The monkey skull.
Photo: AMNH/N. Wong and M. Ellison

Paleontologists operating 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 wildly complex human brains isn’t easy, and there aren’t ancient brains just sitting around in the dirt to compare with. Instead, researchers must look indirectly for evidence of brain shapes, based on the way brains leave imprints on skulls. That’s what makes this finding exciting—it’s a view into what primate brains were like long before humans evolved.

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“It’s pretty remarkable,” John Flynn, one of the study’s authors and curator of fossil mammals at the American Museum of Natural History in New York, told Gizmodo. “We were trying to convince ourselves it was anything but a primate, but it showed a bulbous area where the brain should be... The cleaning and subsequent CT scan analysis all reinforced that and the finding’s significance.”

The anthropoid branch of the evolutionary tree is broken into two groups: the New World monkeys, called the platyrrhines, and the Old World monkeys and great apes (including humans), called the catarrhines. Scientists think the split happened at least 36 million years ago. The fossil skull studied here belongs to one of the preserved early platyrrhines called Chilecebus carrascoensis, perhaps one of the first platyrrhines to diverge 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.

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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 the fossil with high-energy X-rays, allowing them to differentiate 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 around 8 grams. They were also able to estimate the size of the monkey’s olfactory bulb (the region of the brain responsible for processing smells) and shape of the optic canal and optic nerve. The brain also already had some surprisingly complicated folding, Flynn said.

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C. carrascoensis’ brain proportions offer insight into the history of brain evolution and primates. The olfactory bulb was surprisingly small, but wasn’t matched by more complex or larger optical components to make up for it. This tells the researchers that primate brains probably don’t evolve as an entire unit, but changes happen piecemeal.

Though this is an exciting specimen, we’re still talking about something millions of years old and badly weathered over time. It will take more fossils from more species in order to understand the entire picture. Even so, these brain case scans are important tools in reconstructing the primate family tree beyond what brain scans of living monkeys can offer. New scans like the one presented here can tell us what features were lost to evolution, which ones derive from a common ancestor, and which ones evolved independently among different groups. For example, newer platyrrhines like marmosets and tamarins seem to have less brain folding than C. carrascoensis, said Flynn.

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The team hopes to continue analyzing the structure of this brain case in higher resolution.