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Scientists reveal the brain structure of 20 million year old monkey



Monkey skull.
Photo: AMNH / N. Wong and M. Ellison

Paleontologists 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 wild complex human brains is not easy, and there are no old brains that just sit in the dirt to compare with. Instead, researchers must indirectly look for evidence of brain forms based on how brains leave skull imprints. That is what makes this discovery exciting – it is an idea of ​​how primitive brains were like long before humans evolved.

"It's pretty remarkable," John Flynn, one of the study's authors and curators of fossil mammals at the American Museum of Natural History in New York, told Gizmodo. "We tried to convince ourselves that it was anything but a primate, but it showed a bulbous area where the brain would be … The cleaning and subsequent CT scan analysis reinforced all of this and the significance of the finding."

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

A 3D scan of the monkey skull.
GIF: X. Ni / AMNH / MNHN

The researchers, led by Xijun Ni at the Chinese Academy of Sciences, measured fossils with high-energy X-rays, allowing them to distinguish between bone and rock in the sample. They combine the scans into a 3D image showing the structure of the brain imprinted on the fossilized bone. They estimated that such a brain would weigh about 8 grams. They could also estimate the size of the monkey's olfactory bulb (the region of the brain responsible for processing odors) and the shape of the optic canal and optic nerve. The brain also had some surprisingly complex folds, Flynn said.

C. carrascoensis & # 39; brain proportions provide insight into the history of brain development and primates. The light bulb was surprisingly small but was not matched by more complex or larger optical components to compensate for it. Researchers say that primitive brains probably do not develop as a whole unit, but changes do occur in a bit.

While this is an exciting specimen, we are still talking about something millions of years old and badly weathered over time. It will take more fossils from more species to understand the whole picture. Nevertheless, these brain scan scans are important tools for reconstructing the primate tree beyond what brain monkeys of living monkeys can offer. New scans such as those presented here can tell us which functions were lost to evolution, which are derived from a common ancestor, and which developed independently among different groups. For example, newer platyrrins such as marmosets and tamarins appear to have less brain fold than C. carrascoensis, Flynn said.

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


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