CHICAGO (Reuters) - An ancient skull and upper jawbone from two early branches of the human family tree — Homo erectus and Homo habilis — suggest the early human ancestors may have lived close together for half a million years, researchers said on Wednesday.
The fossils, discovered in eastern Africa, challenge the understanding that humans evolved one after another like a line of dominoes, from ancient Homo habilis to Homo erectus and eventually to Homo sapiens, or modern people.
“There has been a view that has suggested habilis very slowly evolved into erectus,” said Susan Anton, a professor of anthropology at New York University. “Now we have the two cohabitating, so that can no longer be the case.”
The research, published in the journal Nature, was conducted by nine scientists including Anton, paleontologist Meave Leakey and her daughter Louise Leakey, both explorers in residence at the National Geographic Society, and Fred Spoor of University College London.
Both fossils were found in 2000 east of Lake Turkana in Kenya as part of the Koobi Fora Research Project, which is affiliated with the National Museums of Kenya.
Their proximity suggests the two species used different food sources and behaviors to live so closely without becoming extinct.
“It’s within two or three minutes walking distance,” said Patrick Gathogo, a doctoral candidate from the University of Utah who helped study the geological layers.
“They must have interacted with each other,” he said in a telephone interview.
The upper jaw bone of Homo habilis dates from 1.44 million years ago, which is earlier than other known fossils of that species.
“The new fossil jaw suggests that Homo habilis was a sister species of Homo erectus, living at broadly the same time, rather than the mother species giving rise to it,” Spoor said in a statement.
The second fossil, found in the same region of northern Kenya, is a well-preserved skull of Homo erectus, which dates from about 1.55 million years ago.
This fossil is striking because of its size. It is the smallest Homo erectus skull found so far and it paints a different picture of the species, suggesting more diversity than researchers had believed.
This diversity could mean that like gorillas, in which males have much larger skulls than females, Homo erectus might have exhibited sexual dimorphism, a primitive trait, the researchers said.
Reduced size differences between the sexes is typically considered a trait acquired during human evolution.
“It makes Homo erectus a bit less like us,” Anton said.
Spoor said all available evidence still suggests that Homo sapiens evolved from Homo erectus, a process that happened in Africa more than 1 million years ago.
He said it is likely the two species will have had a common ancestor living in Africa between 2 million and 3 million years ago.