Sunset on the Savanna

Why do we walk? For decades anthropologists said that we became bipedal to survive on the African savanna.

By James Shreeve|Monday, July 01, 1996
RELATED TAGS: HUMAN EVOLUTION
Once upon a time, there was an ape who lived in the middle of a dark forest. It spent most of its days in the trees, munching languidly on fruits and berries. But then one day the ape decided to leave the forest for the savanna nearby. Or perhaps it was the savanna that moved, licking away at the edge of the forest one tree at a time until the fruits and berries all the apes had found so easily weren’t so easy to find anymore.

In either case, the venturesome ape found itself out in the open, where the air felt dry and crisp in its lungs. Life was harder on the savanna: there might be miles between one meal and another, there were seasons of drought to contend with, and large, fierce animals who didn’t mind a little ape for lunch. But the ape did not run back into the forest. Instead it learned to adapt, walking from one place to another on two legs. And it learned to live by its wits. As the years passed, the ape grew smarter and smarter until it was too smart to be called an ape anymore. It lived anywhere it wanted and gradually made the whole world turn to its own purposes. Meanwhile, back in the forest, the other apes went on doing the same old thing, lazily munching on leaves and fruit. Which is why they are still just apes, even to this day.

The tale of the ape who stood up on two legs has been told many times over the past century, not in storybooks or nursery rhymes but in anthropology texts and learned scientific journals. The retellings have differed from one another in many respects: the name of the protagonist, for instance, the location in the world where his transformation took place, and the immediate cause of his metamorphosis. One part of the story, however, has remained remarkably constant: the belief that it was the shift from life in the forest to life in a more open habitat that set the ape apart by forcing it onto two legs. Bipedalism allowed hominids to see over tall savanna grass, perhaps, or escape predators, or walk more efficiently over long distances. In other scenarios, it freed the hands to make tools for hunting or gathering plants. A more recent hypothesis suggests that an erect posture exposes less skin to the sun, keeping body temperature lower in open terrain. Like the painted backdrop to a puppet theater, the savanna can accommodate any number of dramatic scenarios and possible plots.

But now that familiar stage set has come crashing down under the weight of a spectacular crop of new hominid fossils from Africa, combined with revelations about the environment of our earliest ancestors. The classic savanna hypothesis is clearly wrong, and while some still argue that open grasslands played some role in the origins of bipedalism, a growing number of researchers are beginning to think the once unthinkable: the savanna may have had little or nothing to do with the origins of bipedalism.

The savanna paradigm has been overthrown, says Phillip Tobias, a distinguished paleoanthropologist at the University of Witwatersrand in Johannesburg and formerly a supporter of the hypothesis. We have to look now for some other explanation for bipedalism.

The roots of the savanna hypothesis run deep. More than 100 years ago, Charles Darwin thought that mankind’s early ancestor moved from some warm, forest-clad land owing to a change in its manner of procuring subsistence, or to a change in the surrounding conditions. In his view, the progenitor assumed a two-legged posture to free the hands for fashioning tools and performing other activities that in turn nourished the development of an increasingly refined intelligence. Darwin believed that this seminal event happened in Africa, where mankind’s closest relatives, the African apes, still lived.

By the turn of this century, however, most anthropologists believed that the critical move to the grasslands had occurred in Asia. Though the bones of a primitive hominid--which later came to be called Homo erectus--had been discovered on the Southeast Asian island of Java, the change of venue had more to do with cultural values and racist reasoning than with hard evidence. Africa was the dark continent, where progress was slowed by heat, disease, and biotic excess. In such a place, it was thought, the mind would vegetate--witness the regressive races that inhabited the place in modern times. The plains of central Asia, on the other hand, seemed just the sort of daunting habitat that would call out the best in an enterprising ape. In that environment, wrote the American paleontologist Henry Fairfield Osborn, the struggle for existence was severe and evoked all the inventive and resourceful faculties of man. . . ; while the anthropoid apes were luxuriating in the forested lowlands of Asia and Europe, the Dawn Men were evolving in the invigorating atmosphere of the relatively dry uplands.

In hindsight, the contrasts made between dark and light, forest and plain, slovenly ape and resourceful man seem crudely moralistic. Higher evolution, one would think, was something reserved for the primate who had the guts and wits to go out there and grab it, as if the entrepreneurial spirit of the early twentieth century could be located in our species’ very origins. But at the time the idea was highly influential. Among those impressed was a young anatomist in South Africa named Raymond Dart. In 1925 Dart announced the discovery, near the town of Taung, of what he believed to be the skull of a juvenile man-ape, which he called Australopithecus africanus. While Darwin had been correct in supposing Africa to be the home continent of our ancestors, it seemed that Osborn had been right about the creature’s habitat: there are no forests around Taung, and scientists assumed there hadn’t been any for millions of years. Forests might provide apes with an easy and sluggish solution to the problems of existence, wrote Dart, but for the production of man a different apprenticeship was needed to sharpen the wits and quicken the higher manifestations of intellect--a more open veld country where competition was keener between swiftness and stealth, and where adroitness of thinking and movement played a preponderating role in the preservation of the species.

In the decades that followed Dart’s discovery, more early hominids emerged from eastern and southern Africa, and most researchers concluded that they made their homes in the savanna as well. The question was less whether the savanna played a part in the origin of bipedalism-- that was obvious--than how. Dart originally proposed that Australopithecus had taken to two legs to avoid predators (for sudden and swift bipedal movement, to elude capture), but later he reversed this scenario and imagined his killer ape the eater rather than the eaten, forsaking the trees for the more attractive fleshy foods that lay in the vast savannas of the southern plains.

Later studies suggested that the environments in eastern and southern Africa where early hominids lived were not the vast, unchanging plains Dart imagined. Instead they appeared to be variable, often characterized by seasonally semiarid terrain, a plain studded with scraggly trees and patches of denser woodland. But no matter: This savanna mosaic was still drier and more open than the thick forest that harbors the African apes today. It just made good sense, moreover, that our ancestors would have come down to the ground and assumed their bipedal stance in a habitat where there were not as many trees to climb around in. The satisfying darkness-into-light theme of early hominid development held up, albeit with a little less wattage.

For decades the popularity of the savanna hypothesis rested on the twin supports of its moral resonance and general plausibility: our origin should have happened this way, and it would make awfully good sense if it did. In East Africa, geography seemed to reinforce the sheer rightness of the hypothesis. Most of the earliest hominid fossils have come from the eastern branch of the great African Rift Valley; researchers believed that when these hominids were alive, the region was much like the dry open grasslands that dominate it today. The lusher, more forested western branch, meanwhile, is home to those lazy chimps and gorillas--but to no hominid fossils.

Still, despite such suggestive correspondence, until 20 years ago something was missing from the hypothesis: some hard data to link environment to human evolution. Then paleontologist Elisabeth Vrba of Yale began offering what was the strongest evidence that the drying up of African environments helped shape early human evolution. In studying the bones of antelope and other bovids from hominid sites in South Africa, Vrba noticed a dramatic change occurring between 2.5 and 2 million years ago. Many species that were adapted to wooded environments, she saw, suddenly disappeared from the fossil record, while those suited to grassy regions appeared and multiplied. This turnover pulse of extinctions and origins coincides with a sudden global cooling, which may have triggered the spread of savannas and the fragmentation of forests.

Other investigators, meanwhile, were documenting an earlier turnover pulse around 5 million years ago. For humans, both dates are full of significance. This earlier pulse corresponds to the date when our lineage is thought to have diverged from that of the apes and become bipedal. Vrba’s second, later pulse marks the appearance of stone tools and the arrival on the scene of new hominid species, some with brains big enough to merit inclusion in the genus Homo. The inference was clear: our early ancestors were savanna born and savanna bred.

All the evidence, as I see it, Vrba wrote in 1993, indicates that the lineage of upright primates known as australopithecines, the first hominids, was one of the founding groups of the great African savanna biota.

With empirical evidence drawn from two different sources, the turnover pulse is a great improvement on the traditional savanna hypothesis (which in retrospect looks not so much like a hypothesis as a really keen idea). Best of all, Vrba’s hypothesis is testable. Let’s say that global climate changes did indeed create open country in East Africa, which in turn triggered a turnover of species and pushed ahead the evolution of hominids. If so, then similar turnovers in animal species should have appeared in the fossil record whenever global change occurred.

Over the last 15 years, Andrew Hill and John Kingston, both also at Yale, have been looking for signs of those dramatic shifts at some 400 sites in the Tugen Hills of Kenya. In the heart of the fossil-rich eastern branch of the Rift Valley, the Tugen Hills offer a look at a succession of geologic layers from 16 million years ago to a mere 200,000 years ago-- studded with fragmentary remains of ancient apes and hominids. To gauge the past climate of the Tugen Hills, the researchers have looked at the signatures of the ancient soils preserved in rock. Different plants incorporate different ratios of isotopes of carbon in their tissues, and when those plants die and decompose, that distinctive ratio remains in the soil. Thus grasslands and forests leave distinguishing isotopic marks. When Hill and Kingston looked at soils formed during Vrba’s turnover pulses, however, they found nothing like the radical shifts to grasslands that she predicted. Instead of signs that the environment was opening up, they found that there was a little bit of grass all the time, with no dramatic changes, and no evidence that early hominids there ever encountered an open grassland.

Elisabeth’s turnover pulse hypothesis is very attractive, says Hill. It would have been lovely if it had also been true.

Other research has also contradicted Vrba’s hypothesis. Laura Bishop of Liverpool University in England, for instance, has been studying pig fossils from several East African sites. Some of those fossil animals, she has found, had limbs that were adapted not for open habitats but for heavy woods. Peter deMenocal of Columbia University’s Lamont-Doherty Earth Observatory has been looking at long-term climate patterns in Africa by measuring the concentration of dust in ocean sediments. Over the past 5 million years, he has found, the African climate has cycled back and forth between dry and wet climates, but the pattern became dramatic only 2.8 million years ago, when Africa became particularly arid. Such a change could have played a role in the dawn of Homo, but it came over a million years too late to have had a hand in australopithecines’ becoming bipedal.

What may finally kill the savanna hypothesis--or save it--are the hominids themselves. More than anything else, walking on two feet is what makes a hominid a hominid. If those first bipedal footsteps were made on savanna, we should find fossils of the first hominids in open habitats. For almost 20 years, the earliest hominid known has been Australopithecus afarensis, exemplified by the 3.2-million-year-old skeleton called Lucy. Lucy had a chimp-size skull but an upright posture, which clinches the argument that hominids evolved bipedalism before big brains. But had Lucy completely let go of the trees? It’s been a matter of much debate: oddities such as her curved digits may be the anatomic underpinnings of a partially arboreal life-style or just baggage left over from her tree-climbing ancestry.

Nor did afarensis make clear a preference for one sort of habitat over another. Most of the fossils come from two sites: Hadar in Ethiopia, where Lucy was found, and Laetoli in Tanzania, where three hominids presumed to be afarensis left their footprints in a layer of newly erupted volcanic ash 3.5 million years ago. Laetoli has been considered one of the driest, barest habitats in the eastern rift and thus has given comfort to the savanna faithful. But at Hadar the afarensis fossils appear to have been laid down among woodlands along ancient rivers. Other ambiguous bones that may have belonged to afarensis and may have dated back as far as 4 million years ago have been found at nearby East African localities. Environments at these sites run the gamut from arid to lush, suggesting that Lucy and her kin may not have been confined to one particular habitat but rather lived in a broad range of them. So why give some special credit to the savanna for launching our lineage?

I’ve always thought that there was scant evidence for the savanna hypothesis, based simply on the fact that hominids are extremely plastic behaviorally, says Bill Kimbel, director of science at the Institute for Human Origins in Berkeley, California.

At least afarensis had enough respect for conventional wisdom to stay on the right side of the Rift Valley. Or it did until last year. In November a team led by Michel Brunet of the University of Poitiers in France announced the discovery of a jawbone similar to that of afarensis and, at 3 to 3.5 million years old, well within the species’ time range. The ecology where Brunet’s hominid lived also has a familiar ring: a vegetational mosaic of gallery forest and wooded savanna with open grassy patches. But in one important respect the fossil is out of left field-- Brunet found it in Chad, in north central Africa, more than 1,500 miles from the eastern Rift Valley, where all the other afarensis specimens have been found. This hominid’s home is even farther west, in fact, than the dark, humid forests of the western rift, home to the great apes--clearly on the wrong side of the tracks.

In a normal year the Chad fossil would have been the biggest hominid news. But 1995 was anything but a normal year. In August, Meave Leakey of the National Museums of Kenya and her colleagues made public the discovery of a new hominid species, called Australopithecus anamensis, even older than afarensis. (The fossils were found at two sites near Lake Turkana in Kenya and derive their name from the Turkana word for lake.) The previous spring, Tim White of the University of California at Berkeley and his associates had named a whole new genus, Ardipithecus ramidus, that was older still, represented by fossils found in Aramis, Ethiopia, over the previous three years. The character of anamensis and ramidus could well have decided the fate of the savanna hypothesis. If they were bipeds living in relatively open territory, they could breathe new life into a hypothesis that’s struggling to survive. But if either showed that our ancestors were upright before leaving the forest, the idea that has dominated paleoanthropology for a century would be reduced to little more than a historical artifact.

The best hope for the savanna hypothesis rests with Leakey’s new species. In its head and neck, anamensis shares a number of features with fossil apes, but Leakey’s team also found a shinbone that is quite humanlike and emphatically bipedal--in spite of the deep antiquity of anamensis: the older of the two sites has been dated to 4.2 million years ago. And the region surrounding the sites was a dry, relatively open bushland.

This is good news for those, such as Peter Wheeler of Liverpool John Moores University in England, whose theories of bipedalism depend on an initial movement out of the closed-canopy forest. Wheeler maintains that standing upright exposes less body surface to the sun, making it possible for protohominids to keep cool enough out of the shade to exploit savanna resources. It’s pretty clear that by three and a half million years ago, australopithecines were living in a range of habitats, he says. But if you look at the oldest evidence for bipedalism--Laetoli, and now the anamensis sites--these are actually more open habitats.

However, Alan Walker of Penn State, one of the codiscoverers of anamensis, disagrees. Though the regional climate may have been as hot and arid back then as it is now, he says, the local habitat of anamensis was probably quite different. Back then the lake was much bigger than it is today and would have supported a massive ring of vegetation. Animal fossils found at the anamensis sites--everything from little forest monkeys to grass-eating antelope--were lodged in deltalike sediments that must have been deposited by monstrous great rivers, says Walker, with gallery forests as much as a mile or two wide on both banks.

Even Laetoli is proving to be a mixed blessing for savanna lovers. In the first studies, researchers focused their attention mainly on the abundance of fossils of arid-adapted antelope at the site. Based on these remains, they concluded that Laetoli was a grassland with scattered trees. But according to Kaye Reed of the Institute of Human Origins, these conclusions are debatable. For one thing, the antelope are gregarious herders, so one should expect to find more of their bones than those of more solitary species. Moreover, the original studies underplayed evidence for a more diverse community, which included woodland-dwelling antelope and an assortment of monkeys. In separate studies, Reed and Peter Andrews of the British Natural History Museum took a more thorough look at Laetoli, and both concluded that the original description was far too bleak. Monkeys have to have trees to eat and sleep in, says Reed. I don’t want to give the impression that this was some kind of deep forest. But certainly Laetoli was more heavily wooded than we thought.

Ardipithecus ramidus poses a potentially more devastating blow to the savanna hypothesis. The 4.4-million-year-old species has a skull and teeth that are even more primitive and chimplike than anamensis. Other traits of its anatomy, however, align it with later hominids such as afarensis. It remains to be seen whether ramidus is an early cousin of our direct ancestors or is indeed at the very base of the hominid lineage (its name perhaps reveals the hope of its discoverers, deriving from the Afar word for root). What we do know is that the species lived in a densely wooded habitat along with forest-dwelling monkey species and the kudu, an antelope that prefers a bushy habitat.

We interpret these initial results as evidence that the hominids lived and died in a wooded setting, says Tim White.

A. ramidus would be the final nail in the coffin of the savanna hypothesis, except for one crucial bit of missing information: none of its discoverers will yet say if it was bipedal. Researchers have unearthed a partial skeleton consisting of over 100 fragments of dozens of bones, including hand bones, foot bones, wrist bones--more than enough to determine whether the creature walked like a human or like a chimp. Unfortunately the fragile bones are encased in sediment that must be laboriously chipped away before a proper analysis can begin. Neither White nor any member of his team will comment on what the bones say about locomotion until a thorough study can be completed--which at this point won’t be until 1998 at the earliest.

There are plenty of other puzzling bones to ponder in the meantime. Back in South Africa, where Dart found the first australopithecine, researchers have begun analyzing a horde of some 500 new and previously collected specimens from a site called Sterkfontein. Some of them may have a potent impact on the savanna hypothesis.

The most widely publicized is Little Foot, a tantalizing string of four connected foot bones running from the ankle to the base of the big toe. Little Foot is between 3 and 3.5 million years old, hundreds of thousands of years younger than the ramidus and anamensis specimens. Yet according to Phillip Tobias and his Witwatersrand colleague Ronald Clarke, the fossil demonstrates that this early species of australopithecine--quite probably A. africanus--still spent time in the trees. While the anklebone, Tobias and Clarke say, is built to take the weight of a bipedal stride, the foot is also surprisingly primitive. This is especially true of the big toe, which they contend splayed out to the side like a chimpanzee’s, all the better to grasp tree branches when climbing.

Although not everyone agrees that Little Foot’s foot is so apelike, a host of other fossils from Sterkfontein also speak of the trees. Lee Berger, also at Witwatersrand, has analyzed several shoulder girdles from the collection and found them even better suited to climbing and suspending behavior than those of afarensis. He and Tobias have analyzed a shinbone from the same site and concluded it was more chimplike than human. And in what is perhaps the most compelling finding so far, Berger and Henry McHenry of the University of California at Davis have analyzed the proportions of arms and legs of the Sterkfontein africanus specimens and found that they were closer to chimps than to humans.

The tree-climbing anatomy of africanus has prompted Tobias, long a savanna loyalist, to wonder why such an animal would be out on the South African veld, where today there aren’t any trees big enough to climb around in. A possible answer came from recent reevaluations of the environments at several africanus sites. As in East Africa, the ancient fauna and pollen suggest that conditions there were warmer, wetter, and more wooded then previously thought. Tobias’s colleague Marian Bamford has even recovered traces at Sterkfontein of liana vines, which grow primarily in dense forests. The climate at the site did become drier and more open 2.5 million years ago, but by then hominids had been walking on two legs for at least a million and a half years.

If they live up to their discoverers’ initial claims, the Sterkfontein fossils make hominid history more complicated than we thought. Lucy and her fellow afarensis were traipsing through Ethiopia at about the same time Little Foot and the other africanus hominids were in South Africa--and the two hominids were using different ways of moving around. While Lucy was more committed to life on the ground, Little Foot went for a mixed strategy, sometimes scuttling up trees. What this suggests, says Berger, is that bipedalism may have evolved not once but twice.

And in both cases, the South African researchers argue, bipedalism was not associated with the savanna. The idea that bipedalism evolved as an adaptation to the savanna, declares Tobias, can be thrown out the window.

If so, it shall be missed. For all its shortcomings--a shortage of evidence being the first among them--the savanna hypothesis provided a tidy, plausible explanation for a profound mystery: What set human beings apart from the rest of creation? If not the savanna, what did cause the first hominids to become bipedal? Why develop anatomy good only for walking on the ground when you are still living among the trees? For all the effort it has taken to bring down the savanna hypothesis, it will take much more to build up something else in its place.

I don’t really know why we became bipedal, says Andrew Hill. It’s such an unusual thing.

We’re back to square one, says Tobias.

Square one is not completely empty. Kevin Hunt of Indiana University, for instance, has recently revived the idea that bipedalism was initially an adaptation for woodland feeding rather than a new way of getting around. Chimps often stand while feeding in small trees and bushes, stabilizing themselves by hanging onto an overhanging branch. Hunt suggests that the earliest australopithecines made an anatomic commitment to this specialized way of obtaining food.

Nina Jablonski of the California Academy of Science in San Francisco sees the beginnings of our bipedalism mirrored instead in the upright threat displays of great apes. Perhaps our ancestors resorted to this behavior more than their ape cousins to maintain the social hierarchy. Originally Jablonski and her colleague George Chaplin of the University of Western Australia linked the increase in bipedal threat displays to a move to the savanna, where there would be more competition for resources. But in light of the new evidence for wooded habitats, she now concedes that this essentially primary cause of bipedalism could have emerged in a forest.

There is a stubborn paradox in such models, based as they are on living primates: since chimps and gorillas have presumably been performing these behaviors for millennia without the evolution of bipedalism, how could the same behaviors have driven just such an evolution in hominids? In the early 1980s, Owen Lovejoy of Kent State in Ohio proposed an elegant explanation for bipedalism that bypassed this logical difficulty and had nothing to do with moving about on the savanna.

Bipedality is a lousy form of locomotion, says Lovejoy. It’s slower and more awkward, and it puts the animal at greater risk of injury. The advantage must come from some other motivating selective force. To Lovejoy, the force is reproduction itself. In his view, what separated the protohominids from their ape contemporaries was a wholly new reproductive strategy, in which males provided food to females and their mutual offspring. With the males’ assistance, the females could forage less and give birth more frequently than their ape counterparts because they could care for more than one child at a time. In return a male gained continual sexual access to a particular female, ensuring that the children he provisioned were most likely his own.

This monogamous arrangement would have provided an enormous evolutionary advantage, says Lovejoy, since it would directly affect the number of offspring an individual female could bear and raise to maturity. But the males would need the anatomic apparatus to carry food back to be shared in the first place. Bipedalism may have been a poor way of getting around, but by freeing the hands for carrying, it would have been an excellent way to bear more offspring. And in evolution, of course, more offspring is the name of the game.

If the savanna hypothesis is barely kicking, it’s worth remembering that it isn’t dead. While scientists no longer believe in the classic portrait of protohominids loping about on a treeless grassy plain, there’s treeless, and then there’s treeless. Despite the revisionism of recent years, the fact remains that Africa as a whole has gradually cooled and dried over the past 5 million years. Even if the trees did not disappear completely, early hominids may still have faced sparser forests than their ancestors. Bipedalism may still have been an important part of their adjustment to this new setting.

We have to be careful about what we call savanna, says Peter Wheeler. Most savanna is a range of habitats, including bushland and quite dense trees. My arguments for bipedalism being a thermoregulatory adaptation would still apply, unless there was continuous shade cover, as in a closed-canopy forest. Nobody is saying that about anamensis.

Unfortunately they are saying that about anamensis’s older cousin, Ardipithecus ramidus, in its deep forest home. Which brings us full circle to the business of storytelling. Every reader knows that a good story depends on having strongly drawn characters, ones the author understands well. Rather suddenly, two new protagonists have been added to the opening chapters of human evolution. About anamensis we still know very little. But about ramidus we know next to nothing. Once upon a time, nearly four and a half million years ago, there was a hominid that lived in the middle of a dark forest. Did it walk on two legs or four? If it walked on two, why?

The locomotor habit of ramidus is crucial, says Lovejoy, who is among those charged with the enviable task of analyzing its bones. If it is bipedal, then the savanna hypothesis in all its mundane glory would be dead. But if it is quadrupedal, then the old idea, even though I think it is inherently illogical, would not be disproved.
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