By Marlowe Hood
Paris - The last ancestor shared by monkeys and humans probably lived between 28 and 24 million years ago, several million later than previously thought, fossils unveiled on Wednesday have revealed.
A partial skull of the unknown species, found in western Saudi Arabia, rewrites the timeline of primate evolution and fills in a yawning gap in the fossil record, the researchers said.
Up to now, genome-based analysis put the split between hominoids - which includes apes and humans - and cercopithecoids, or so-called Old World monkeys, at 35 to 30 million years ago.
But the new species, dubbed Saadanius hijazensis, has been accurately dated to about 28 million years ago, and may have persisted even longer before the split occurred.
Its distinctive features show that the last common ancestor of monkeys, apes and humans - called catarrhines - existed further up the evolutionary tree than the genetic approach suggested.
The discovery also makes it possible for the first time to identify the mysterious fossil of another primate that lived some four million years later as clearly belonging to a post-split ape.
"The shift in age does not change how we think of human origins," said lead scientist William Sanders, a professor at the University of Michigan.
"But it does help us to narrow down the time period in which the group that ultimately produced humans and their direct ancestors arose. We can now search in this 28-to-24 million year time frame," he said in an email exchange.
One telltale trait in particular - a bony ear tube - told paleontologists that their landmark discovery lived just before the genetic parting of the ways between monkey and man.
"Saadanius shares most of its features with archaic catarrhines, and displays none of the advanced features characteristic of apes or Old World monkeys," he explained.
Apes, for example, have frontal sinuses, and large canines in males, while Old World monkeys have special molars in the back of their jaws for puncturing and grinding seeds.
But the bony ear tube is characteristic of both groups, and absent in the earlier catarrhines, meaning that Saandanius had to have come in between, Sanders said.
The findings, published in Nature, should also go a long way toward settling a stubborn debate about the facial profile of the ancestor to both apes and Old World monkeys.
One theory, based on the study of modern-day animals, suggested a short face with a smooth, rounded forehead.
But Saadanius lends credence to a competing theory, based on the fossil record, which postulates a long, projecting face and a narrow, triangular-shaped forehead.
The partial skull of the new species was discovered last year at the Shumaysi Formation at Harrat Al Ujayfa in Al Hijaz Province by University of Michigan researcher Iyad Zalmout along with a team from the Saudi Geological Survey.
The specimen probably weighed 15kg to 20kg. - Reuters