A 20-million-year-old skull suggests complex brain evolution in monkeys and monkeys

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It has long been thought that the size of the brain of anthropoid primates – a diverse group of modern and extinct monkeys, human beings and their close relatives – has gradually increased over time. New research on one of the oldest and most complete fossil primate skulls in South America shows that the pattern of brain evolution in this group was much more complex. The study, published today in the journal Progress of science and led by researchers from the American Museum of Natural History, the Chinese Academy of Sciences and the University of California at Santa Barbara, suggests that the brain has been enlarged so repeated and independent during the history of the anthropoid, and that it was more complex in some group than previously recognized.

"Humans have an unusually wide brain, but we do not really know how much this trait began to develop," said lead author Xijun Ni, an associate researcher at the Museum and a researcher at Chinese Academy of Sciences. "It's partly because of the scarcity of well-preserved fossil skulls belonging to much older parents."

As part of a long-standing collaboration with John Flynn, curator of the Museum of Mammal Fossils, Frick, Ni led a detailed study of an exceptional anthropoid fossil dating back 20 million years discovered in the Chilean Andes, the skull and known specimen of Chilecebus carrascoensis.

"Through more than three decades of partnership and close collaboration with the National Museum of Chile, we have recovered many new and remarkable fossils from unexpected places in the rugged Andean volcanic terrain," said Flynn. "Chilecebus is one of those rare and truly spectacular fossils, revealing new insights and surprising conclusions every time new analytical methods are applied to its study. "

Previous research by Flynn, Ni and their colleagues on Chilecebus provided a rough idea of ​​the encephalization of the animal or the size of the brain relative to the size of the body. A high encephalization quotient (EQ) means a large brain for an animal of a given body size. Most primates have high EQs compared to other mammals, although some primates, particularly humans and their closest relatives, have an even higher EQ than others. The last study goes further in this understanding by illustrating the trends observed in the more general genealogical tree of anthropoids. The resulting "PEQ" – or phylogenetic encephalization quotient, to correct the effects of close evolutionary relationships – during Chilecebus is relatively small at 0.79. In comparison, most living monkeys have a PEQ between 0.86 and 3.39, with humans reaching an extraordinary level of 13.46 and having developed significantly increased brain size, even compared to their close relatives. With this new framework, the researchers confirmed that brain enlargement occurred repeatedly and independently in anthropoid evolution, in both the New and Old World lineages, with occasional decreases in size.

High-resolution X-ray tomography scanner and 3D digital reconstruction of the interior of Chilecebus& # 39; skull gave the research team new information about the anatomy of his brain. In modern primates, the size of the visual and olfactory centers in the brain is negatively correlated, reflecting a potential evolutionary "compromise", which means that visually acute primates generally have a lower sense of smell. Surprisingly, the researchers discovered that a small olfactory bulb in Chilecebus has not been counterbalanced by an amplified visual system. This discovery indicates that, during the evolution of primates, the visual and olfactory systems were much less closely related than previously supposed.

<li data-thumb = "https://scx1.b-cdn.net/csz/news/tmb/2019/4-20millionyea.jpg" data-src = "https://scx2.b-cdn.net/ gfx / news / hires / 2019 / 4-20millionyea.jpg "data-sub-html =" This illustration compares brain size of various primates, including humans (top left) and fossil Chilecebus (middle), based on a new method (phylogenetic encephalization quotient or PEQ) that takes into account both body size and evolutionary relationships of the species. The size of each primate species reflects its QPE value (large head equals high QPE, small head equals low QPE), and not its actual brain size or body / head size. For example, a high PEQ (larger heads in this image) means a larger brain than expected for an animal of a given body size. Credit: © Xiaocong Guo / Xijun Ni ">



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Other findings: The size of the opening of the optic nerve suggests that Chilecebus was diurnal. In addition, the folding pattern (sulcus) of the brain Chilecebus, although much simpler than most modern anthropoids, has at least seven pairs of sulcus sulci and is surprisingly complex for such an ancient primate.

During his epic journey on the Beagle, Charles Darwin explored the mouth of the canyon where Chilecebus was discovered 160 years later. Closed by the snow in winter, Darwin is inspired by the "scenes of the highest interest" presented by his vision. This exquisite fossil, discovered a few kilometers east of Darwin's position, would have delighted him, "said co-author André Wyss of the University of California, Santa Barbara.