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.