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Humans evolved large brains and flat faces at a surprisingly rapid pace compared to other apes, likely reflecting the evolutionary advantages of these traits, finds a new analysis of ape skulls by UCL researchers.

The paper, published in the journal Proceedings of the Royal Society B: Biological Sciences, analyzed how the evolutionary diversity of the skulls of humans and other related apes evolved over millions of years. They found that the skull structure for humans evolved substantially faster than that of any other closely related ape species.

Lead author, Dr. Aida Gomez-Robles (UCL Anthropology) said, "Of all the ape species, humans have evolved the fastest. This likely speaks to how crucial skull adaptations associated with having a big brain and small faces are for humans that they evolved at such a fast rate. These adaptations can be related to the cognitive advantages of having a big brain, but there could be social factors influencing our evolution as well."

The researchers examined three-dimensional virtual models of the skulls of different modern species of primates, including seven hominids, or "great apes," such as humans, gorillas and chimpanzees, and compared them to nine species of hylobatids, or "lesser apes," such as gibbons.

Hominids and hylobatids evolutionarily split from each other about 20 million years ago. During that time, the anatomical diversity of hominids exploded, while that of hylobatids remained surprisingly restricted. As a result, the skulls of different species of hylobatids look extremely similar to each other, while species of hominids look much more distinct. However, even within hominids, humans evolved faster than other species.

To measure skull variation, the researchers broke down each species' skulls into four principal sections: the upper face, the lower face, the front of the head and the back of the head. Using a computer to compare the 3D scans, they numerically identified how different each section was between species. It's the most detailed comparative analysis of 3D skull structure across these closely related ape species.

Most great apes have big and forwardly projecting faces with relatively small brains, while humans have flatter faces and large round heads. Gibbons, in some ways similar to humans, likewise have relatively flat faces as well and a round head, but a much smaller brain.

The team used the slow evolution and low diversity of hylobatids as a kind of control to compare the variation in hominid skulls. By comparing the species, the researchers found that humans changed about twice as much as would be expected if there wasn't some additional factor encouraging additional changes.

The researchers cautioned that while it can be tempting to conclude that the evolutionary advantages of greater intelligence from bigger and more complex brains was the primary driver for human's rapid evolution, social factors could be affecting these changes as well.

Dr. Gomez-Robles added, "After humans, gorillas have the second fastest evolutionary rate of their skulls, but their brains are relatively small compared to other great apes. In their case, it's likely that the changes were driven by social selection, where larger cranial crests on the top of their skulls are associated with higher social status. It's possible that some similar, uniquely human social selection may have occurred in humans as well."