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A 20 million year old skull shed new light on the mystery of brain evolution in primates.
It has long been thought that the size of the brain of anthropoid primates, a group of modern and extinct monkeys, human beings and their close relatives has gradually increased over time.
But research on one of the oldest and most complete fossil primate skulls in South America indicates that enlargement has occurred repeatedly and independently, with decreases occasional size.
The study, published in the journal Science Advances, included a detailed study of an anthropoid fossil 20 million years old discovered in the heights of the Chilean Andes.
This is the only known specimen of Chilecebus carrascoensis.
Lead author, Xijun Ni, associate researcher at the American Museum of Natural History, said, "Brains are unusually hypertrophied, but we know very little about how this essential trait has begun to develop.
Previous research gave a rough idea of the encephalization of the animal, or the size of the brain in relation to body size.
A high encephalization quotient (EQ) means a large brain for an animal of a given size [/caption]
Most primates have high EQs compared to other mammals, although some primates, particularly humans and their closest relatives, have even higher EQs than others.
The resulting phylogenetic encephalization quotient (PEQ), which corrects the effects of close evolutionary relationships, is relatively low in Chilecebus at 0.79.
Most living monkeys have a PEQ ranging from 0.86 to 3.39, with humans entering at 13.46.
Using this, the researchers confirmed that brain enlargement occurred repeatedly and independently in anthropoid evolution, with occasional decreases in size.
For the first time, researchers used digitization and digital reconstruction methods to analyze the fossilized cranial cavity of Chilecebus.
Their findings revealed a complex brain structure of unexpected proportions, suggesting that the internal structures of the primate brain may not have grown in proportion to the evolution of brain size.
The researchers also measured the orbit of the skull and the opening of the optic channel, where the optic nerve would have been located.
The size of the olfactory bulb, a structure implicated in the sense of smell, was also measured.
Although the olfactory bulb is relatively small, suggesting a bad sense of smell, the researchers were surprised to find that this lower sensitivity was not offset by an improved visual system, as in primates today. .
This discovery provides some first clear evidence that visual and olfactory systems have been decoupled during the course of the anthropoid brain, say the authors.
John Flynn, Frick curator at the Museum's Museum of Fossils, said: "Chilecebus is one of those rare and truly spectacular fossils, revealing new insights and surprising conclusions every time new analytical methods are applied to it. study."
– Press Association
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