How primates developed large brains decoded

Scientists have reconstructed virtual brains from ancient, kiwi-sized primate skulls that could help solve one of the most intriguing evolutionary mysteries: how modern primates developed large brains.

Paleontologists at University of Florida (UF) in the US found clues in the remarkably preserved skulls of adapiforms, lemur-like primates that scurried around the tropical forests of Wyoming about 50 million years ago.

Thought to be a link between primitive and advanced primates, their fossil skulls were the best evidence available for understanding the neuroanatomy of the earliest ancestors of modern primates.

But there was just one problem - the brain cavities of the fragile skulls contained only rock and dust.
 

Arianna Harrington, previously a UF undergraduate student, used CT technology to create the first virtual 3-D brain casts of the early primates.

The eight virtually reconstructed and dissected brains - the most ever created for a single study - show an evolutionary burst including improved vision and more complex neurological function preceded an increase in brain size, said Harrington, now a Duke University doctoral student.

"It may be that these early specialisations allowed primate brains to expand later in time," said Harrington, the study's lead author.

"The idea is that any patterns we find in primate brain evolution could lead to a better understanding of the early evolution that led to the human brain," Harrington said.

Adapiforms, which are not directly related to humans, evolved after the earliest primate ancestors, called plesiadapiforms, which lived about 65 million years ago.

Harrington and colleagues created virtual endocasts for three different species of adapiforms: Notharctus tenebrosus and Smilodectes gracilis from the middle Eocene Bridger formation of Wyoming and a late Eocene European specimen named Adapis parisiensis.

Adapiforms' skulls differ from the earlier plesiadapiforms in a few ways including having more forward-facing eyes.

Thanks to the new virtual endocasts, scientists were able to take a closer look at anatomical features which showed that, while adapiforms placed relatively less emphasis on smell more similar to modern primate brains, the relative brain size was not so different from that of plesiadapiforms.

"As our study shows, the earliest primates actually had relatively small brains. So they did not start out with large brains and maintain them," said study co-author Jonathan Bloch, curator of vertebrate paleontology at the Florida Museum of Natural History.

The findings were published in the Journal of Human Evolution.