In the latest of a stunning series of fossil finds in East Africa, scientists reported Wednesday the discovery of a new human ancestor that lived about 4 million years ago.
Any new species of hominid, or prehuman, joins the rarest, most precious jewels in science. And each one prompts furious debate about what it is, how it should be classified, and what place it occupies in the human family tree.
The new study, published in the current issue of the journal Nature, describes a primitive creature that walked erect, instead of on its knuckles like an ape. It has been named Australopithecus anamensis and predates the famous "Lucy" family.
It is the earliest direct evidence of upright walking, or bipedalism, among fossil hominids, said Dr. Alan Walker, a Penn State paleoanthropologist and member of the research team.
"These fossils place the emergence of bipedalism further back in time by a half-million years," Walker said. Upright walking is the key trait that many scientists believe separated us from the African apes.
The oldest direct evidence of upright walking had been a set of famous footprints left by Lucy-type hominids in wet volcanic ash about 3.6 million years ago in Laetoli, Tanzania.
"This (hominid) gets close to the hypothesized time of the splitting of the ape and human lineages and fills in more of the gap in our knowledge of human evolution," Walker said.
Such discoveries provide graphic evidence that supports genetic studies indicating humans and apes diverged much later than previously suspected-only between 5 million and 6 million years ago, an eyeblink in geological time.
The new hominid actually has been showing up in bits and pieces since 1965 and consists of several individuals from two sites in Kenya.
Bones and teeth have been dated to between 4.1 and 3.9 million years ago. Parts of a shinbone and upper armbone indicate the owner had walked on two legs.
The specimens were recovered near Lake Turkana by a research team from the National Museums of Kenya, directed by Dr. Meave Leakey. She and her husband, Richard, scion of the famous fossil-hunting Leakey clan, have spent 27 years scouring the Lake Turkana basin for the secrets of human origins.
Their latest find follows last year's celebrated discovery of an even earlier human forebear, Ardipithecus ramidus, by longtime rival Dr. Tim White, of the University of California at Berkeley.
At 4.4 million years old, A. ramidus, which means "root" in the region around the Aramis River in Ethiopia where the fossils turned up, is the earliest and most primitive human ancestor discovered so far.
Ramidus was very apelike with an upright posture, a tiny brain and chimpanzee-like teeth. Both it and the Leakey's A. anamensis had lived in woodlands, not the open savanna where other hominids resided.
The need to stand up and watch for predators may have given bipedalism a boost in such a setting, the scientists speculated.
A. anamensis, which takes its name from the Turkana word for "lake," serves as a good intermediate between A. ramidus and the later A. afarensis, represented by the famous Ethiopian skeleton, Lucy.
Anamensis looked a lot like a chimp when it perished 4 million years ago. That is revealed by its jaws-parallel mandibular tooth rows set close together, large canines with long roots, wide, flaring molar teeth-and a small elliptical external opening for the ear.
In contrast, the leg and arm bones are quite humanlike, and suggest it walked upright and weighed about 128 pounds, big for a hominid.
However, the path to human ancestry is unclear and the new fossils will add fuel to an already fiery debate.
Until ramidus appeared last year, Lucy grudgingly had been accepted by science as the mother of us all. White and co-discoverer Donald Johanson had spent 20 years establishing Lucy's reputation as the most famous fossil in the world.
But the Leakeys had never been fond of Lucy and her kind. They always had contended that the extinct, brutish creatures with softball-sized brains called australopithecines-southern apes-that kept turning up in Africa since 1924 represented merely a quaint experiment in evolution that led to a dead-end.
The true line leading to humankind was much older and nobler, the Leakeys maintained, and sooner or later they would find it.
Now, however, the scientists are suggesting that the bones and teeth of their new fossil resemble those of A. afarensis from Laetoli, Tanzania, more closely than afarensis specimens from Hadar, Ethiopia, Lucy's hometown.
The implication is that afarensis represents not one, but at least two different species.
Not only that, Walker and Leakey suggest in their Nature paper that White's ramidus fossils actually may belong to species indirectly related to all later hominids and should represent a different branch of the hominid family tree.