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 unveiling new-found fossils said on Thursday.
Revealing an ancient skull and a jawbone from two early branches of the human family tree - Homo erectus and Homo habilis - a team of Kenyan scientists said they were surprised to find that early female hominids were much smaller than males.
The skull was the first discovery of a female Homo erectus. It suggests mankind's upright ancestors may have been physiologically closer to modern gorillas and chimpanzees, which also exhibit big differences in size between males and females, than had been supposed.
"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 is considered a primitive character because it occurs in other apes," she said, standing in front of the bones at Kenya's National Museum.
She said this could also mean the sexual behaviour 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.
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. This means they must have co-existed, exploiting different habitats at the same time, they 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 palaeontologist Meave 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, so would 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.