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Fossils of a new pre-human species discovered in Kenya show that our ancestors were walking upright 4 million years ago, much earlier than previously thought, anthropologist Meave Leakey said Wednesday.

"They show very clearly that 4 million years ago, our ancestors did not swing around like apes," Leakey told a news conference in Nairobi to announce the discovery."The discoveries add very important new insights into the very earliest stages of human evolution and establish that upright posture and bipedalism go back in time to beyond 4 million years."

The fossils were found in northwestern Kenya's Lake Turkana basin by a National Museums of Kenya research team headed by Leakey and have been named Australopithecus anamensis.

The finds include jaws and teeth, dating between 3.9 and 4.1 million years ago, a piece of skull including the ear region and a lower leg bone.

Until Tim White of the University of Berkeley in California and his colleagues last year announced the discovery of a new species of early hominid from 4.4 million-year-old deposits in Ethiopia, the earliest known human ancestor was Australopithecus afarensis.

The partial skeleton, nicknamed "Lucy," was discovered by Meave Leakey's husband Richard in the 1970s.

Meave Leakey said the newlyfound fossils had features consistent with them being a cross between Lucy and another early human, Ardipithecus ramidus, the highly touted "missing link" discovered last year, and being Lucy's direct ancestors.

Previously, the earliest evidence of pre-humans walking were footprints found in 3.75 million-year-old volcanic ash at Laetoli in Tanzania.

Leakey said primitive features distinguishing the species from other known early human ancestors were in the crania and dentition and included almost parallel mandibular tooth rows set close together, large canines with long strong roots, wide flaring molar teeth and a small elliptical external opening of the ear.

"In contrast, the leg and arm bones have relatively modern morphology and the shape of the former indicates that at this early date, human ancestors were walking bipedally," Leakey said.

The most complete specimens of Australopithecus anamensis were recovered from a site southwest of Lake Turkana. With the exception of one specimen the fossils were recovered between 1988 and 1995.

Leakey said the national museum research work around Lake Turkana was a long-term project in an area investigated for 27 years since her husband led an initial expedition in 1968.

The fossils were found on the surface, and Leakey said she planned to start excavating in the next expedition period from May to August next year for any possible additional evidence.

Leakey's husband, conservationist Richard Leakey, has written a book, "The Origin of Humankind," putting forward the theory that between 3 million and 7 million years ago at least six pre-human species co-existed in East Africa.