WASHINGTON (Reuters) - A monkey-like animal seen as an ancestor of monkeys, apes and humans was not as brainy as expected, according to scientists who analyzed its nicely preserved 29-million-year-old skull.
The finding indicated that primate brain enlargement evolved later than once thought, the researchers said on Monday.
They analyzed a remarkably well-preserved fossilized skull of the little primate Aegyptopithecus 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 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 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.
The condition of the earlier skull -- “smashed up,” as Simons put it -- prevented the analysis possible with the newer one.
“YOU NEED TO BE SMARTER”
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.
Aegyptopithecus is sometimes called “Dawn Ape.” Simons said it looked somewhat like an ape, particularly in its teeth and skull, and said it is thought to be close to the ancestry of monkeys, apes and humans.
“Because of the proportions of having a fairly robust chewing mechanism and a small brain, its skull looks like an ape’s skull. It looks like a miniaturized gorilla,” Simons said.
The new skull fits easily into a person’s palm and is less than half the size of the 1966 skull. The researchers think it was from a female weighing perhaps 5.5 pounds (2.5 kg), while the earlier one was from a male more than twice as big.
They said this size difference between the sexes of this species is similar to that of gorillas.
Simons said that he previously overestimated its brain size based on features of the 1966 skull, which has a bigger snout and more pronounced crests. Those features now seem to be attributes of a male of the species.
Other aspects of its remains indicate it was branching off from its lemur-like ancestors.
For example, its skull indicates the brain’s visual cortex was large, suggesting it had very good vision -- an important characteristic of higher primates. Its eye sockets also indicate it was active during the day. Many more primitive primates are nocturnal.
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