Genetic evidence suggests that chimps, gorillas and humans are all descended from a single unknown species that lived 5 million to 7 million years ago. Until recently, the oldest fossils from the prehuman line went back only 3.6 million years. Our oldest known ancestor was Australopithecus afarensis, a small creature that came to light in 1974 with the discovery of the “Lucy” skeleton in Hadar, Ethiopia. Lucy had many chimp-like features–small brain, small ears, large canines–but her upright design showed the lineage was well on its way to becoming human. For 20 years, no one found a likely ancestor to Lucy. But last year, Berkeley paleoanthropologist Tim White was working just 50 miles from the Lucy site when he discovered bones from a primate species that lived nearly a million years earlier. It wasn’t clear whether White’s 4.5 million-year-old creature (Ardipithecus ramidus) had walked upright, and its teeth were more apelike than Lucy’s. But it seemed a plausible forerunner.
Last week’s findings suggest White’s primates were not Lucy’s ancestors but members of some different lineage. Working both sides of Lake Turkana, Leakey’s so-called Hominid Gang has recovered 21 bone specimens, 3.9 million to 4.2 million years old, from creatures that were more apelike than Lucy, yet far more Lucy-like than the species that White identified last year. Despite their chimplike upper jaw, large canines and tiny ear openings, these animals had the broad molars and thick tooth enamel that distinguish prehuman primates from apes. And unlike White’s species, this one was unmistakably bipedal. Its fibula (the smaller of the two shinbones) was thicker than a modern human’s, but no longer sturdy enough to support the muscles that enable an ape to use its toes as thumbs. And its tibia (the main shinbone) had grown thicker at both ends to boar the weight, and the shock, of upright walking.
It’s possible, of course, that the newly discovered species was just an intermediate step between White’s older species and Lucy. But as Lucy’s discoverer, Berkeley paleontologist Donald Johanson, observes, it appears that the Leakey team has found Lucy’s true grandparents. That doesn’t change our standing in the world, but it fleshes out the story of where we came from. Now if someone could just tell us where we’re going.
