by Brian Switek
In 2001, Brigitte Senut and Martin Pickford announced that the hominin Orrorin tugenensis had been discovered in the Tugen Hills of Kenya. The leftovers from a carnivore kill (as evidenced by toothmarks on the bones), there was not much left of Orrorin, but enough of its skeleton was recovered to tell that it was relevant to early human evolution.85 At 5.8 to 6.1 million years old, Orrorin falls into the proposed time range during which the last common ancestor of chimpanzees and humans would have existed, and it appears to have possessed a key human trait.
Among the Orrorin bones was the top portion of a femur, including the surface that articulated with the hip. This bone was very similar to the femurs of australopithecines in having a long femoral neck that would have dictated where the gluteal muscles from the hip attached to a knob of bone called the greater trochanter. This is important, as the gluteal muscles help us keep balanced as we walk, and from the available evidence Orrorin probably stood upright and walked in a manner similar to that of the australopithecines.
Another, even older, potential early human was described the following year. During the latter part of the twentieth century it was thought that Africa’s Rift Valley was the crucible of human evolution, as all the earliest hominins had been found there. But in 2001, early hominin fossils were found far to the west in the Sahel desert of Chad. Discovered by Djimdoumalbaye Ahounta, a local man working with several of his countrymen and French paleoanthropologist Alain Beauvilain, the creature was represented by a crushed, but complete, skull, part of a jaw, and a few teeth. It was dubbed Sahelanthropus tchadensis, and it caused quite a stir in the media and scientific circles. Its crushed skull soon appeared in magazines, on book covers, and on television programs as a definite human ancestor. This fascination with Sahelanthropus is not that surprising. Whereas the other apes were represented by bits and pieces, Sahelanthropus had a face, and it was said to be seven million years old. This would make it the oldest hominin yet known, but the interpretation of Sahelanthropus is not so straightforward.
FIGURE 92 - The virtual reconstruction of the skull of Sahelanthropus to correct for the distortion seen in the original skull (clockwise from upper left: the skull as seen from the front, top, right side, and bottom). Note the placement of the foramen magnum directly beneath the skull in the view of the skull’s underside.
The skull of Sahelanthropus was not found embedded in sediment, but on the wind-blown surface of the desert. This makes its age questionable, as the winds are so powerful they can transport and even destroy fossils. Its identification as a hominin is also tenuous. Even though much was made initially made of its flat face, a reconstruction undertaken by Cristoph Zollikofer and colleagues in 2005 revealed that Sahelanthropus did have a lower face that jutted out. (The announcement of a twelve-million-year-old, flat-faced fossil ape Anoiapithecus from Spain in 2009 showed that a flat face was not a useful trait in distinguishing early humans in any case.) Although the reconstructed skull was not unlike those of other hominins, the skull of Sahelanthropus also closely resembled that of a fourteen-million-year-old ape from Spain named Pierolapithecus. Both had straight lines of molars and premolars in the jaw, like non-human apes, and the faces of the two genera are very similar. Given that Pierolapithecus has been suggested as being close to the common ancestor of gorillas and hominins, it is possible that Sahelanthropus represents a non-human ape that predates the chimpanzee-human split.
But there was yet another early hominin, named but not fully described, that was of high importance to the debates over the earliest humans. During the early 1990s a team led by Tim White discovered the fragments of a new kind of hominin in the 4.4-million-year-old strata of the Afar Depression in Ethiopia. It was briefly described as Ardipithecus ramidus in 1994, but everyone knew that White’s team had found much more. The trouble was that most of the fossils, including a crushed skull, were extremely delicate and required a lot of preparation, and so (somewhat like Dubois’s Pithecanthropus) they were studied in secret for a decade and a half.
The importance of Ardipithecus ramidus was played up even before its full description. With the description of a related species, Ardipithecus kadabba in 2004, the Afar fossils were groomed to fall in a straight line of early human evolution. It would have started 5.6 million years ago with Ardipithecus kadabba, giving way to Ardipithecus ramidus by 4.4 million years ago, leading into another new hominin called Australopithecus anamensis by 4.2 million years ago, which transitioned into Australopithecus afarensis around four million years ago. Some paleoanthropologists took this temporal arrangement and the proximity in which these species were found to suggest that there was a single, linear march of these early hominin species in northeastern Africa, with a major split only occurring after the evolution of Lucy and her kind.
This was the context through which Ardipithecus ramidus, nicknamed “Ardi,” was introduced to the public on October 2, 2009. An entire issue of the journal Science was devoted to the fossils, covering everything from what sort of animals lived alongside Ardi to inferred social behaviors of our 4.4-million-year-old relative.86 But what was most fascinating was that Ardipithecus ramidus did not look like something approaching a chimpanzee. It was a different kind of ape that ran counter to popular perceptions of human evolution.
The origin of bipedalism has been a vexing question for anthropologists for decades. No other mammal walks like we do, and even our closest living relatives (gorillas and chimpanzees) are slouched over to walk on their knuckles. According to different hypotheses, early humans stood up to see predators, to carry tools, to pick fruit, to hold babies while walking, to be better long-distance runners, among other activities—with the focus always being on some evolutionary pressure that made us stand up. Since chimpanzees were taken as a general model for what our ancestors might be like, it was assumed that the progenitors of the first humans would have been knuckle-walking apes, too; there had to be some powerful explanation for the shift.
The skeleton of Ardipithecus ramidus suggests something very different. It had a combination of traits such as long arms, curved fingers, and a divergent big toe that point toward a life in the trees. Furthermore, based upon the recovered pelvis fragments, the hips of Ardipithecus ramidus were probably wider and more bowl-shaped than what is seen in chimpanzees. And while the ends of femur that would make up the knee joint was missing, the upper leg bone did have an inward slant suggestive of the arrangement seen in fully bipedal hominins such as Australopithecus afarensis. Further down, a tiny bone of the foot called the os perineum also provided an interesting contrast with living apes. In monkeys and other primates this tiny bone is embedded in a tendon that allows them to keep the foot rigid, which is important to securely landing on a branch when jumping from one tree to another. Chimpanzees and gorillas lack this “rigid foot” feature, however, and the fact that Ardipithecus ramidus had it might have made an important difference in the way it moved.
Altogether the anatomy of Ardipithecus ramidus is suggestive of an ape that lived in the trees and got around by grasping branches with both its hands and feet. It did not show adaptations in the wrist or hips to being a knuckle walker as seen in living chimpanzees and gorillas, but the adaptations Ardipithecus ramidus had for life in the trees might have allowed it to also walk on two legs when on the ground. Keeping a rigid foot would have helped the hominin keep its balance, just as the shape of its femur and hips would. These small differences might have made it easier to stand and move upright while on the ground than to move around on all fours‚so bipedalism (rather than knuckle-walking) evolved among hominins.87 This means that our upright posture would be an exaptation: an adaptation that was jury-rigged from already existing anatomical traits for a new purpose.
But Ardipithecus ramidus is not the only early hominin to show hints of bipedalism (or traits that could be co-opted to compel upright walking). Orrorin possessed modifications of the femur closely tied to walking upright, and the position of the foramen magnum undernearth the skull of Sa
helanthropus also hints that when the rest of its skeleton is found it may express other traits indicative of bipedalism. While the fossils currently known did not overlap in time they could all represent an early diversification of arboreal hominids that possessed traits that could be easily co-opted for life on the ground, which also makes it possible that there might have been bipedal apes not directly ancestral to us. Take Sahelanthropus, for example. Let’s assume that the rest of its skeleton will show other traits associated with bipedalism but that it turns out to be a hominid, not a hominin. This would mean that, when looking at fossils from the time the first humans were thought to have evolved (still between about seven and five million years ago), we could no longer assume that an ape with bipedal traits was automatically an early human. As with other groups of fossil organisms careful comparisons would be required to tease out the relationships of early fossils based upon shared, derived characteristics, and it is entirely possible that early members of any group did not express the definitive characteristics seen in later members of the group. To put it more simply, there would be no single trait that would draw a stark line between “ape” and “human,” and focusing too heavily on bipedalism might obscure matters more than illuminate them.
The systematic status of Ardipithecus, Orrorin, and Sahelanthropus will no doubt remain controversial for some time, but these new genera illustrate the complex nature of human evolutionary history. Hominins, like the anthropoids and apes, appear to have undergone an early diversification, and those so far discovered hint that there are certainly other as-yet-unknown early hominin genera waiting to be found. Even though the news media, and some anthropologists, seem obsessed with finding our direct ancestors, the reality presented by the fossil record is much more complicated, and it is highly unlikely that we have yet discovered our earliest hominin ancestor. The evolutionary history of hominins is best viewed as a branching bush, not a straight line of ascent.
The general pattern of hominin evolution as presently understood looks something like this. What Ardipithecus, Orrorin, and Sahelanthropus may indicate is that there was a radiation of hominin types around six million years ago. That diversity appears to have given way to a bottleneck by about 4.2 million years ago, at which time Australopithecus anamensis was living in the woodlands of East Africa. One population of these hominins may have given rise to Australopithecus afarensis, which is the only known hominin present from about four to three million years ago. After that, hominin evolution branched out again.
FIGURE 93 - A hypothetical family tree of hominins. There are probably many more species yet to be discovered, and in the coming decades some of these relationships may change as others become better supported. Even so, the overwhelming picture is that humans have had a “bushy” evolutionary history that has left only one living species.
On the one side there was Australopithecus africanus, a gracile australopithecine related to the robust forms like Paranthropus aethiopicus and Paranthropus robustus (akin to the Leakeys’ “Dear Boy”). These robust australopithecines resembled their gracile predecessors in overall form, but they had massive teeth and jaws that might have allowed them to consume a wide variety of food in the open, grasslandlike habitat in which they lived. This would have been especially important during harsh seasonal changes when there may not have been much other than tubers to eat. The first of their kind appeared 2.6 million years ago and persisted until about one million years ago, but ultimately they left no descendants.
The other side of the split, which included our ancestors and close relatives, was also undergoing a diversification around 2.5 million years ago. At that time, the hominin Australopithecus garhi lived in what is now Ethiopia, temporally overlapping with Australopithecus africanus and possibly the earliest members of our genus, Homo. It was not one of our ancestors, but a close relative, who quickly disappeared as hominins like Homo habilis came on the scene. At places like Olduvai Gorge, in fact, Homo habilis lived alongside other hominins like Paranthropus boisei, and there were indeed several kinds of human running around Africa at once. What had caused this proliferation may have been a change in climate. From about 2.8 to 2.4 million years ago there was a climatic pulse that caused the further spread of open grassland habitats dominated by a cooler, drier climate. This change is correlated with the split in the human family tree, and the ecological shift may have created new selective pressures that led to the flowering of hominin species.
This branching pattern is exactly what would be expected if natural selection was a primary mechanism in evolution. Entire species do not follow a “March of Progress” from a “lower” form into a “higher” one; evolution happens due to differences in gene flow and selective pressures on different populations. In this way, one population can remain relatively unchanged while another undergoes rapid evolution, producing a new species that overlaps in time with their ancestral species. This applied to hominins as to any other organism.
In 2007, Fred Spoor and his colleagues reported that the 1.4-million-year-old site of Ileret, Kenya, presented evidence that Homo habilis and Homo erectus had coexisted there for about 500,000 years. Media outlets were thrown into a tizzy over this discovery, and the find was mistakenly said to show that Homo erectus could not have evolved from Homo habilis. How could ancestors live alongside their descendents?
The truth of the matter is that the paper confirmed that evolution is a branching process that occurs in populations that may overlap with each other temporally and even spatially. If the two species really do constitute an ancestor-descendent relationship then a population of Homo habilis gave rise to Homo erectus, perhaps as a result of another climatic pulse that led to the further expansion of grasslands around two million years ago. But other populations of Homo habilis persisted for a time before dying out around 1.6 million years ago. This fits the punctuated equilibrium pattern of evolution predicted by Eldredge and Gould, wherein there is a rapid evolutionary change, resulting in a new species, followed by little to no change at all.
By the time Homo habilis perished Homo erectus was already a world traveler. The Homo erectus bones from Java are as ancient as the oldest Homo erectus from Africa, and the discovery of 1.9-million-year-old Homo erectus skulls from Georgia have shown that this hominin dispersed from Africa early.88 There may have been more than one wave of dispersal, but Homo erectus quickly spread through Africa, the Middle East, Asia, and Indonesia. (Much like the Channel Islands mammoths, one population of Homo erectus in Indonesia appears to have become dwarfed and adapted into a different species, Homo floresiensis .) It was a species adapted for life in open landscapes, and due to their increased energy needs (which included a larger brain) Homo erectus required higher quality foods. It was among the first humans to regularly obtain meat through hunting, and later individuals of the species may have been among the first humans to use fire for cooking.
Eventually, however, the widespread populations of Homo erectus died out, with the last population in Asia persisting until about 200,000 years ago. Before that occurred, though, a population of Homo erectus gave rise to a species called Homo heidelbergensis that appeared in Europe about 600,000 years ago. This is the probable ancestor to our close relative Homo neanderthalensis, the Neanderthal, which evolved in Europe about 130,000 years ago.89 They were robustly built with deep, chinless lower jaws and low-domed skulls that held brains as large, if not larger, for their body size than ours. They made advanced tools, buried their dead, and perhaps even made music; Neanderthals were much more intelligent than the club-wielding imbeciles they are often portrayed as. By 30,000 years ago, however, they had disappeared.
No one is sure what happened to the Neanderthals, but our own species has always been a prime suspect for their extinction. During the time that Neanderthals evolved in Europe, our species was evolving in Africa, sometime between 200,000 and 100,000 years ago. At 70,000 years ago, one population left Africa and, like Homo erectus, our species spread over the globe. There was not a s
eparate origin for different races of Homo sapiens in different parts of the world, but a common African ancestry that links all living people together.
Ancient members of our species in Europe, the Cro Magnons, began their habitation there about 40,000 years ago and may have come into contact with the Neanderthals. Did we decimate their populations in combat? Did we carry diseases that wiped them out? Did we hybridize with them (making us part Neanderthal)? Or was it something else entirely, like climate change, that did in our sister species? No Neanderthal skull has been found with a Cro Magnon spear point in it, so it is difficult to tell how Homo sapiens and Neanderthals may have competed. Neither have supposed hybrid skeletons held up to scrutiny, and while a study published in early 2010 hints that we mayl have interbred with Neanderthals, this hypothesis requires further study and does not explain their extinction. Given that the Neanderthals survived the harsh climatic changes of the Ice Age, it also seems unlikely that they would have died when the glaciers began to recede. The reason for their extinction remains elusive, but when they did disappear it was the first time in millions of years that there was only one kind of human.
In our lonely isolation we have often wondered, “What makes us human?” We now know the answer: “Surprisingly little.” The remains of extinct hominins have exquisitely documented our family history, and our understanding of living apes has further assaulted every fortification meant to defend our uniqueness. From the conformation of our bones to the intricacies of our genetic code, our bodies testify to our ancient history.
I can only wonder how we would perceive ourselves if the Neanderthals or robust australopithecines survived today. Would they help us better understand our origins, or would we destroy them out of revulsion? Perhaps it is best that we will never know. We may want to identify the epitome of “humanness,” to find comfort in some unassailable characteristic that makes us superior (be it in the eyes of God or our own), but there is no bright line dividing us from the apes. Since we are apes ourselves, I do not expect one to be found. For to ask “What makes us human?” assumes that there was some glorious moment, hidden in the past, in which we transcended some boundary and left the ape part of ourselves behind. We forget that those are labels we have created to help organize and understand nature, as if sometime in the Pleistocene there was a glorious moment when language, art, and culture swept into the world and gave us dominion over it.