Scientists working in Ethiopia have found what may be the oldest known traces of human-like life - teeth and bones from up to 5.8 million years ago - in a discovery that challenges the long-held belief that man's earliest ancestors first emerged on the grassy plains.
The remains are believed to be those of forest-dwelling creatures that walked upright. They are about 1 million years older than any other known fossils definitively identified as those of hominids, the group that includes humans, the researchers said.
The fossils come from a point in time tantalizingly close to the evolutionary split between the lineage leading to humans and the one that produced chimpanzees. Scientists believe that split took place between 5 million and 8 million years ago.
"This evidence appears to be on the human line - one of the earliest human ancestors. Not only is the dating very solid, but what the report tells us about the environments of the time is really critical," said Brian Richmond, an anthropologist at the University of Illinois, Urbana-Champaign, who was not connected to the study. "This is a windfall of information compared to what we've had."
The bones were found in a remote Ethiopian desert that was wet and forested - and rattled by volcanic eruptions - when the creatures lived there, the researchers reported in Thursday's issue of the journal Nature.
That discovery clashes with the widely held theory that the drying up of forests millions of years ago was critical to human evolution. This theory holds that early human ancestors learned to walk upright, and diverged forever from their apelike cousins, because their forests were gone and they had to survive on the treeless plains.
Bernard Wood, a human origins professor at George Washington University, said it is not entirely proven that the creatures were hominids or that their habitat was really a forest. "But that doesn't diminish the importance of what they've found," he said.
The research team, led by Yohannes Haile-Selassie of the University of California at Berkeley, made the discovery 140 miles northeast of Addis Ababa, and about 50 miles south of where the fossil "Lucy" was found about three decades ago. Lucy is some 3.2 million years old and is believed to be a member of the species from which all modern humans are descended.
Haile-Selassie and his colleagues found 11 specimens, including a jawbone with teeth, hand and foot bones, fragmentary arm bones and a piece of collarbone. They represent at least five individuals, Haile-Selassie said.
Dating was done by measuring trapped argon gas in volcanic ash that had been mixed in with the bones. It found the fossils to be between 5.2 million and 5.8 million years old.
Haile-Selassie said the specimens revealed a primitive version of Ardipithecus ramidus, an early hominid species whose oldest known fossils were previously found in 4.4-million-year-old sediment in Ethiopia.
He said with further research, the bones might turn out be a new species altogether.
The researchers said it is impossible to get much of an idea of what the creatures looked like because no skull or intact limb was found. The lower jaw is roughly the same size as a chimpanzee's, but the back teeth are bigger and the front teeth narrower, indicating they ate less fruit and leaves and more fibrous food.
One of the fossils, a 5.2-million-year-old toe joint, suggests the creature walked upright, Haile-Selassie said. Upright walking is considered a hallmark trait of the human lineage.
The discovery is sure to fuel the debate over early human evolution. In recent months, two other research teams have challenged long-held assumptions about what ancient creatures were truly human ancestors.
In February, a French team led by researcher Brigitte Senut described remains of a new species, Orrorin tugeneniss, found in Kenya. The fossils are 6 million years old, or older than Haile-Selassie's. Senut said they represented a hominid species.
But Haile-Selassie questioned whether the French team had enough evidence to classify their fossils as those of a hominid. Other scientists have split over the issue.
In March, a team led by Meave Leakey, a member of the fossil-hunting Leakey family, announced the discovery of a 3.5 million-year-old skull in Kenya. Leakey said it is about the same age as Lucy but appeared to be a completely different species with a more human-like face. Leakey said it may be this species - and not that of Lucy - that was an early direct ancestor of humans.