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Your hands than mine are quicker for a fray,
My legs are longer though, to run away.
— SHAKESPEARE , A Midsummer Night’s Dream
The forest, as usual, is quiet apart from the muted sounds of rustling leaves, buzzing insects, and a few chirping birds. Suddenly, pandemonium breaks out as three chimpanzees tear through the trees high above the forest floor, leaping spectacularly from branch to branch, hair bristling, screaming wildly as they chase a group of colobus monkeys at breakneck speed. In less than a minute, an experienced older chimp makes a magnificent jump, catches a terrified monkey that was heading his way, and dashes its brains out against a tree. The hunt is over as suddenly as it started. As the victor rips his prey into pieces and starts to consume the flesh, other chimps hoot with excitement. Any humans watching, however, are likely to be shocked. Observing chimps hunt can be disturbing, not just because of the violence, but also because we prefer to think of them as gentle, intelligent cousins. Sometimes they seem mirrors of our better selves, but when hunting, chimps reflect humanity’s darker tendencies in their craving for flesh, their capacity for violence, and even their lethal use of teamwork and strategy.
The scene also highlights fundamental contrasts between human and chimp bodies. Apart from the obvious anatomical differences such as fur, snouts, and walking on all fours, chimps’ spectacular hunting skills underscore how athletically pathetic humans are in many ways. Humans almost always hunt with weapons because no person alive could possibly match a chimp for speed, power, and agility, especially in the trees. Despite my desire to be like Tarzan, I climb trees clumsily, and even practiced tree climbers must ascend and descend gingerly and cautiously. The ability to scamper up a tree trunk as if it were a ladder, leap between precarious branches, and make a flying grab through the air at a fleeing monkey while landing safely on a bough or branch is far beyond the skill of the most highly trained human gymnast. Although watching a chimp hunt is disturbing, I find it impossible not to admire the inhuman acrobatic capabilities of these chimps with which we share more than 98 percent of our genetic code.
Humans are comparatively poor athletes on land as well. The world’s speediest humans can sprint about 23 miles (37 kilometers) per hour for less than half a minute. For most of us plodders, such speeds seem superhuman, but numerous mammals, including chimps and goats, easily run at twice that speed for many minutes without the help of coaches or years of intense training. I can’t even outrun a squirrel. Running humans are also unwieldy and unsteady, unable to make rapid turns. Even the slightest bump or nudge can cause a runner to tumble to the ground. Finally, we lack power. An adult male chimp weighs 15 to 20 kilograms (33 to 44 pounds) less than most human males, yet efforts to measure their strength indicate that a typical chimp can muster more than twice as much muscle force as the brawniest of elite human athletes. 1
As we start our exploration of the human body’s story in order to ask what humans are adapted for, a key first question is: why and how did humans become so ill adapted to life in trees, as well as feeble, slow, and awkward?
The answer begins with becoming upright, apparently the first major transformation in human evolution. If there was any one key initial adaptation, a spark that set the human lineage off on a separate evolutionary path from the other apes, it was likely bipedalism, the ability to stand and walk on two feet. In his typically prescient fashion, Darwin first suggested this idea in 1871. Lacking any fossil record, Darwin made his conjecture by reasoning that the earliest human ancestors evolved from apes; by becoming upright, they emancipated their hands from locomotion, freeing them for making and using tools, which then favored the evolution of larger brains, language, and other distinctive human features:
Man alone has become a biped; and we can, I think, partly see how he has come to assume his erect attitude, which forms one of his most conspicuous characters. Man could not have attained his present dominant position in the world without the use of his hands, which are so admirably adapted to act in obedience to his will.… But the hands and arms could hardly have become perfect enough to have manufactured weapons, or to have hurled stones and spears with a true aim, as long as they were habitually used for locomotion and for supporting the whole weight of the body, or, as before remarked, so long as they were especially fitted for climbing trees.… If it be an advantage to man to stand firmly on his feet and to have his hands and arms free, of which, from his pre-eminent success in the battle of life, there can be no doubt, then I can see no reason why it should not have been advantageous to the progenitors of man to have become more and more erect or bipedal. They would thus have been better able to defend themselves with stones or clubs, to attack their prey, or otherwise to obtain food. The best built individuals would in the long run have succeeded best, and have survived in larger numbers. 2
A century and a half later, we now have enough evidence to suggest that Darwin was probably right. Thanks to a peculiar set of contingent circumstances—many of them initiated by climate change—the oldest known members of the human lineage developed several adaptations to stand and walk on just two legs more easily and frequently than apes. Today, we are so thoroughly adapted to being habitually bipedal, we rarely give our unusual way of standing, walking, and running much thought. But look around you: how many other creatures, apart from birds (or kangaroos if you live in Australia), do you see tottering or hopping about on just two legs? The evidence suggests that of all the human body’s major transformations over the last few million years, this adaptive shift was one of the most momentous, not only because of its advantages, but also because of its disadvantages. Therefore, learning about how our early ancestors became adapted to being upright is a principal starting point for recounting the human body’s journey. As a first step, let’s meet those primordial ancestors, beginning with the last ancestor we shared with apes.
The term “missing link,” which goes back to the Victorian era, is a frequently misused word that generally refers to key transitional species in the history of life. Although many fossils are glibly labeled missing links, there is one especially fundamental species in the record of human evolution that is well and truly missing: the last common ancestor (LCA) of humans and the other apes. To our great frustration, this important species so far remains entirely unknown. Like chimps and gorillas, the LCA most likely lived, as Darwin inferred, in an African rain forest, an environment inhospitable to the preservation of bones, and thus to the creation of a fossil record. Bones that fall to the forest floor quickly rot and then dissolve. For this reason, there are few informative fossil remnants of the chimpanzee and gorilla lineages, and the chances are slim of finding fossil remains of the LCA. 3
Although absence of evidence is not evidence for absence, it sure does lead to rampant speculation. A dearth of fossils from the part of the family tree where the LCA belongs has occasioned much conjecture and debate regarding this elusive missing link. Even so, we can make some reasonable inferences about when and where the LCA lived and what it was like by making careful comparisons of the similarities and differences between humans and apes in conjunction with what we know about our evolutionary tree. This tree, illustrated in figure 1 , shows that there are three living species of African apes, and that humans are more closely related to the two species of chimpanzees, common chimps and pygmy chimps (also known as bonobos), than to gorillas. Figure 1 , which is based on extensive genetic data, also shows that the human and chimp lineages diverged about 8 to 5 million years ago (the exact date remains the subject of debate). Strictly speaking, humans are a special subset of the ape family termed hominins , defined as all species more closely related to living humans than to chimpanzees or other apes. 4
FIGURE 1. Evolutionary tree of humans, chimpanzees, and gorillas. This tree shows the two species of chimps (bonobos and common chimpanzees); some experts divide gorillas into more than one species.
Our especially close evolutionary relationship to chimps came as a surprise to scientists in the 1980s when the molecular evidence necessary to resolve this tree became available. Before then, most experts assumed that chimps and gorillas were more closely related to each other than to humans because chimps and gorillas look so similar. Yet, the counterintuitive fact that we are evolutionary first cousins with chimps but not gorillas provides valuable clues for reconstructing the LCA, because even though humans and chimps share an exclusive LCA, chimps, bonobos, and gorillas are much more like one another than they are like humans. Although gorillas weigh two to four times as much as chimps, if you were to grow a chimp to the size of a gorilla, you’d get something that sort of (though not completely) resembles a gorilla. 5 Adult bonobos are also shaped like and even behave like adolescent chimpanzees. 6 In addition, gorillas and chimps walk and run in the same peculiar fashion known as knuckle walking , in which they rest their forelimbs on the middle digits of the hand. Therefore, unless the many similarities between the various species of African great apes evolved independently, which is highly improbable, the LCA of chimps and gorillas must have been somewhat chimplike or gorillalike in terms of anatomy. By the same logic, the LCA of chimps and humans was also probably anatomically like a chimp or a gorilla in many respects.
Put crudely, when you look at a chimp or a gorilla, the chances are that you are regarding an animal that vaguely resembles your very distant ancestor—that all-important missing species—from several hundred thousand generations ago. I must emphasize, however, that this hypothesis is impossible to test definitively without direct fossil evidence, leaving plenty of room for differing opinions. Some paleoanthropologists think that the way humans stand and walk upright is reminiscent of the way that gibbons, a more distantly related ape, swing below and travel on top of branches. In fact, for more than one hundred years, when chimps and gorillas were thought to be first cousins, many scholars reasoned that humans evolved from an unknown species that was sort of gibbonish. 7 Alternatively, a few paleoanthropologists speculate that the LCA was a monkeylike creature that walked on top of branches and climbed trees using all four limbs. 8 These views notwithstanding, the balance of evidence suggests that the very first species in the human lineage evolved from an ancestor that wasn’t considerably different from today’s chimps and gorillas. This inference, it turns out, has major implications for understanding how and why the first hominins apparently evolved to be upright. Fortunately, unlike the still-missing LCA, we have tangible evidence of these very ancient ancestors.
When I was a student, there were no useful fossils to record what happened during the first few million years of human evolution. Lacking data, many experts had no choice but to assume (sometimes blithely) that the oldest fossils then known, such as Lucy, who lived about 3 million years ago, were good stand-ins for earlier, missing hominins. However, since the mid-1990s we have been blessed by the discovery of many fossils from the first few million years of the human lineage. These primordial hominins have abstruse, unmellifluous names, yet they have caused us to rethink what the LCA was like, and, more important, they reveal much about the origins of bipedalism and other features that made the first hominins different from the other apes. Currently, four species of early hominins, two of which are shown in figure 2 , have been found. Before discussing what these species were like, what they were adapted for, and their relevance to later events in human evolution, here are some basic facts about who they were and where they came from.
The oldest known proposed species of hominin is Sahelanthropus t chadensis , discovered in Chad in 2001 by an intrepid French team under the leadership of Michel Brunet. Recovering fossils of this species required years of grueling, dangerous fieldwork because they had to be excavated from beneath the sands of the southern part of the Sahara Desert. Today, this area is a barren, inhospitable place, but millions of years ago it was a partly wooded habitat near a giant lake. Sahelanthropus is mostly known from a single, nearly complete cranium (nicknamed Toumaï, which means “hope of life” in the language of the region it was found) shown in figure 2 , as well as some teeth, jaw fragments, and a few other bones. 9 According to Brunet and his colleagues, Sahelanthropus is at least 6 million years old and may be as old as 7.2 million years. 10
Another proposed species of early hominin from Kenya, named Orrorin tugenensis , is about 6 million years old. 11 Unfortunately, there are only a few scraps of this enigmatic species: a single jaw fragment, some teeth, and some limb bone fragments. We still know little about Orrorin , in part because there is not much to study, and in part because the fossils have not yet been comprehensively analyzed.
FIGURE 2. Two early hominins. Top, cranium of Sahelanthropus tchadensis (nicknamed Toumaï); bottom, a reconstruction of Ardipithecus ramidus (nicknamed Ardi). The angle of the foramen magnum in Toumaï indicates a vertically oriented upper neck, a clear sign of bipedalism. The reconstruction of the partial Ardipithecus skeleton suggests that she was adapted for bipedal walking as well as climbing trees. Image of Sahelanthropus courtesy of Michel Brunet; drawing of Ardipithecus copyright © 2009 Jay Matternes.
The richest trove of early hominin fossils was discovered in Ethiopia by an international team led by Tim White and colleagues from the University of California, Berkeley. These fossils have been assigned to two different species from yet another genus, Ardipithecus . The older species, Ardipithecus kadabba , is dated to between 5.8 and 5.2 million years ago and is so far known from a handful of bones and teeth. 12 The younger species, Ardipithecus ramidus , dated to 4.5 to 4.3 million years ago, includes a much larger collection of fossils, including a remarkable partial skeleton of a female nicknamed Ardi, shown in figure 2 . 13 This species is also represented by numerous fragments (mostly teeth) of more than a dozen other individuals. Ardi’s skeleton is the focus of intense research because it gives us a rare, exciting opportunity to figure out how she and other early hominins stood, walked, and climbed.
You could fit all the fossils from Ardipithecus , Sahelanthropus , and Orrorin in a single shopping bag. Even so, they yield concrete glimpses of the earliest phases of human evolution during the first few million years after we split from the LCA. One unsurprising revelation is that these early hominins are generally apelike. As predicted by our close relationship to the African great apes, they bear many resemblances to chimps and gorillas in details of the teeth, crania, and jaws, as well as their arms, legs, hands, and feet. 14 For example, their skulls have small brains in the size range of chimps, a substantial browridge above the eyes, big front teeth, and long, projecting snouts. Many features of the feet, arms, hands, and legs of Ardi are also similar to what one sees in African apes, especially chimps. In fact, some experts have suggested these ancient species are too apelike to actually be hominins. 15 I think, however, that they are bona fide hominins for several reasons, the most important of which is that they bear indications that they were adapted to walking upright on two legs.
Egocentric creatures that we are, humans often mistakenly consider our quintessential features to be special when in fact they are simply unusual. Bipedalism is no exception. Like many parents, I fondly remember when my daughter took her first triumphant steps, which suddenly made her seem so much more human than our dog. A common belief (especially among proud parents) is that walking upright is particularly challenging and difficult, perhaps because it takes human children many years to learn to walk well, and because few other animals are habitual bipeds. In actual fact, the reason children don’t toddle until they are about a year old and then walk and run awkwardly for a few more years is that many of their neuromuscular skills also require considerable time to mature. 16 Just as it takes years for our big-brained children to walk properly, it also takes them years to speak rather than babble, control their bowels, and manipulate tools with skill. In addition, although habitual bipedalism is rare, occasional bipedalism is unexceptional. Apes sometimes stand and walk on two legs, as do many other mammals (including my dog). Yet human bipedalism is different from what apes do in one key respect: we habitually stand and walk very efficiently because we gave up the ability to be quadrupeds. Whenever chimps and other apes walk upright, they lurch about with an awkward and energetically costly gait because they lack a few key adaptations, shown in figure 3 , that enable you and me to walk well. What is especially exciting about the first hominins is that they, too, have some of these adaptations, indicating that they were also upright bipeds of some sort. However, if Ardi is generally representative of these hominins, they still retained many ancestral features useful for climbing trees. Although we are struggling to reconstruct precisely how Ardi and other early hominins walked when they weren’t climbing, there is no question that they walked very differently from you and me in a much more apelike fashion. This type of early bipedalism was probably a critical intermediate form of upright locomotion that set the stage for later, more modern gaits, and it was made possible by several adaptations we still retain in our bodies today.
The first of these adaptations is the shape of the hips. If you watch a chimpanzee walk upright, observe that it keeps its legs far apart and its upper body sways from side to side like an unstable drunkard. Sober humans, in contrast, sway their torsos almost imperceptibly, which means we can spend most of our energy moving forward instead of stabilizing the upper body. Our steadier gait is largely attributable to a simple change in the shape of the pelvis. As figure 3 shows, the large, broad bone that forms the upper part of the pelvis (the ilium) is tall and faces backward in apes, but this part of the hip is short and faces sideways in humans. This sideways orientation is a crucial adaptation for bipedalism because it allows the muscles on the side of the hips (the small gluteals) to stabilize the upper body over each leg during walking when only one leg is on the ground. You can demonstrate this adaptation for yourself by standing on one leg as long as possible while keeping your trunk upright. (Go ahead and try!) After a minute or two, you’ll feel these muscles tire. Chimps cannot stand or walk this way because their hips face backward, permitting the same muscles only to extend the leg behind them. The sole way a chimp can avoid falling sideways when one leg is on the ground is by markedly tilting its trunk to the side above that leg. Not so Ardi. Although Ardi’s pelvis was badly distorted and had to be reconstructed extensively, she appears to have a shortened and sideways-facing ilium, just like a human. 17 In addition, the femur of Orrorin has an especially large hip joint, a long neck, and a wide upper shaft, features that allowed its hip muscles to stabilize the torso efficiently when walking and to withstand the high side-to-side bending forces this action causes. 18 These features inform us that the first hominins didn’t have to lurch from side to side when walking.
FIGURE 3. Comparison of a human and chimpanzee highlighting some of the adaptations for upright standing and walking in humans. Figure adapted from D. M. Bramble and D. E. Lieberman (2004). Endurance running and the evolution of Homo. Nature 432: 345–52.
Another important adaptation for being a biped is an S-shaped spine. Like other quadrupeds, apes have spines that curve gently (the front side is slightly concave), so when they stand upright, their trunks naturally tilt forward. As a result, the ape’s torso is positioned unstably in front of its hips. In contrast, the human spine has two pairs of curves. The lower, lumbar curve is made possible by having more lumbar vertebrae (apes usually have three or four, whereas humans usually have five), several of which have a wedged shape in which the top and bottom surfaces are not parallel. Just as wedge-shaped stones allow architects to construct arched structures like bridges, wedged vertebrae curve the lower spine inward above the pelvis, positioning the torso stably above the hips. Human chest and neck vertebrae create another, gentler curve at the top of the spine, which orients the upper neck downward rather than backward from the skull. Although we have yet to find any early hominin lumbar vertebrae, the shape of Ardi’s pelvis hints at a long lumbar region. 19 An even more telling clue of having an S-shaped spine adapted for bipedalism comes from the shape of the Sahelanthropus cranium. The necks of chimps and other apes emerge from near the backs of their skulls at a slightly horizontal angle, but Toumaï’s cranium, shown in figure 2 , is so complete we can deduce confidently that his upper neck was nearly vertical when he was standing or walking. 20 This configuration could be possible only if Toumaï’s spine had a backward curve in the lower spine, the neck, or both.
Yet more crucial adaptations for upright locomotion that appear in early hominins are at the other end of the body, in the foot. Walking humans usually land first on the heel and then, as the rest of the foot makes contact with the ground, we stiffen the foot’s arch, enabling us to push the body upward and forward at the end of stance, mostly with the big toe. The shape of the human arch is created by the shapes of the foot’s bones, as well as by many ligaments and muscles that secure the bones in place like cables in a suspension bridge, and which become taut (to varying extents) when the heel comes off the ground. In addition, the surfaces of the joints between the toes and the rest of the foot in humans are very rounded and point slightly upward, helping us bend our toes at an extreme angle (hyperextend) when we push off. The feet of chimps and other apes lack an arch, preventing them from pushing off against a stiffened foot, and their toes are unable to extend as much as humans’.
Importantly, Ardi’s foot (along with a younger partial foot that could belong to the same genus) bears some traces that the middle was partly stiffened, and it has toe joints that were capable of bending upward at the end of stance. 21 These features suggest that Ardi, like humans but unlike chimps, had feet capable of generating effective propulsion when walking upright.
The evidence I just summarized for bipedalism in the first hominins is electrifying but admittedly scant. There is a great deal we don’t know about how these species stood, walked, and ran because we lack much of Ardi’s skeleton, and we know almost nothing about the skeletons of Sahelanthropus and Orrorin. Nonetheless, there is sufficient evidence to indicate that these ancient species stood and walked differently than you and me to a large extent because they retained numerous ancient adaptations for climbing trees. Ardi’s foot, for example, had a highly muscular and divergent big toe that was very capable of grasping around branches or tree trunks. Its other toes were long and fairly curved, and its ankle tilted slightly inward. These features, which are useful for climbing, caused her foot to function differently than modern feet. When walking, she probably used her feet more like a chimpanzee, keeping her weight along the outside of the foot rather than rolling it in ( pronating ) like a human. 22 Ardi also had short legs, and if she walked along the outside of her feet, then she might have walked with a wider stance than people today. Perhaps she had slightly bent knees as well. As you might expect, there is plenty of other evidence for tree-climbing abilities in Ardi’s upper body, which had long, powerfully muscled fore arms and long, curved fingers. 23
Standing back from the details, the overall picture that emerges of the first hominins is that they were certainly not quadrupeds when they were on the ground but instead were occasional bipeds who stood and walked upright in a distinctively nonhuman manner when they were not climbing trees. They could not stride as efficiently as humans, but they were probably able to walk upright with more efficiency and stability than a chimp or a gorilla. However, these ancient ancestors were also adept climbers who likely spent a considerable portion of their time aloft. If we could observe them climbing, we’d probably marvel at their ability to scamper up boughs and jump from branch to branch, but they might have been less agile than a chimp. If we could observe them walking, we’d think their gait was slightly odd as they stepped on the sides of their long, inwardly angled feet, taking short strides. It is tempting to imagine them wobbling about unstably on two legs like upright chimps (or drunken humans), but this is unlikely. I suspect they were proficient at both walking and climbing, but they did so in a distinctive fashion unlike any creature alive today.
Animals move about for many reasons, including to escape predators and to fight, but a principal reason to walk or run is to get dinner. Accordingly, before we consider why bipedalism initially evolved we need to highlight one additional suite of features, all related to diet, which distinguishes the first hominins.
For the most part, the earliest hominins like Toumaï and Ardi have apelike faces and teeth, suggesting that they ate a rather apelike diet that was dominated by ripe fruit. For example, they have wide front teeth shaped like spatulas, which are well suited for biting into fruits just as you do when you sink your teeth into an apple. They also have cheek teeth with low cusps that are perfectly shaped for crushing the flesh of fibrous fruits. However, there are a few subtle hints that these early members of the human lineage were slightly better adapted than chimps to eating low-quality foods in addition to fruit. One difference is that their cheek teeth are moderately bigger and thicker than those of apes such as chimps and gorillas. 24 Larger, thicker molars would have been better able to break down harder, tougher items of food like stems and leaves. Second, Ardi and Toumaï are a little less snouty because of slightly more forward-placed cheekbones and more vertical faces. 25 This configuration positions the chewing muscles so they produce higher bite forces for breaking down tougher and harder foods. Finally, the canines ( fangs) of early hominin males are smaller, shorter, and less dagger-shaped than those of male chimps. 26 Although some researchers believe that smaller male canines suggest that males fought less with one another, an alternative and more convincing explanation is that smaller canines were adaptations to help them chew tougher, more fibrous food. 27
Putting the evidence together, we can conjecture with some confidence that the first hominins probably gorged as much as they could on fruit, but natural selection favored those better able to resort to eating less desirable, tough, fibrous foods, like the woody stems of plants, which require lots of hard chewing to break down. These diet-related differences are frankly subtle. However, when we consider them in combination with what we know about their locomotion and the environments in which they lived, we can begin to hypothesize why the first hominins became bipedal, thus setting the human lineage off on a very different evolutionary path from our ape cousins.
Plato once defined humans as featherless bipeds, but he didn’t know about dinosaurs, kangaroos, and meerkats. In actual fact, we humans are the only striding, featherless, and tailless bipeds. Even so, tottering about on two legs has evolved only a few times, and there are no other bipeds that resemble humans, making it hard to evaluate the comparative advantages and disadvantages of being a habitually upright hominin. If hominin bipedalism is so exceptional, why did it evolve? And how did this strange manner of standing and walking influence subsequent evolutionary changes to the hominin body?
It is impossible to ever know for sure why natural selection favored adaptations for bipedalism, but I think the evidence most strongly supports the idea that regularly standing and walking upright was initially selected to help the first hominins forage and obtain food more effectively in the face of major climate change that was occurring when the human and chimpanzee lineages diverged.
Climate change is a topic of intense interest today because of evidence that humans are warming the earth by burning massive quantities of fossil fuel, but it has long been an influential factor in human evolution, including during the time when we split from the apes. Figure 4 graphs the temperature of the earth’s oceans over the last 10 million years. 28 As you can see, between 10 and 5 million years ago, the entire earth’s climate cooled considerably. Although this cooling happened over millions of years and with endless fluctuations between warmer and colder periods, the overall effect in Africa was to cause rain forests to shrink and woodland habitats to expand. 29 Now imagine yourself as the LCA—a large-bodied, fruit-eating ape—during this period. If you were living in the heart of the rain forest, you probably would not have noticed much of a difference. But if you had the misfortune to be living at the margins of the forest, then this change must have been stressful. As the forest around you shrinks and becomes woodland, the ripe fruits you hunger after become less abundant, more dispersed, and more seasonal. These changes would sometimes require you to travel farther to get the same amount of food, and you’d resort more frequently to eating fallback foods, which are more abundant but lower in quality than preferred foods such as ripe fruit. Typical fallback foods for chimpanzees include the fibrous stems and leaves of plants, as well as various herbs, 30 and the evidence for climate change suggests that the first hominins would have needed to find and eat such foods more often and more intensely than chimps do. Perhaps they were more like orangutans, whose habitats are not as continuously bountiful as those of chimps, requiring them to eat very tough stems and even bark when fruit is unavailable. 31
Just as the tough get going when the going gets tough, natural selection acts most strongly not during times of plenty, but during times of stress and scarcity. If, as we think, the LCA was a mostly fruit-eating ape that lived in a rain forest, then natural selection would have favored the two major transformations we see in very early hominins such as Toumaï and Ardi. The first shift is that hominins with bigger, thicker cheek teeth and the ability to chew more forcefully would have been better able to consume more tough, fibrous fallback foods. The second but more extensive shift, bipedalism, is a little harder to appreciate as an adaptation to climate change but was probably even more important in the long run for several reasons, one of which may be surprising.
FIGURE 4. Climate change during human evolution. The left graph plots how global sea temperatures have fallen over the last 20 million years, with a major cooling event around the time the human and chimpanzee lineages diverged. This graph is expanded on the right to highlight the last 5 million years. The mean temperature indicated by the central line is an average of the many large, rapid fluctuations (shown by the zigs and zags). Note the major cooling at the beginning of the Ice Age. The graph is modified from J. Zachos et al. (2001). Trends, rhythms, and aberrations in global climate 65 Ma to present. Science 292: 686–93.
One obvious advantage of bipedalism is that standing on two feet can make it easier to forage for certain fruits. Orangutans, for example, sometimes stand nearly upright on branches when feeding in trees, reaching for precariously hanging foods by keeping their knees straight and holding on to at least one other branch. 32 Chimps and some monkeys also stand in a similar fashion when feeding on low-hanging berries and fruits. 33 So, bipedalism initially might have been a postural adaptation. Maybe competition for food was so intense that early hominins better able to stand upright gathered more fruit during seasons of scarcity. In this context, early hominins with more sideways facing hips and other features that helped them remain upright might have had an advantage over others when standing because they spent less energy, had more stamina, and were more stable. By the same token, being able to stand and walk upright more effectively might have helped hominins to carry more fruit, as chimps sometimes do when competition is intense. 34
A second, more surprising, and possibly more important advantage of bipedalism is that walking on two legs may have helped early hominins save energy when traveling. Recall that the LCA was probably a knuckle walker. Knuckle walking is a decidedly peculiar way to walk on all fours, and it is also energetically costly. Laboratory studies that have enticed chimps to walk on treadmills while wearing oxygen masks have found that these apes spend four times more energy to walk (on either two or four limbs) a given distance than humans. 35 Four times! This extraordinary difference occurs because chimps have short legs, they sway from side to side, and they always walk with bent hips and knees. As a result, chimps constantly spend lots of energy contracting their back, hip, and thigh muscles to keep from toppling over and collapsing to the ground. Not surprisingly, chimps walk comparatively little, only about 2 or 3 kilometers a day (about 1 to 2 miles). 36 For the same amount of energy, a human can walk between 8 and 12 kilometers (5 to 7.5 miles). Therefore, if early hominins were able to walk bipedally with less lurching and with straighter hips and knees, they would have had a substantial energetic advantage over their knuckle-walking cousins. Being able to walk farther using the same amount of energy would have been a very beneficial adaptation as the rain forests shrank, fragmented, and opened up, causing preferred foods to become rarer and more dispersed. Keep in mind, however, that although the way humans walk on two legs is vastly more economical than the way chimps knuckle walk, the first hominins may have been only slightly more efficient than chimps and not as efficient as later hominins.
As one might expect, other selective pressures are hypothesized to have favored bipedalism in the first hominins. Additional suggested advantages of being upright include improved abilities to make and use tools, to see over tall grasses, to wade across streams, and even to swim. None of these hypotheses bear up under scrutiny. The oldest stone tools don’t appear until millions of years after bipedalism evolved. In addition, apes can and do stand up just fine to wade and look about, and it takes considerable imagination to convince oneself that humans are well adapted for swimming either in terms of cost or speed. (Spending much time in some African lakes or rivers is also a surefire way to become a crocodile’s meal.) Another longstanding idea is that bipedalism was initially selected to help hominins carry food, perhaps so males could provision females, just as hunter-gatherer men do today. In fact, one formulation of this idea is that bipedalism evolved to favor males who exchanged food for sex with females. 37 Titillating as the idea may seem—especially in light of the fact that human females, unlike their chimp counterparts, display no overt signals when they are ovulating—the hypothesis is unconvincing for several reasons, not the least of which is that human females often provision males. In addition, we don’t yet know how much bigger early hominin males were than females, but in later species of hominins, males were about 50 percent bigger than females. 38 This kind of size difference between sexes is strongly associated with males vigorously competing with one another for sexual access to females rather than wooing females through cooperation and food sharing. 39
In short, many lines of evidence suggest that climate change spurred selection for bipedalism in order to improve early hominins’ ability to acquire the fallback foods they needed to eat when fruit was not available. More evidence is needed to test this scenario fully, but whatever its cause, the shift to standing and walking upright was the first major transformation in human evolution. But why was bipedalism such a big deal for what followed in human evolution? What makes it such a fundamentally important adaptation?
The tangible world around us usually appears so normal and so natural that it is tempting and sometimes comforting to assume that everything we perceive has a purpose, perhaps by design, and that things are as they should be. This way of thinking can lead one to believe that humans are as much a certainty as the moon in the sky and the laws of gravity. Although selection for bipedalism played an initial, fundamental role in the first stages of human evolution, the contingent circumstances by which it arose highlight the fallacy of its inevitability. Had early hominins not become bipeds, then humans would never have evolved as they did, and you would probably not be reading this. Further, bipedalism initially evolved because of an improbable series of events, all of which were contingent on earlier circumstances that were driven by chance shifts in the world’s climate. Bipedal hominins probably neither could have nor would have evolved if knuckle-walking, fruit-eating apes hadn’t previously evolved to live in the African rain forest. In addition, had the earth not cooled substantially those many millions of years ago, the conditions that favored the beginnings of bipedalism among these apes might never have existed. Our being here is the result of many rolls of the dice.
Whatever its causes, was habitually standing and walking on two legs the spark that ignited later developments in human evolution? In some ways, the kind of intermediate bipedalism we see in Ardi and company seems like an improbable trigger for what followed. As we have seen, the first hominins were like their African ape cousins in many respects, with the major exception of being upright on the ground. If a surviving relict population of very early hominins were to be discovered, we’d be more likely to send them to zoos than boarding schools because they had modest, chimp-size brains. In this respect, Darwin was prescient to speculate in 1871 that, of all the characteristics that make humans distinct, it was bipedalism rather than big brains, language, or tool use that first set the human lineage off on its separate path from the other apes. Darwin’s reasoning was that bipedalism initially emancipated the hands from locomotion, allowing natural selection to subsequently favor additional capabilities such as making and using tools. In turn, these capabilities selected for bigger brains, language, and other cognitive skills that have made humans so exceptional in spite of our lack of speed, strength, and athletic prowess.
Darwin appears to have been right, but a major problem with his hypothesis was that he did not account for how or why natural selection favored bipedalism in the first place, and he could not explain why freeing the hands then selected for tool making, cognition, and language. After all, kangaroos and dinosaurs also have unencumbered hands, but they didn’t evolve big brains and tool-making abilities. Such arguments led many of Darwin’s successors to argue that it was big brains rather than bipedalism that led the way in human evolution.
More than one hundred years later, we now have a better idea of how and why bipedalism initially evolved and why it was such a monumental and consequential shift. As we have seen, the first bipeds didn’t get up on two feet in order to free their hands; instead they probably became upright in order to forage more efficiently and to reduce the cost of walking (if the LCA was a knuckle walker). In this respect, bipedalism was probably an expedient adaptation for fruit-loving apes to survive better in more open habitats as Africa’s climate cooled. Moreover, the evolution of habitual bipedalism did not require an immediate radical transformation of the body. Although few mammals habitually stand and walk on two legs, the anatomical features that make hominins effective bipeds are actually just modest shifts that were evidently subject to natural selection. Consider lumbar regions. In any population of chimps, you’ll find that about half of them have three lumbar vertebrae, the other half have four, and a very tiny number have five, thanks to heritable genetic variations. 40 If having five lumbar vertebrae gave some apes a few million years ago a slight advantage when standing and walking, they would have been more likely to have passed that variation on to their offspring. The same selective processes must have applied to other features that improved the LCA’s ability to be bipedal, such as how wedged its lumbar vertebrae were, the orientation of its hips, and the stiffness of its feet. How long it took for selection to transform a population of the LCA into the first bipedal hominins is unknown, but it could have occurred only if the initial intermediate stages had some benefit. Put differently, the first hominins must have had a slight reproductive advantage from being just partly better at standing or walking upright.
Change always generates new contingencies and new challenges. Once bipedalism evolved, it created new conditions for further evolutionary change to occur. Darwin, of course, understood this logic, but he mostly considered how bipedalism led to further evolutionary change by focusing on its advantages rather than the disadvantages. Yes, bipedalism did free the hands and set the stage for subsequent selection based on tool making. But these additional selective changes don’t seem to have become important for millions of years, and they didn’t inevitably follow from having a spare pair of limbs. What Darwin didn’t give much consideration to was that bipedalism also posed new and substantive challenges for hominins. We are so used to being bipedal—it seems so normal—that we sometimes forget what a problematic mode of locomotion it can be. Ultimately, these challenges may have been just as important as its benefits for subsequent events in human evolution.
One major drawback with being bipedal is coping with pregnancy. Pregnant mammals, four-legged or two-legged, have to carry a lot of extra weight not only from the fetus but also from the placenta and extra fluids. By full term, a pregnant human mother’s weight increases by as much as 7 kilograms (15 pounds). But unlike in quadrupedal mothers, this extra mass has a tendency to cause her to fall over because it shifts her center of gravity well in front of the hips and feet. As any pregnant mother-to-be will tell you, she becomes less stable and less comfortable as her pregnancy progresses, requiring her either to contract her back muscles more, which is tiring, or to lean backward, shifting her center of mass back over her hips. Although this characteristic pose saves energy, it places extra shearing stresses on the lumbar vertebrae of the lower back as they try to slide away from one another. Lower back pain is thus a common, debilitating problem for human mothers. Yet we can see that natural selection helped hominin mothers cope with this extra load by increasing the number of wedged vertebrae over which females arch their lower spines: three in females versus two in males. 41 This extra curving reduces shearing forces in the spine. Natural selection also favored females whose lumbar vertebrae have more reinforced joints to bear these stresses. And, as you would predict, these adaptations for coping with the unique problems of being a pregnant biped are very ancient and can be seen in the oldest vertebral columns of hominins so far discovered.
Another consequential disadvantage of bipedalism is loss of speed. When early hominins became bipeds they surrendered the ability to gallop. By any conservative estimate, not being able to gallop limited our early ancestors to being about half as fast as a typical ape when sprinting. In addition, two limbs are much less stable than four and make it harder to turn quickly when running. Predators such as lions, leopards, and saber-toothed tigers probably had a field day hunting hominins, making it especially perilous for our ancestors to venture into open habitats (and risk not being anyone’s ancestor). Bipedalism probably also hampered the ability to climb trees with as much agility as a quadrupedal ape. It is hard to tell for sure, but early bipeds were probably unable to hunt the way chimpanzees do, by leaping through the trees. Giving up speed, power, and agility set the stage for natural selection to eventually (millions of years later) make our ancestors tool makers and endurance runners. Becoming bipedal also led to other quintessential human problems like sprained ankles, lower back pain, and knee troubles.
Yet in spite of the many disadvantages of being bipedal, the benefits of walking and standing upright must have outweighed the costs at every evolutionary stage. Early hominins apparently trudged about parts of Africa in search of fruits and other foods in spite of their lack of speed and agility on the ground. These hominins were also probably quite adept at climbing trees, and as far as we can tell, their overall way of life endured for at least 2 million years. But then another burst of evolution occurred around 4 million years ago that gave rise to a diverse group of hominins known collectively as the australopiths. The australopiths are important not only because they are a testament to the initial success and subsequent importance of bipedalism, but also because they set the stage for later, even more revolutionary shifts that further transformed the human body.