Newly discovered African fossils could resolve questions over the age and evolution of a species thought to be the most ancient known upright-walking ancestor of humans, according to research to be published in the May 7 issue of the journal Nature.
The research team includes Meave G. Leakey, curator of paleontology at the National Museums of Kenya; Craig S. Feibel, assistant professor of anthropology at Rutgers University; Ian McDougall, professor of earth sciences at the Australian National University; Carol Ward, assistant professor of anthropology at the University of Missouri; and Alan C. Walker, distinguished professor of anthropology and biology at Penn State.
The researchers, who first named the new species Australopithecus anamensis, in a paper published in Nature in 1995 based on 22 fossils discovered in northern Kenya, now have unearthed 38 additional fossils that paint a more complete picture of the species. Some scientists questioned the antiquity of the fossil bones described in the 1995 paper, which were found sandwiched at different depths between layers of rock and ancient soils, because a definite time of origin could not then be measured for the youngest layer. "The fossils found in younger deposits were the most human-looking, so some people said we could have two species instead of just one — an older one and a much younger one," Walker says.
"In 1995 we could not firmly establish the age of the youngest geologic layer associated with these fossils because the best dating technique, the 40 Argon/39 Argon method, needs crystals and this particular layer is mostly powdery ash," Walker explains. "Since then — after much sifting through the ashes — we have managed to get enough good crystals to determine quite firmly that these fossils are between 4.1 and 4.2 million years old," he says. "We also have discovered 38 more fossils at this site that clearly show us how very primitive this species was."
Walker says he is interested in the Australopithecus anamensis species because it is an important branch on the human family tree. According to Walker, the new fossils reveal that the ancient species had primitive jaws shaped more like a chimpanzee's than like a modern human's. It also had another characteristically primitive feature known as sexual dimorphism — large differences between the sexes in overall body size and the shape of certain body parts such as teeth. "These males have very large canine teeth, while we humans have lost our canine dimorphism, to a large extent," Walker explains. "Human males still do have slightly bigger canines, on average, and their enamel is thinner than a female's — a characteristic of male apes, which sharpen their teeth to use as weapons," he adds.
Among the new fossils is a wrist bone, which Walker says has the primitive features of a chimpanzee's. On the other hand, he says, the leg bone is more like a modern human's, with a structure that allowed the species to walk upright on two legs instead of on all fours like a chimpanzee. "It just shows you that we don't evolve all at once in every part of our body — we do it in little bits and pieces like a mosaic, depending on which piece natural selection is acting on at the time," he explains.
Although some scientists believe that the human family tree is bushy at its base, with multiple species evolving at the same time, Walker says all the early fossils discovered so far support a more tree-like picture of evolution for hominids — the ancient ancestors on the human family tree. "We don't yet have any convincing evidence early in the fossil record of multiple hominid species," Walker says. "The simplest explanation for the hominid fossil record from 4.5 to 3 million years ago is that there is only one single evolving species and that it had a primitive degree of body-size sexual dimorphism." He adds, "eventually we will find the last common ancestor of chimps and humans."
This research was supported by the National Geographic Society in Washington, D.C.; the U.S. National Science Foundation; the National Museum of Kenya; the Leakey Foundation; the Australian Institute of Nuclear Science and Engineering; the Australian Nuclear Science and Technology Organization, and Caltex in Kenya.
Questions about the overall project: Meave G. Leakey, (+) 254-2-74-21-61, e-mail: firstname.lastname@example.org (Kenya)
Questions about geology: Craig S. Feibel, (+) 732-932-8853, email@example.com (U.S.A.)
Questions about Argon Dating: Ian McDougall, (+) 61-26-249-4136, e-mail: Ian.McDougall@anu.edu.au (Australia)
Questions about anatomical analysis of the fossils: Carol Ward, (+) 573-882-5407, e-mail: firstname.lastname@example.org (U.S.A.)
Alan C. Walker (on May 4 and then not until after May 17) (+) 814-865-3122 or (+) 814-865-0796, email@example.com (U.S.A.)