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.

科學(xué)家對(duì)一個(gè)保存良好的2900萬年前的頭骨進(jìn)行研究，發(fā)現(xiàn)這種類猴的猴子、猿和人類祖先并沒有預(yù)想得那么聰明。

周一，研究人員們說此發(fā)現(xiàn)顯示靈長類大腦進(jìn)化時(shí)間比原來認(rèn)為的要晚。

這個(gè)小巧的古埃及猿頭骨化石保存十分完好。此種靈長類動(dòng)物生活在2900萬年前的溫暖埃及叢林，以樹為家，以果為食，但是現(xiàn)在這片叢林已經(jīng)變成沙漠了。

一項(xiàng)被稱為微型電子計(jì)算機(jī)X線斷層掃描的技術(shù)可以幫助測(cè)算動(dòng)物大腦尺寸，它也可被稱為微CT。

杜克大學(xué)靈長類動(dòng)物學(xué)家Elwyn Simons在電話采訪中說：“我們很驚訝它的大腦竟然如此之小。” 由他負(fù)責(zé)的本次研究被刊登在美國國家科學(xué)院報(bào)中。

Simons說：“它雖然是一個(gè)靈長類頭骨，但是比猴子或者猿的都要小。也就是說靈長類動(dòng)物大腦的飛速發(fā)展比我們?cè)瓉硗扑愕囊稽c(diǎn)。”

這個(gè)小的女性頭骨在2004年被發(fā)掘于開羅西南部一個(gè)采石場(chǎng)中。它的保存狀況好于1966年在同一地點(diǎn)發(fā)現(xiàn)的另一個(gè)稍大的同類男性頭骨。

根據(jù)之前的研究，科學(xué)家們推斷這種動(dòng)物應(yīng)該擁有相對(duì)大的大腦。但是它的大腦卻比具有原始特點(diǎn)的現(xiàn)代狐猴的還小。

Simons說這種動(dòng)物生活的時(shí)候非洲還是一個(gè)島嶼，沒有生存競(jìng)爭(zhēng)。Simons認(rèn)為他們的大腦發(fā)展發(fā)生在亞非大陸相連的時(shí)候，因?yàn)楹芏鄤?dòng)物包括新的和危險(xiǎn)的食肉動(dòng)物都來到了這里。

他說：“競(jìng)爭(zhēng)促進(jìn)腦容量的擴(kuò)展，因?yàn)槟闳绻蛔兊酶斆骶偷盟馈！?BR>