Ten million-year-old fossils discovered in Ethiopia show that humans and apes probably split six or seven million years earlier than widely thought, according to landmark study released Wednesday.
The handful of teeth from the earliest direct ancestors of modern gorillas ever found -- one canine and eight molars -- also leave virtually no doubt, the study's authors and experts said, that both humans and modern apes did indeed originate from Africa.
The near total absence to date of traces on the continent of apes from this period had led many scientists to conclude that the shared line from which humans and living great apes emerged had taken a long evolutionary detour through Eurasia.
But the study, published in the British journal Nature, "conclusively demonstrates that the Last Common Ancestor (of both man and ape) was strictly an African phenomenon," commented paleoanthropologist Owen Lovejoy of Kent State University in Ohio.
Lovejoy described the fossils as "a critically important discovery," a view echoed by several other scientists who had read the paper or seen the artifacts.
"This is a major breakthrough in our understanding of the origin of humanity," Yohannes Haile-Selassie, a physical anthropologist at the Cleveland Museum of Natural History, told AFP.
The most startling implication of the find, the scientists agree, is that our human progenitors diverged from today's great apes -- including gorillas, orangutans and chimpanzees -- several million years earlier than widely accepted research based on molecular genetics had previously asserted.
The trail in the hunt for physical evidence of our human ancestors goes cold some six or seven million years ago.
Orrorin -- discovered in Kenya in 2000 and nicknamed "Millennium Man" although its sex remains unknown -- goes back 5.8 to 6.1 million years, while Sahelanthropus, found a year later in Chad, is considered by most experts to extend the human family tree another one million years into the past.
Beyond that, however, fossils of early humans from the Miocene period, 23 to five million years ago, disappear. Fossils of early apes especially during the critical period of 14 to eight million years ago were virtually non-existant -- until now.
"We know nothing about how the human line actually emerged from apes," the authors of the paper noted.
But the new fossils, dubbed "Chororapithecus abyssinicus" by the team of Japanese and Ethiopian paleoanthropologists who found them, place the early ancestors of the modern day gorilla 10 to 10.5 million years in the past, suggesting that the human-ape split occurred before that.
There is broad agreement that chimpanzees were the last of the great apes to split from the evolutionary line leading to man, after gorillas and, even earlier, orangutans.
Conventional scientific wisdom, based on genetic "distances" measured by molecular geneticists, had placed the divergence between chimps and humans some five to six million years ago. Orangutans are thought to have parted company with our ancestors 13 to 14 million years ago.
"If the new discovery is in the gorilla lineage, then this will definitely substantially push back the split time between apes and humans," Halie-Selassie at Kent State told AFP.
The scientists leading the team that found the fossils -- Gen Suwa of the University of Tokyo, and Ethiopian paleontologists Berhane Asfaw and Yonas Beyene -- calculated that the human-orangutan split "could easily have been as old as 20 million years."
They determined that the teeth belonged to gorilla ancestors based on unique shared characteristics of the molars, which had evolved for a diet of fibrous foods such as stems and leaves.
The match is not exact, however, and could prompt some scientists to challenge the findings.
The teeth fragments, found in barren scrubland some 170 kilometres (100 miles) east of Ethiopia's capital Addis Ababa, almost went unnoticed.
Asfaw recalled the chance discovery.
"It was our last day of field survey in February 2006, and our sharp-eyed field assistant, Kampiro, found the first ape tooth, a canine," he said.
"He picked it up and showed it to me, and I knew that this was something new -- Ethiopia's first fossil great ape."