UNIVERSITY OF CHICAGO PRESS, OCTOBER 2013But bipedality has evolved considerably since the first appearance of bipeds: it did not appear all at once. The awkward gait of the very primitive fossil hominin Ardipithecus ramidus (at 4.4 million years old, the earliest for which good skeletal evidence is known) shows that the first bipeds were not as refined as modern humans. They could stand upright, they could walk, though not as upright as modern humans, but they probably could not run very well. However, footprints attributed to the fossil human Australopithecus afarensis (Lucy) from a million years later show that by this time, creatures close to the human lineage could walk just about as well as modern humans. Even so, the skeleton of this creature was still very different from modern humans: Lucy could walk, but her skeleton suggests that she might have been a better tree climber than modern humans are.
The act of standing upright was followed, in sequence, by walking and then running—two gaits that demand very special, and rather different, adaptations. Daniel Lieberman and Dennis Bramble have recently proposed that many features of modern humans appear to be adaptations not to walking, as such, but to long-distance running. These include a range of features throughout the body not directly connected with the legs and feet.
Here are just two examples. Homo erectus and modern humans have barrel-shaped rib cages, in contrast to the cone-shaped, wide-bellied rib cages of earlier hominins. This means that later hominins had “waists,” which would have allowed the counterrotation of the arms relative to the legs while running. This is an extremely important aid to balance. Such counterrotation, however, would move the head from side to side with each stride, if it weren’t for a corresponding reduction in the neck musculature to allow the head to be suspended independently. In human beings there is a ligament—the nuchal ligament—that connects the back of the skull with the back and shoulders. This allows the posture of the head to be maintained without effort. This ligament is not found in apes. It is found, however, in predators such as dogs, which track and hunt over long distances without tiring—just as traditional hunters do.
The current consensus is that bipedality was the first distinctive feature to have evolved in the human lineage, long before the expansion of the brain. Before many fossils had been discovered, of course, the view was that the large brain of humans evolved before the upright, bipedal stance: this conceit explains why the Piltdown forgery was so effective.
Bipedality might be a distinctively human feature—but is it “special”? Not really—apes have a variety of peculiar modes of locomotion, from quadrupedal knuckle walking (gorillas and chimps) to movement with all four limbs as hands (orangutan) or the forelimbs alone (gibbons). The evidence from Ardipithecus ramidus suggests that the distinctive modes of locomotion in each modern ape species are products of their own very special evolutionary circumstances, and not some relics of ancient times. Some extinct apes, not directly related to hominins, were even bipedal.
As a final note in this chapter, I refer you to the strange case of Oreopithecus. This ape lived in the Late Miocene (7– 9 million years ago) and was endemic to a Mediterranean island whose fabric now forms parts of the Italian region of Tuscany. Oreopithecus was, in its own way, a biped, so much so that its hands were sufficiently free to allow for a precision grip, in which the tips of the fingers and thumb can be pressed together, allowing fine manipulation—something often assumed to be exclusive to toolmaking humans. But Oreopithecus was a very distant cousin of hominins, not an ancestor. Its bipedality was not a harbinger of technology, holding babies close to its chest, or anything else. The free hands of Oreopithecus were not, as far as we know, employed in making tools, thereby refining “planning depth,” swiftly followed by the conquest of the earth. Whether bipedality in the species was connected with sexual display will probably remain forever unknown. We do know that bipedality did not save it. As far as we know, Oreopithecus remained confined to its island home, where it quietly became extinct. For Oreopithecus, bipedality was a trait as individual as any other variety of ape locomotion, not the first step in some progressive path of transformation between Ape and Angel.
And the same is true for us.
Excerpted from The Accidental Species: Misunderstandings of Human Evolution by Henry Gee. Copyright © 2013 by Henry Gee. Published by The University of Chicago Press.