Writing in tomorrow's issue of the journal Nature, lead study author Gen Suwa of the University of Tokyo and colleagues dub the new creature Chororapithecus abyssinicus.
The scientists found the ape's canine on the last day of their field work in early 2006.
An assistant named Kampiro picked the fossil out of the rocky terrain after the team had done surveys by foot over about 62 miles (100 kilometers).
The team found the molars during a second dig in 2007.
"When I saw the first big molar in the March survey, wow, was I surprised," said Yonas Beyene, a member of the research team.
In terms of size and proportion, it's impossible to tell the difference between the teeth from these "proto-gorillas" and teeth belonging to some modern gorilla subspecies, the researchers write.
The team believes the C. abyssinicus fossils represent a great ape ancestor because the molars suggest that the animal had started to adapt toward eating a highly fibrous diet similar to that of today's gorillas.
Richard Potts, director of the Human Origins Program at the Smithsonian Institution, said it is possible that the teeth belonged to another type of ape ancestor whose eating habits simply resemble a gorilla's.
"They have a really good case that this has a gorillalike diet," Potts said. "The question is: Is it a gorilla?
"Maybe the similarities of the tooth are due to the similarities of a diet of a different line of ape."
Other researchers agree that the data Suwa's team has right now are very limited.
Nonetheless, the experts say, the find does seem poised to overturn old ideas that were based on the lack of a fossil record rather than on solid evidence.
"We just can't use the absence of evidence as an evidence of absence," said Yohannes Haile-Selassie, a paleontologist with the Cleveland Museum of Natural History.
It's possible Suwa and his colleagues will accumulate more fossils that might support the theory of an earlier evolutionary split between apes and humans, Haile-Selassie said.
"It's just a matter of dedicated work and spending a lot of time and energy in looking for those tiny fossils."
The newfound ape species is the latest in a spate of new fossil announcements to come out of East Africa in recent months.
In July researchers found a roughly 3.5-million-year-old jawbone in Ethiopia that helps bridge a gap between two human ancestors.
And earlier this month researchers in Kenya found fossils that suggest the two human ancestors Homo habilis and Homo erectus—previously believed to have lived at different times—actually coexisted for half a million years.
(Read "Kenyan Fossils May Add New Branch to Human Family Tree" [August 8, 2007].)
Researchers say the finds are partly coincidence and partly the result of better fossil-hunting techniques coupled with better access to the region.
"In the old days we had to depend on aerial photographs, and some of the areas were not covered by aerial photographs," study co-author Asfaw said. "We were working totally blind."
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