ew, more definitive age estimates, combined with the discovery of more fossils, have apparently dispelled lingering doubts that skeletal remains found in Africa three years ago are indeed those of the earliest known human ancestors to walk erect, scientists reported last week.
The discovery in northern Kenya of 38 more fossil specimens, including skull and tooth fragments, has enabled paleoanthropologists to draw a more complete picture of the ancestral species, Australopithecus anamensis.
These prehuman individuals had apelike jaws, teeth and wrist bones, and a small brain. The males were considerably larger than the females, a characteristically primitive pattern of sexual dimorphism that is pronounced today among gorillas.
But in other aspects, particularly their leg and arm bones, the individuals had a more humanlike anatomy that would allow them to walk upright on two legs instead of on all fours.
"It just shows you that we don't evolve all at once in every part of our body," said Dr. Alan C. Walker, a paleoanthropologist at Pennsylvania State University, who is a member of the science team reporting the new research. "We do it in little bits and pieces like a mosaic."
When the initial findings about A. anamensis were announced in 1995, geologists could not firmly establish the age of the sediments in which the fossils were embedded. Questions were raised about their antiquity.
Some scientists suspected that the fossils were a mix from two species. The more humanlike specimens, for example, could have come from younger geologic deposits and from a more advanced species than the one represented by the more primitive fossils.
Since then, geologists sifting through volcanic ash at the discovery sites near Lake Turkana extracted enough crystals to conduct a more definitive argon dating analysis. The new tests put the age of the A. anamensis fossils at between 4.07 million and 4.17 million years old, some 500,000 years earlier than previous evidence for the emergence of walking on two legs.
"These tests basically remove any doubt that the origins of bipedalism go back well over four million years," Dr. Craig S. Feibel, a geologist at Rutgers University, said in a telephone interview last week.
A research team led by Dr. Meave G. Leakey of the National Museums of Kenya in Nairobi reported the new findings in the current issue of the journal Nature. The other members of the team, besides Walker and Feibel, were Dr. Carol Ward of the University of Missouri at Columbia and Dr. Ian McDougall of the Australian National University in Canberra.
In the report, the scientists said the fossils "show that this species is demonstrably more primitive than A. afarensis," which emerged as early as 3.6 million years ago and previously was the earliest known upright-walking human ancestor.
The famous Lucy skeleton, found in Ethiopia in the early 1970s, was a member of the afarensis species, which some paleoanthropologists consider the common ancestor of all later species on the human family tree.
The findings should also be critical to research aimed at filling the gap of knowledge about human origins before afarensis and after the lineages of modern apes and humans split from their last common ancestor.
Genetic studies by molecular biologists put the fateful split at five million to seven million years ago.
The only other fossils from that time gap, in addition to anamensis, are those of Ardipithecus ramidus, dated at 4.4 million years ago. This probable missing-link species was discovered in Ethiopia in 1995 by Dr. Tim D. White of the University of California at Berkeley. Scientists are still waiting for White to describe the fossils in more detail before they can determine whether ramidus is the earliest known direct ancestor of anamensis and afarensis and presumably human beings or whether it belongs on a different branch of the evolutionary tree.
The new research by Leakey's group suggested that the anamensis species is the most primitive australopithecine, intermediate in anatomy as well as age between White's ramidus and Lucy's afarensis family.