Ancestor of humans not so brainy

 　 人类祖先并不聪明
 

A monkey-like animal seen as an ancestor of monkeys, apes and humans was not as brainy(1) as expected, according to scientists who analyzed its nicely preserved 29-million-year-old skull.

The finding indicated that primate(2) brain enlargement evolved later than once thought, the researchers said on Monday.

They analyzed a remarkably well-preserved fossilized(3) skull of the little primate Aegyptopithecus(4) zeuxis, which lived in the trees and ate fruit and leaves about 29 million years ago in warm forests in what is now an Egyptian desert.

A technique called microcomputerized tomography(5) scanning -- a computerized X-ray method also called micro-CT -- allowed them to determine the dimensions of the animal's brain.

"What was astonishing is how small this brain is," Duke University primatologist Elwyn Simons, who led the study in the Proceedings of the National Academy of Sciences, said in a telephone interview.

"You can also see it's a pretty darn primitive brain. It would be small for a monkey or an ape," Simons added. "So it's telling us that the speed of achievement of brain enlargement in primates was a little slower than perhaps we had thought."

This skull of a small female was uncovered in a quarry(6) southwest of Cairo in 2004. It was better preserved than another skull of a larger male of the species found in the same area in 1966.

Based on earlier finds, scientists had theorized the species had a relatively large brain. Instead, it had a brain that might have been even smaller than that of a modern lemur, a primate with primitive traits.

Simons said that when this primate lived, Africa was an island, limiting the competition for survival. Simons said brain enlargement may have evolved in this lineage after Africa became connected to Asia, bringing in more animals including new and dangerous predators.

"Brain-volume enlargement is favored under conditions of competition because you need to be smarter," Simons said.