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.

科学家对一个保存良好的2900万年前的头骨进行研究，发现这种类猴的猴子、猿和人类祖先并没有预想得那么聪明。

周一，研究人员们说此发现显示灵长类大脑进化时间比原来认为的要晚。

这个小巧的古埃及猿头骨化石保存十分完好。此种灵长类动物生活在2900万年前的温暖埃及丛林，以树为家，以果为食，但是现在这片丛林已经变成沙漠了。

一项被称为微型电子计算机X线断层扫描的技术可以帮助测算动物大脑尺寸，它也可被称为微CT。

杜克大学灵长类动物学家Elwyn Simons在电话采访中说：“我们很惊讶它的大脑竟然如此之小。” 由他负责的本次研究被刊登在美国国家科学院报中。

Simons说：“它虽然是一个灵长类头骨，但是比猴子或者猿的都要小。也就是说灵长类动物大脑的飞速发展比我们原来推算的要慢一点。”

这个小的女性头骨在2004年被发掘于开罗西南部一个采石场中。它的保存状况好于1966年在同一地点发现的另一个稍大的同类男性头骨。

根据之前的研究，科学家们推断这种动物应该拥有相对大的大脑。但是它的大脑却比具有原始特点的现代狐猴的还小。

Simons说这种动物生活的时候非洲还是一个岛屿，没有生存竞争。Simons认为他们的大脑发展发生在亚非大陆相连的时候，因为很多动物包括新的和危险的食肉动物都来到了这里。

他说：“竞争促进脑容量的扩展，因为你如果不变得更聪明就得死。”