An extraordinarily complete skull of a 30-million-year-old human ancestor once held a brain about the size of a lime, according to a new study.
The skull—of a species related to apes, humans, and monkeys—is evidence that the more advanced and bigger brains of African primates developed later than previously believed, researchers said.
Elwyn Simons, a primatologist at Duke University, and colleagues discovered only the second intact cranium of Aegyptopithecus zeuxis found to date.
The skull is "extraordinarily unusual," he said, mainly because it’s complete and uncrushed.
The completeness of the fossil skull allowed Simons and colleagues to take computerized x-rays and create a virtual model of the specimen’s tiny brain.
Based on analyses of previous fossil skulls collected at the dig site outside Cairo, Egypt, scientists had assumed the ancient monkey’s brain was larger and more advanced.
The new fossil indicates Aegyptopithecus had a relatively primitive brain compared to its descendants: humans, chimpanzees, gorillas, and other so-called Old World primates.
(Related: "Did Earliest Human Ancestors Have More Apelike Faces?" [April 5, 2007.)]
Nevertheless, the brain region responsible for vision, called the visual cortex, was large. This suggests that, like many primates, Aegyptopithecus had good vision.
"Although its eye sockets were small," Simons added, "it was daytime active. It has high visual acuity unlike noctural animals [such as] lemurs and bush babies."
In addition, the new skull is less than half the size of the only other nearly complete Aegyptopithecus skull, found by Simons and colleagues found at the same Egyptian site in 1966.
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.