NAIROBI , Kenya ( Reuters ) -- The skull of a female Homo erectus -- the first-ever discovery -- suggests the upright ancestors of humans may have been physiologically closer to modern gorillas and chimpanzees . Frederick Kyalo Manthi holds the Homo erectus complete skull he discovered in 2000 near lake Turkana in Kenya . Homo erectus , long viewed as a crucial evolutionary link between modern humans and their tree-dwelling ancestors , may have been more ape-like than previously thought , scientists said Thursday . Unveiling newfound fossils , a team of Kenyan scientists said they were surprised to find that early female hominids were much smaller than males . Gorillas and chimpanzees also exhibit big differences in size between males and females . The fossils , an ancient skull and a jawbone from two early branches of the human family tree -- Homo erectus and Homo habilis , were revealed at Kenya 's National Museum . Both fossils were found in 2000 east of Lake Turkana . But the Homo erectus skull , dating back 1.55 million years , was slightly older than the Homo habilis jawbone , which was found to be 1.44 million years old , the scientists said . `` Prior to the discovery of the new specimens , scientists did not know that Homo erectus males were far larger than the females , '' said Dr Emma Mbua , one of the team . `` This sexual dimorphism [ two forms of the same species ] is considered a primitive character because it occurs in other apes , '' she said , standing in front of the bones at the museum . She said this could also mean the sexual behavior of Homo erectus was more like that of apes , where individuals , especially males , mate with several partners , sometimes in a few hours , than that of its more monogamous human successors . The fossils , discovered in east Africa 's Rift Valley , regarded as the `` cradle of humankind '' , challenge the idea that human prototypes evolved one after the other in a linear fashion from Homo habilis to Homo erectus , ending with modern humans . This means they must have co-existed in different habitats at the same time , the scientists added . `` They were kind of sisters , if you like , '' said Frederick Manthi , the scientist who discovered the fossils . `` Homo habilis never gave rise to Homo erectus . These discoveries have completely changed the story . '' The research , first published in the journal Nature , was conducted by nine scientists including well-known paleontologist Maeve Leakey and her daughter Louise Leakey . The scientists think both Homo erectus and Homo habilis must have evolved from a common ancestor 2-3 million years ago . The most famous such ancestor is Ethiopia 's `` Lucy , '' a fossil more than 3 million years old that set off this week for a tour of museums in the United States . The basic evolutionary story -- that all humans came `` out of Africa '' after evolving from apes in the Rift Valley around 5 million years ago -- remains unchanged and may even be strengthened , the scientists said . `` The more fossils we find in Kenya , the more we justify the story that east Africa is the cradle of mankind , '' Manthi said . `` These hominids tell us there was a large diversity within this species , which strengthens that convention . '' The researchers said Homo habilis was largely a herbivore , likely to have foraged for fruits in greener , more heavily forested areas than Homo erectus , who is thought to have been a hunter who thrived in east Africa 's open savannah . Manthi said the team would have to find more fossils to confirm the findings . `` The story of human evolution has not yet been [ told ] , '' said Kenya Museum director Farah Idle . `` There are many missing links . The more discoveries you make , the more questions you raise . '' E-mail to a friend . Copyright 2007 Reuters . All rights reserved.This material may not be published , broadcast , rewritten , or redistributed .
Female : Skull of female , erectus is more ape - like . Newly discovered bones show female hominids were much smaller than males . Bones from both erectus , and habilis were found in east Africa 's Rift Valley .