July 15, 2010

Primates, Humans Split Later Than Thought

Fossils unveiled on Wednesday reveal that the last ancestor shared by monkeys and humans lived most likely between 24 and 28 million years ago, which is several million years later than previously assumed.

A partial skull of an unknown species was found in western Saudi Arabia that rewrites the timeline of primate evolution and fills in a vast gap in the fossil record, researchers said.

Previous genome-based analysis put the split between hominoids (apes and humans) and cercopithecoids (Old World monkeys) at 35 to 30 million years ago.

But now, the new species, dubbed Saadanius hijazensis, has been precisely dated to nearly 28 million years ago, and may have endured even longer before the split occurred.

The unique features of the fragment show that the last common ancestor of monkeys, apes and humans existed further up the evolutionary tree than the genetic approach had originally suggested.

The new discovery makes it possible for scientists, for the first time, to identify the mysterious fossil of another primate that lived four million years later as clearly belonging to a post-split ape.

Professor William Sanders of the University of Michigan said the "shift in age does not change how we think of human origins."

However, "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.

One trait in particular -- a bony ear tube -- confirmed that the discovery lived just before the genetic parting of monkey and man. "Saadanius shares most of its features with archaic catarrhines [primates], and displays none of the advanced features characteristic of apes or Old World monkeys," Sanders 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 found in both groups, and absent in the earlier catarrhines, which means Saadanius had to have come in between, said Sanders.

The findings should also add to a stubborn debate over the facial profile of the ancestor to both apes and Old World monkeys. One theory, based on the study of modern-day animals, suggests a short face with a smooth, rounded forehead.

But Saadanius adds weight to a competing theory, 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. University of Michigan researcher Iyad Zalmout and a team from the Saudi Geological Survey are credited with its discovery.

The findings are published in the journal Nature.


Image 1: Saadanius hijazensis  is a new genus and species of primate that lived in the Arabian Peninsula 29-28 million years ago. The fossil, found in 2009, preserves most of the face, the front upper portion of the skull, the temporal bone, and the palate, with some of the left and right upper teeth. The specimen was found with the palate and teeth facing upward, imbedded in an iron-rich clastic conglomerate in the middle part of the Shumaysi Formation. Credit: Iyad S. Zalmout, University of Michigan Museum of Paleontology

Image 2: Still image taken from videoscan of the face of Saadanius hijazensis, a new genus and species of primate that lived in the Arabian Peninsula during the late Oligocene epoch, 29-28 million years before present. Credit: Julia Fahlke, University of Michigan Museum of Paleontology


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