Comparing the two skulls allowed the researchers to conclude the new one is from a female, which may have weighed about 5.5 pounds (2.5 kilograms). The first is from a male twice that size, they say.
The size difference between the sexes is similar to that among larger Old World monkeys or gorillas—the second closest human ancestor, after chimpanzees.
Simons said a large size difference between sexes in primates is a sign they live in large social groups.
Dean Falk is an anthropologist at Florida State University in Tallahassee and an expert on brain evolution. She was not involved in the new study.
She said the new study, which appeared online today in the journal Proceedings of the National Academy of Sciences, challenges "perceived truths" that large brain size was required for things like daytime activity and living in large social groups.
"[The new study] is saying you don't have increased brain size back when you have some of these things," she said.
In fact, Falk believes the virtual model of Aegyptopithecus' brain, used in the new study, suggests the brain was even less advanced than the researchers propose.
However, Falk agrees that the brain model does confirm an enlarged area for vision, suggesting good eyesight was important early on for our ancestors.
"All of this makes sense to me," she said, "and it's nifty."
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