Paleontologists working with CT scanners have discovered the brain structure of primates at 20 million years, thanks to an extremely well preserved fossil skull.
Understanding the evolution of our wildly complex human brains is not easy and there are no ancient brains that just sit in the dirt to compare. Instead, researchers should indirectly search for evidence of brain shapes based on how the brain imprints on the skulls. This is exactly what makes this discovery exciting – it is an overview of what the brain of primates was before humans evolved.
"It's quite remarkable," John Flynn, co-author 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 everything else is not primate, but it shows a bulbous area where the brain must be… The purification and subsequent CT analysis reinforces this and the significance of the find."
The anthropoid branch of the evolutionary tree is broken down into two groups: New World monkeys called platyrhins, and Old World monkeys and Great Monkeys (including humans) called Catharines. Scientists estimate that the split happened at least 36 million years ago. The fossil skull investigated here belongs to one of the preserved early platyrups called Chilicebus carrascoensis, perhaps one of the first platypus to cross the common ancestor of the group, according to a paper published in Science Advances. The creature would be similar in size to a modern tamarin or marmoset, but with a smaller brain.
Researchers led by Xijun Ni at the Chinese Academy of Sciences measure high-energy X-ray fossils. which allows them to distinguish bone and rock in the sample. They combine scanning into a 3D image that shows the structure of the brain imprinted on the petrified bone. They estimated that such a brain would weigh about 8 grams. They were also able to gauge the size of the monkey's olfactory bulb (the area of the brain responsible for odor processing) and the shape of the optic canal and optic nerve. The brain also already had a surprisingly complex fold, Flynn said. C. Brain proportions of carrascoensis offer an insight into the history of brain evolution and primates. The olfactory bulb was surprisingly small, but was not combined with more complex or larger optical components to compensate for it. This tells the researchers that primate brains probably do not develop as a whole unit, but changes do occur in parts.
Although this is an exciting copy, we are still talking about something millions of years old and badly weathered. More fossils of more species will be needed to understand the whole picture. However, these brain scans are important tools for reconstructing the primate family tree beyond what the live monkey brain scans have to offer. New scans like the ones presented here can tell us which features have been lost in evolution, which ones come from a common ancestor, and which evolved independently among different groups. For example, newer ones, such as marmosets and tamarins, appear to have less brain folding than C. carrascoensis Flynn said.
The team hopes to continue to analyze the structure of this higher resolution brain case.