Scientists reveal the brain structure of a monkey aged 20 million years

Palaeontologists using CT scanners have revealed the structure of a 20 million-year-old primate brain, thanks to an extremely well-preserved fossil skull.

Understanding the evolution of our extremely complex human brains is not easy, and there are no old brains sitting in the dust with which to compare. Instead, researchers must look indirectly for evidence of brain shape, based on how the brain leaves imprints on the skull. That's what makes this discovery exciting: it's a snapshot of what the primate brain was like long before human evolution.

"It's quite remarkable," John Flynn, one of the authors of the study and curator of fossil mammals at the American Museum of Natural History in New York, told Gizmodo. "We were trying to convince ourselves that it was something other than a primate, but it showed a bulbous area where the brain had to be … The cleansing and subsequent CT scan all reinforced this finding and the importance of the discovery."

The anthropoid branch of the evolutionary tree is divided into two groups: New World monkeys, called platyrrhines, and Old World monkeys and great apes (including humans). ), called catarrhines. Scientists believe that the split occurred at least 36 million years ago. The fossil skull studied here belongs to one of the earliest preserved platyrrhins called Chilecebus carrascoensis, perhaps one of the first platyrrhines to diverge from the group's common ancestor, according to the article published in Science Advances. The creature would have been similar in size to a tamarind or modern marmoset, but with a smaller brain.

The researchers, led by Xijun Ni at the Chinese Academy of Sciences, measured the fossil with high-energy X-rays, thus allowing them to differentiate bone and rock from the sample. They combine the scans into a 3D image, showing the structure of the brain printed on the fossilized bone. They estimated that such a brain would weigh about 8 grams. They were also able to estimate the size of the olfactory bulb of the monkey (the region of the brain responsible for the treatment of odors) and the shape of the optic channel and the optic nerve. The brain also had already surprisingly complicated folds, Flynn said.

C. carrascoensis' Brain proportions provide insight into the history of brain and primate evolution. The olfactory bulb was surprisingly small, but it did not have more complex or bulky optical components. This tells the researchers that the primate brain will probably not evolve as a complete unit, but that the changes will be piecemeal.

Although this is an exciting specimen, we are still talking about something that goes back millions of years and has struggled badly over time. It will take more fossils belonging to more species to understand the situation as a whole. Nevertheless, these cerebral case analyzes are important tools for rebuilding the primate family tree beyond what brain tests of living monkeys can offer. New analyzes like the one presented here can tell us which features were lost during evolution, which are derived from a common ancestor and which evolved independently between different groups. For example, new platyrrhins such as marmosets and tamarins appear to have fewer brain folds than C. carrascoensis, Flynn said.

The team hopes to continue to analyze the structure of this brain case with a higher resolution.