times more energy per unit body mass per unit distance when walking than humans (Sockol et al., 2007). Fragmentary fossils of human ancestors dated between 7 and 4 Ma exhibit early signs of the shift to bipedalism, and there is abundant fossil evidence of habitual bipedality among Australopithecus species that lived in Africa between 4 and 1 Ma. The general picture of Australopithecus is that their bodies were adapted for a mixture of activities on both the ground and in trees, with their lower extremities displaying key adaptations for bipedal walking, but their upper extremities retaining many features useful for climbing (Fig. 1.1) (Stern, 2000; Ward and Hammond, 2016). Although Australopithecus bipedalism was probably not entirely human-like, natural selection apparently favored adaptations that allowed them to travel and obtain food more efficiently in open, nonforested habitats, which were increasingly common in Africa during the Pliocene epoch (Lieberman, 2013). To what extent Australopithecus walking was slower, less stable, and less energetically efficient than modern human walking is debated (Stern, 2000; Thompson et al., 2015; Hatala et al., 2016), but their form of bipedalism was effective enough to persist for several million years. Multiple species of early Homo appeared in Africa between approximately 3 and 2 Ma, of which the best known is Homo erectus. H. erectus was the earliest known human ancestor with a body that was essentially human-like (Fig. 1.1) (Walker and Leakey, 1993; Bramble and Lieberman, 2004). Unlike Australopithecus, who retained adaptations for life in the trees, H. erectus was a fully committed biped. H. erectus was probably the first species to disperse outside of Africa, with fossil evidence from the Caucasus Mountains by 1.8 Ma and eastern Asia by 1.6 Ma. In addition, H. erectus was the first species to practice a hunting and gathering way of life (Lieberman, 2013). Hunting and gathering is a uniquely human strategy for acquiring food that combines the pursuit of animals and the extraction of wild plants with a heavy reliance on food sharing and cooperation among group members (Marlowe, 2005; Kelly, 2013). From the time of H. erectus up until farming first began roughly 10 thousand years ago (kya), all humans were hunter-gatherers. The emergence of hunting and gathering meant a heavy dependence on activities requiring endurance, most notably long-distance walking. Today, few human groups continue to live by hunting and gathering, but those who do and who inhabit hot, arid African
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Fig. 1.1. Important locomotor adaptations in chimpanzees, Australopithecus, and Homo erectus. Highlighted are adaptations for climbing in chimpanzees, adaptations for bipedal walking in Australopithecus, and adaptations for running in Homo erectus.
Fig. 1.2. San hunter-gatherers walking across the Kalahari Desert in southern Africa. (Courtesy of L.K. Marshall and L.J. Marshall. Copyright President and Fellows of Harvard College, Peabody Museum #2001.29.390.)
environments like those occupied by H. erectus must travel, on average, 9–15 km/day, typically while carrying children, food, and tools (Fig. 1.2) (Marlowe, 2005). Given its similar body size, H. erectus likely walked similar distances. In addition, H. erectus probably engaged in long bouts of digging up edible tubers with sticks, an activity to which modern hunter-gatherers devote 2–3 hours/day (Marlowe, 2010). Modern huntergatherers expend, on average, 22 kcal per unit body mass daily on physical activity (Pontzer et al., 2015), approximately twice what chimpanzees spend (Pontzer et al., 2016). It is thus not surprising that natural selection favored adaptations in H. erectus associated with greater locomotor energy efficiency, particularly its long legs (Pontzer et al., 2010). Another pivotal adaptation of H. erectus was its capacity for endurance running, most likely for hunting
(Bramble and Lieberman, 2004). Today, huntergatherers hunt with weapons like bows and arrows, but these were not invented until after 100 kya (Shea, 2016), and even simple stone spear points appear only 500 kya (Wilkins et al., 2012). Instead, H. erectus probably dispatched their prey using a strategy known as persistence hunting (Carrier, 1984; Bramble and Lieberman, 2004; Liebenberg, 2006). Persistence hunting is made possible by humans’ ability to run long distances at speeds that require quadrupedal mammals to gallop. One advantage humans have at this speed is the uniquely evolved ability to cool the body by sweating. All other mammals must cool by panting, which they cannot do while galloping (Bramble and Jenkins, 1993). Thus, when chased for lengthy periods, especially in hot conditions, animals will overheat and hide to cool down. Human hunters intermittently chase their prey while running and then track them while walking; eventually the animals collapse from hyperthermia, at which point they become easy targets. By chasing animals over long distances at speeds that force their prey to gallop, persistence hunters also can exhaust their prey or drive them into either natural or manufactured traps where the animals can be safely killed. Although now rare, persistence hunting is still occasionally practiced by modern hunter-gatherers in Africa and elsewhere (Lieberman et al., 2009). A study of persistence hunts by San hunter-gatherers in the Kalahari documented that the average temperature typically was 39°C and total distance was 27.8 km. Hunters ran only about half the time, resulting in a total average speed of 6.2 km/hour (a fast walk below the walk/run transition speed) (Liebenberg, 2006). The fossil record of H. erectus suggests that it was probably the first species
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capable of persistence hunting; anatomic adaptations specifically for endurance running that do not benefit walking include short toes and an expanded gluteus maximus (Fig. 1.1) (Bramble and Lieberman, 2004).
HOW HAVE HUMAN BRAINS CO-EVOLVED WITH PHYSICAL ACTIVITY PATTERNS? Human brains are approximately three times bigger than expected for an ape of our body size, and approximately five times bigger relative to body size than all mammals. Although Australopithecus had slightly larger brains than chimpanzees, brain enlargement was pronounced in the genus Homo, especially in H. erectus (Fig. 1.3) (Lieberman, 2011). On average, brain size was at least 60% larger in H. erectus than in chimpanzees after controlling for body size. Over the nearly 2 million years of this species’ existence, brain size increased from about 600–700 to approximately 1200 cm3. Brains grew even larger in the descendants of H. erectus, including H. neanderthalensis (1170–1740 cm3) and modern humans, H. sapiens (1100–1900 cm3). The coincidence between the initial spike in brain size in H. erectus and the emergence of adaptations for endurance and the hunter-gatherer lifestyle strongly suggests these phenomena are evolutionarily linked (Bortz, 1985; Lieberman, 2013; Raichlen and Polk, 2013). Hunter-gatherer subsistence depends critically on our uniquely complex cognitive skills. Among the most vital skills is an enhanced ability to cooperate, which must have been enabled in H. erectus and more recent ancestors by brain expansion (Lieberman, 2013; Henrich, 2016), as well as changes in neurochemistry that do not readily fossilize (Raghanti et al., 2018). Effective hunting and gathering are impossible without intense and regular cooperation among members of a group. In hunter-gatherer societies, everyone helps everybody regardless of whether they are kin (Hill et al., 2011),
Fig. 1.3. Expansion of brain size (endocranial volume) throughout human evolution.
and almost all activities are collective efforts, including foraging and food processing, tool production and camp maintenance, and caring for children and the sick (Hill, 2002; Gurven, 2004; Ivey et al., 2005). To achieve this requires a brain equipped for cultural learning, in which a person’s social behavior is shaped by information gained from other group members (Henrich, 2016). Cultural learning, in turn, requires a fully developed theory of mind (the ability to understand the minds of others), the power to reason, the faculty to communicate through symbolic behavior and language, the means to keep track of complex social interactions, and the wherewithal to curb selfish and aggressive impulses. All of these human cognitive abilities are absent or poorly developed in other primates (Whiten and Erdal, 2012). Apes occasionally cooperate, but only in limited contexts. For example, male chimpanzees will share meat with other males after a hunt, but primarily to appease beggars and thus avoid having to defend their kills (Gilby, 2006). Sharing among chimpanzees contrasts starkly to sharing within huntergatherer societies in which the yields of foraging are generally redistributed according to the needs of each group member, young or old, female or male, healthy or sick (Gurven, 2004). No other primate relies on prosocial food sharing the way that humans do (Jaeggi and Gurven, 2013). Another key cognitive skill essential to hunting and gathering is the ability to acquire and mentally process complex ecologic information in different habitats and conditions. The hunter-gatherer subsistence strategy permitted humans to exploit animal and plant resources across diverse and changing environments, and thus eventually to disperse to nearly every corner of the world. Brains had to be capable of storing and organizing massive amounts of culturally acquired ecologic knowledge accumulated over generations, and extending this knowledge via inference to solve unanticipated challenges (Henrich, 2016). Even in arid African environments, hunter-gatherer diets consist of as many as 100 different animal and plant species (Marshall, 1976; Marlowe, 2010). Exploitation of these resources requires learning and knowing where and when to find them, as well as how to make them edible by processing them with tools and cooking (Wrangham, 2009; Shea, 2016). Hunting poses the additional and demanding cognitive challenge of having to anticipate the movement patterns of evasive and often cryptic prey, not just to locate the animal in the first place but also to then track it. Accomplishing these feats requires both inductive and deductive thinking: inductive logic to find and follow the animal based on clues from footprints, spoor, and other sights and smells, and deductive logic to formulate hypotheses about how the animal is likely to behave, and to use clues to test these predictions. The cognitive elements employed in
SPORTS AND THE HUMAN BRAIN: AN EVOLUTIONARY PERSPECTIVE animal tracking may represent the roots of scientific thinking (Liebenberg, 1990; Carruthers, 2002). Although human brains comprise only about 2% of body weight, they expend at least 20% of resting metabolism. In absolute numbers, an average human adult brain requires approximately 300–400 kcal/day, more than three times as much as a chimpanzee’s brain (Lieberman, 2011). A typical H. erectus brain would have required at least 200 kcal/day more than a chimpanzee’s brain. Bigger brains presumably paid for themselves by giving our ancestors the cognitive means to exploit a broader range of high-quality foods. The initial increase in brain size among H. erectus, coupled with its bodily adaptations for endurance, likely set in motion a positive-feedback loop in which the emergence of hunting and gathering enabled additional brain enlargement, which in turn further enhanced subsistence skills, which permitted more brain growth, and so on (Lieberman, 2013). Over time, as cognition advanced in parallel with brain size, culturally acquired knowledge became progressively more complex and sophisticated, and cultural skills and practices became increasingly elaborate and vital to survival (Henrich, 2016). Based on evolutionary theory and data, any major evolutionary change in animal behavior requires natural selection for adaptations that not only allow but also compel individuals to engage in the new behavior (Rhodes and Kawecki, 2009; Wallace and Garland, 2016). Thus, the evolution of hunting and gathering presumably selected for brain changes that enhanced thinking abilities as well as increased motivation to participate in activities essential to hunter-gatherer survival. Neurologic adaptations that encouraged endurance-based physical activities would have been especially important, given the strong pressure to avoid such activities imposed by their energetic costs (Raichlen et al., 2012). Although the neurophysiologic underpinnings of humans’ motivation and propensity for particular kinds of physical activity are not fully understood, one crucial component is likely the brain’s natural reward circuit (Rhodes and Kawecki, 2009). This highly sensitive system is responsive to stimuli generated by physical activity, especially aerobic exercise, and appears to play a major role in motivating individuals to be physically active and improve performance (Sparling et al., 2003; Dietrich and McDaniel, 2004; Boecker et al., 2008; Raichlen et al., 2012). Neurobiologic rewards could thus be an important target for natural selection favoring particular physical activity patterns, especially endurancebased activities (Rhodes and Kawecki, 2009; Raichlen et al., 2012). Consistent with this hypothesis, in an experiment in which mice were selectively bred across several generations for high levels of voluntary wheel running, many of
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the most consequential traits that were favored by artificial selection were in the brain and primarily related to natural rewards (Rhodes et al., 2005; Wallace and Garland, 2016). Although much remains unknown about how the human brain’s neurobiologic reward system was shaped by the evolution of hunting and gathering, recent research has identified one feature that appears to partly explain at least our species’ propensity for endurance running. Specifically, compelling experimental evidence indicates that humans exhibit markedly greater upregulation of endocannabinoid signaling in response to longdistance running than animals trained to run the same proportional distance that do not normally use strenuous aerobic activity in their foraging behaviors (Raichlen et al., 2012), suggesting that human brains are adapted to better encourage running with natural rewards. In all likelihood, alterations to the endocannabinoid system were just one of many neurophysiologic factors affecting behavioral motivation that were required for hunting and gathering to evolve.
FOR WHAT TYPES OF PHYSICAL ACTIVITY ARE HUMAN BRAINS NOT WELL ADAPTED? Although clinicians may think of exercise as a strategy for preventing or helping to treat illness, evolutionary biologists think of physical activity as an ancient adaptation. The human propensity for physical activity almost certainly did not evolve because it promoted health, but because it increased individuals’ ability to gain resources and produce offspring (Lieberman, 2013). This brings us back to one of the two questions posed by the sports neurology conundrum: why is it that human brains depend on physical activity to function optimally? From an evolutionary perspective, until very recently, humans never could avoid physical activity; thus, human brains require physical activity simply because we evolved to be physically active. The more interesting and perhaps less obvious question is why human brains evolved to function so poorly without physical activity. Evolutionary mechanisms will always be favored that limit energy waste and allocate as much energy as possible to producing offspring (Raichlen and Alexander, 2017). As a result, many energetically costly physiologic systems require a functional stimulus to adjust capacity to demand to reserve energy for reproduction. A familiar example is the relationship between physical activity and muscle size. Because muscles consume approximately 20% of the body’s resting energy budget, muscles grow bigger and stronger primarily when they are used, and waste away under conditions of disuse. Similar physiologic responses to varying levels of demand exist in almost every system in the body. In the brain, increased
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levels of physical activity, especially aerobic exercise, result in increased production of neurotrophic factors, which, in turn, increase neurogenesis, neuroprotection, and cognitive function (Hillman et al., 2008; van Praag, 2008; Raichlen and Polk, 2013; Chieffi et al., 2017). Conversely, in individuals who avoid physical activity, brains are weaker and more susceptible to degeneration. The argument that human brains are poorly adapted to too much physical inactivity because evolution favored physically active but energy-limited individuals leads to the concept of mismatch diseases. Mismatches are diseases that are more prevalent or severe today because our bodies and brains are poorly or inadequately adapted to modern environments (Lieberman, 2013; Stearns and Medzhitov, 2015). Just as cavities were once rare but are now common because our teeth are poorly adapted to diets rich in sugar and starch, numerous chronic noninfectious diseases have become more prevalent and severe because humans did not evolve to be as physically inactive as many people are today. The two principal criteria for identifying mismatch diseases are that: (1) the disease is actually more prevalent or severe today than among past human populations; and (2) preventable determinants of the disease have become more common in modern environments (Lieberman, 2013). Several neurologic and psychiatric diseases are good candidates for mismatch diseases, especially those for which physical inactivity is a risk factor or may hasten disease progression, including Alzheimer disease, Parkinson disease, multiple sclerosis, anxiety, and depression (Dunn et al., 2001; Rovio et al., 2005; Goodwin et al., 2008; Motl and Gosney, 2008). Since the prevalence of some of these diseases increases with age, longer life expectancy today might be expected to be responsible for the current burden of particular conditions on the elderly. However, although this may be partly true, hunter-gatherer longevity is greater than commonly assumed, with the modal age of adult death in modern hunter-gatherers being 68–78 years old (Gurven and Kaplan, 2007). That said, there is little good evidence regarding neurologic and mental health among past populations, so hypotheses about mismatch diseases of the brain must be considered cautiously. Achieving a better understanding of the neurologic and mental health of living hunter-gatherers and other contemporary traditionalliving, nonindustrialized populations should be a goal of future research. An evolutionary perspective is also relevant to addressing why human brains are so susceptible to harm from particular activities that are common in some sports. Throughout evolutionary history, human skulls became increasingly fragile and prone to fracture. Compared to earlier species of Homo, modern humans have skulls with neurocrania that are markedly thinner (Lieberman, 1996)
as well as facial skeletons that lack protective bony buttresses such as browridges and flaring zygomatics (Carrier and Morgan, 2015). Although the mechanisms responsible for this increase in skull fragility are not well understood, one important factor might be the reduction in androgen reactivity in modern humans, which may have been favored by natural selection because it facilitated hunting and gathering by promoting social tolerance (Cieri et al., 2014). Regardless of causes, however, the consequence of this evolutionary change is that human brains are especially vulnerable to hemorrhages from skull fractures. This vulnerability was no more apparent than during the first half of the twentieth century when fracture-induced brain death claimed the lives of many American football players, largely due to direct trauma to the skull coupled with the lack or inadequacy of protective headgear. By the mid twentieth century, hard plastic helmets began to be used on a more regular basis, which had the positive effect of dramatically decreasing skull fractures. However, improvements in helmet design allowed individuals to repeatedly hit their heads against their opponent with less painful consequences, which gave way to another maladaptive neurologic health concern, namely, enhanced risk of concussion and repetitive head impact exposure. Although humans are not the only animals susceptible to concussion or repetitive head impact exposure, the extraordinary expansion of the human brain throughout evolution has made it prone to damage from acceleration and deceleration forces (Gibson, 2006; Lieberman, 2011). Such forces produce a combination of rotational and linear movements of the brain; the potential for either type of motion to damage brain tissue depends on brain size. Forces generated by a given rotational movement, which can cause shearing of axons and neuronal cell bodies, increase as a function of the brain’s mass times the square of its distance from the center of rotation (Wainwright et al., 1976). Forces generated by a given linear movement increase as a function of the ratio of brain mass to surface area (Gibson, 2006). As a result, while brain enlargement was a vital adaptation for hunting and gathering, it came at the cost of a severely decreased ability to withstand brain accelerations and decelerations, which commonly occur among individuals participating in contact/collision sports.
CONCLUSION Human brains require physical activity to function optimally because they evolved among our huntergatherer ancestors who were almost never able to avoid regular, endurance-based physical activity. Moreover, because energy from food was limited among our ancestors, human brains – like most energetically costly
SPORTS AND THE HUMAN BRAIN: AN EVOLUTIONARY PERSPECTIVE physiologic systems – evolved to require stimuli from physical activity, to adjust capacity to demand. As a result, human brains are poorly adapted to the extreme physical inactivity typical of many people today, which in all likelihood contributes to the current high prevalence and severity of many neurologic and mental health disorders. Thanks to our evolutionary history as physically active hunter-gatherers, human brains are adapted to reap major health benefits from sports and other athletic activities, especially those involving aerobic exercise. Unfortunately, however, humans’ exceptionally large brains are also vulnerable to being harmed by certain sports, especially contact/collision sports, which increase risk of concussion and repetitive head impact exposure.
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