You look familiar
Another piece of humanity’s family tree is fitted into place
Sep 10th 2011 | from the print edition
THE opening scene of Mel Brooks’s film “History of the World: Part One” dispenses with human origins in one line: “And the ape stood, and became man.” Would that it were that easy for palaeontologists to sort out. The transition to humanity is generally agreed to have occurred between Australopithecus, a genus of small-brained, bipedal primates whose most famous member is a fossil nicknamed “Lucy”, and the big-brained species Homo erectus. But pinning down when precisely this took place, and which of the various australopithecine species were involved, has been challenging. Now the most human-like australopithecine found to date is clarifying things—and staking a claim to be the species from which early humans evolved.
Fossils of the new species, Australopithecus sediba, were discovered in 2008 in a cave in South Africa. Initial research, led by Lee Berger of the University of the Witwatersrand, in Johannesburg, concluded that the species came too late in the fossil record to be the ancestor of the Homo lineage. This week, however, a range of new research into sediba, again led by Dr Berger, has been published in Science. These studies conclude that sediba did in fact predate Homo erectus and, moreover, that parts of its anatomy are surprisingly similar to modern man.
The fossils examined in the Science papers are of an adolescent boy and an adult woman. They are well preserved, and encased in sediments that allow uncommonly precise dating. They lived 1.977m years ago, predating the appearance of Homo erectus by 77,000 years. The period is an especially muddled one for palaeontology, being full of fragmentary fossils that are difficult to assign either to Homo or to Australopithecus. The sediba fossils, by contrast, have some of the most complete features in the early human record.
The new studies centre on the most telling bits of anatomy in the story of human evolution: the brain, pelvis, hands and feet. The brain itself does not fossilise, but the inside of the cranium retains an impression of its contours. The researchers mapped these with high-powered X-ray beams to create a three-dimensional model of the surface of sediba’s brain. They found that its size was on a par with other australopithecines, but its shape was more like that of a human brain. Specifically, the frontal lobes, which are the seat in modern humans of higher cognitive functions such as abstract reasoning, looked more humanlike in sediba’s brain than they do in the brains of other australopithecines. That suggests the neurological changes which gave rise to humanity may have predated the brain’s expansion—an event that had, hitherto, been regarded as crucial to the emergence of humans.
The hands, feet and pelvis of sediba indicate that it both climbed trees and walked upright, though with a different gait from that of humans or chimpanzees. The species may have been a toolmaker, as its hand allowed for a human-like grip. Sediba’s pelvis, an upright butterfly shape, is likewise reminiscent of the human one. It therefore sheds light on a longstanding debate: whether it was bipedalism or giving birth to babies with large heads that drove changes in the shape of the human pelvis. Adult sediba (and therefore, presumably, their babies) had small heads, which indicates that walking upright was the advantage brought by the modern pelvis.
All of which makes sediba more similar to modern humans than are other australopithecines—and more similar, even, than Homo habilis, until now seen as one of the earliest humans. The consensus had been that habilis was a transitional form between Australopithecus and Homo erectus. Dr Berger posits that sediba may have evolved directly into Homo erectus, leaving habilis as an evolutionary sideline, and not even part of the genus Homo. Slowly, then, the origin of the strange assemblage of characters that makes a human being human is emerging. As the oracle said, the beginning of wisdom is: know thyself.