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Even if ramidus didn't walk upright, however, another of the recently discovered human ancestors certainly did. Less than a year after A. ramidus made headlines, a team led by Meave Leakey of the National Museums of Kenya (wife of well-known fossil hunter Richard Leakey) and Alan Walker of Pennsylvania State University revealed that it too had found fossils of an ancient human ancestor at two sites near Lake Turkana, in Kenya. Not only is the new hominid very old, dating to 4.2 million years B.P., but it is similar in some ways to A. afarensis--though clearly more primitive. Given the family resemblance, Leakey and Walker assigned the fossils to the same genus, Australopithecus, and gave the new species the name anamensis (anam is the Turkana word for lake).
Several of the bones underscore that A. anamensis did indeed walk upright, some 500,000 years before the next oldest two-legged hominid known. But these creatures didn't walk in the modern sense. As Leakey explains, "They weren't nearly as efficiently upright as we are, and they had relatively short legs. They had a form of locomotion that we don't know today because there isn't anything equivalent."
Precisely where do A. ramidus and A. anamensis fit into the scheme of human evolution? Leakey believes the latter is a direct ancestor of A. afarensis and thus a direct ancestor of modern humans. White and his colleagues have tentatively labeled the older ramidus a "sister species" of all later hominids; it's either our direct ancestor or a close relative of that ancestor. Whichever ramidus turns out to be, it's clear that paleontologists are closing in on the split between apes and humans. "We're in the ballpark. Five or 10 years ago, we couldn't even have conceived of this," asserts White. "Ardipithecus is the closest thing we currently have to the common ancestor of African apes and humans, but its derived characteristics, particularly its teeth, suggest that it postdates that ancestor."
As for the ancestor, White hints that his team has already discovered hominid fossils that are more than 5 million years old, though he refuses to elaborate before detailed studies are completed. But Leakey and Walker readily acknowledge that they are studying two 5.5 million-year-old hominid teeth and a similarly ancient jaw fragment with an embedded tooth from a site in northern Kenya. "They look like australopithecines with lots of primitive features," Walker says, but there isn't enough evidence from these fossils alone to claim a new species.
THE EARLIEST HUMANS
Given their 2 million-year-plus life-span, the australopithecines were surely one of evolution's better experiments. But nature is an inveterate tinkerer, even with successful species. Between 3 million and 1.9 million years B.P., several variations on the Australopithecus theme popped up in eastern and southern Africa, including A. africanus, A. aethiopicus, A. robustus and A. boisei. (Just to complicate matters, the last three are assigned by some experts to an entirely different genus, Paranthropus.)