Evolutionary findNewly discovered pieces of an ancient primate skull, including a face and frontal braincase shown here from the front and side, suggest to scientists that Old World monkeys and apes diverged between 29 million and 24 million years ago.I. Zalmout, W. Sanders

A slope-faced, big-toothed creature from the distant past has inspired scientists to recalibrate the ancient evolutionary split between apes and Old World monkeys.

Discoverers of a partial apelike skull in western Saudi Arabia say that it now appears that a poorly understood parting of major primate groups occurred between 29 million and 24 million years ago. A 2004 analysis of DNA from living apes and monkeys in Africa and Asia had estimated an earlier divergence, between 34.5 and 29.2 million years ago.

An intriguing mosaic of features on the newly unearthed fossil, which dates to between 29 million and 28 million years ago, suggests that it lived shortly before a common ancestor that gave rise to hominoids — a primate lineage that includes apes and humans — and the monkeys of Africa, Asia and Europe. A team led by anthropology graduate student Iyad Zalmout of the University of Michigan in Ann Arbor reports the find in the July 15 Nature.

Zalmout and his colleagues assign the skull to a new primate genus and species, Saadanius hijazensis.

“This is wonderful discovery, a real missing link that fills in a gap in our understanding of the timing and pattern of anatomical change involved in the evolution of Old World monkeys and apes,” remarks anthropologist John Fleagle  of Stony Brook University in New York.

Zalmout, working with members of the Saudi Geological Survey in Jeddah, found the partial Saadanius skull on February 17, 2009, in a section of Saudi Arabia’s Shumaysi Formation framed by previously dated volcanic ash layers. Based on the specimen’s size and shape, the researchers estimate that Saadanius weighed 15 to 20 kilograms (33 to 44 pounds), making it a medium-sized primate for its time.

Saadanius sports a projecting snout, a relatively tall face with long, narrow nasal bones, broad cheek teeth and other traits resembling those of older primates previously unearthed at a geological formation on the edge of Egypt’s Sahara Desert. Researchers estimate that those creatures lived between 35 million and 30 million years ago.

But a few critical anatomical features, including a long, tube-shaped ear canal, distinguish Saadanius from its primate predecessors, the scientists say. And unlike Old World monkeys and hominoids that evolved after about 24 million years ago, Saadanius — which Zalmout’s group identifies as a male based on dental characteristics — lacked nasal sinuses and large canine teeth typical of later ape and monkey males.

For that reason, the researchers use 24 million years as the most recent estimate of when Old World monkeys diverged from apes.

Anthropologist Brenda Benefit of New Mexico State University in Las Cruces proposed in 1993 that a common ancestor of Old World monkeys and apes, or its close relative, would look much like the new fossil find. “Saadanius strikingly matches her prediction,” says Michigan’s William Sanders, a coauthor of the new study.

Saadanius apparently lived not long before Old World monkeys and apes diverged from a common ancestor, Fleagle says. Unfortunately, a 5- to 10-million-year gap separates Saadanius from the earliest known Old World monkeys and hominoids, so key pieces of the evolutionary transition are still missing.

That makes it difficult to narrow down the timing of an ape–Old World monkey divergence, says anthropologist Elwyn Simons of Duke University in Durham, N.C. But the new fossil “fits in exactly as it should” in primate evolution, he holds.

Although additional fossil finds are needed, the Saudi specimen offers a more reliable age estimate for a crucial shift in primate evolution than can be gleaned from DNA studies of living apes and monkeys, comments anthropologist David Begun of the University of Toronto.