The earliest known common ancestor of apes, monkeys and humans was a squirrel-sized animal that lived 36-million years ago, according to an analysis of skull, jaw and teeth fossils found in an Egyptian desert.
Elwyn Simons of Duke University said the fossils bear the distinctive teeth and skull shape of a creature that clearly was "at the bottom of the human family tree, near the roots."
"This is the earliest animal known from the higher primates, the group that humans, apes and monkeys are in," Simons said Thursday.
The ancient and extinct animal, known as Catopithecus, was first reported by Simons and his colleagues three years ago. But the teeth and skull fossils that unmistakably mark it as a higher primate were found more recently.
A report on the discovery is to be published today in Science, journal of the American Association for the Advancement of Science.
Simons' claim of finding the oldest higher primate fossil is plausible but unprovable, said Bert Covert of the University of Colorado, Boulder.
"This is fantastic material, the best from early higher primates," said Covert. "The question is, is it really the oldest" from the evolutionary branch that includes humans.
Covert said that specimens found in China and Algeria are older, but are "extremely fragmentary" and their species classification is uncertain.
"Simons' material is between plausible and compelling" as the oldest higher primate fossil, said Covert. "The other material I would place between unlikely and plausible. We cannot resolve the issue right now."
Simons said he has no doubt about his find because of the distinctive features of the Catopithecus fossils. He said they have the higher primate characteristics that include shovel-shaped front upper teeth, a skull with a flattened face, forward-looking eye sockets and a fused forehead bone. In lower primates, such as lemurs, this forehead bone is separated.
"We don't have the skull from the specimens from Algeria," he said. "What if the skull doesn't have the higher primate eye sockets or the fused forehead? That's why I say Catopithecus is the oldest with established credentials."
Simons said the fossils were found in a desert, but Catopithecus lived in a monkey paradise.
Millions of years ago, he said, Egypt's Fayum desert was covered with tall trees growing on the banks of a large lake not far from the Mediterranean Sea. There were flowers and fruit and many types of lush vegetation. There were also many animals: crocodiles, fish, water birds, rodents and a creature called hyrax that was the size of a small cow, he said.
Catopithecus lived in the treetops, dining on fruit and occasional insects. It was about the size of a North American squirrel and probably had a long tail and "a monkey-like face, like a marmoset," said Simons.
If his interpretation is correct, higher primates, such as gorilla, chimpanzee and humans, evolved from the tiny monkey-like animal that lived among the trees of ancient Egypt, Simons said.