Skull find connects monkey and man

29-million-year-old primate skull with a surprisingly small brain

Elwyn Simons with two fossil skulls. © Duke University
Read out

The discovery of a well-preserved, 29-million-year-old skull reveals surprising facts about the earliest ancestors of humans and monkeys. Her brain was apparently smaller and the gender differences more pronounced than previously thought. Researchers report in the current issue of the Proceedings of the National Academy of Sciences.

Aegyptopithecus zeuxis has been regarded as a common ancestor of humans and monkeys since the discovery of a first skull of this species in 1966 in a quarry near Cairo in Egypt. Based on the measurements of the former fund, the researchers assumed that this species of prey monkeys must have already had a relatively large brain. But a new find has now refuted this assumption. Elwyn Simons, professor of biological anthropology and anatomy at the American Duke University discovered again in the same excavation area an Aegyptopithecus skull.

Half the size of a previous find

This is in an amazingly good state of preservation, especially the skull roof is completely intact. This enabled the researchers to use micro-computer tomography to accurately measure the cranial volume - and thus the size of the monkey's brain. The result was obtained in the form of a three-dimensional computer reconstruction of the brain. "The species had a brain that could be even smaller than that of a modern lemur, " explains Simons. "That means that the big-headed monkeys and apes did not develop their big brains until later."

Small enough to sit comfortably in the explorer's palm, the new skull is actually less than half the size found in 1966. Simons and his colleagues had initially assumed that this could possibly be a different species. But more detailed analyzes spoke against it. From the comparison of the two skulls the same age, the scientists conclude that the new must come from a female of the species, the old one from a male.

Gender difference similar to the gorillas

The enormous size difference between the sexes is comparable to that of the gorillas, the genetically second closest relatives of humans. In modern primates, such differences are usually typical of species that live in larger groups of 15 to 20 males and females. "We conclude that Aegyptopithecus also lived in a larger social group. That, in turn, suggests he had enough brain to separate the group members, "explains Simons. display

Sight already strong

The skull of the Aegyptopithecus is thus overall smaller than originally assumed. However, the shape of the eyes of both skulls indicates that Aegyptopithecus, like most modern primates, was active during the day. Many prosimians, the group that includes the lemurs, are nocturnal.

Other properties of the skull and other fossilized Aegyptopithecus fossils collected at the site over the past four decades, however, indicate that this primate had already moved away from its lemur-like ancestors in its development, says Simons. "We also find that the visual cortex was very large, which means that this species, like many primates, has a very good sense of sight. This is considered to be a very important feature of the higher primates and evidently already evolved in Aegyptopithecus.

(Duke University, 15.05.2007 - NPO)