An article published in the journal “Science Advances” reports a study of the known fossil skull of Chilecebus carrascoensis, an extinct monkey that lived about 20 million years ago, in the Miocene period, in today’s Chile. A team of researchers subjected it to a high resolution CT scan to obtain a 3D digital reconstruction that allowed to study its internal structure and therefore the brain’s structure. The new information on this primitive relative of monkeys and hominids that evolved later suggest that the evolution of these primates’ brains was non-linear and that it grew several times independently.
The skull that led to the identification of the species Chilecebus carrascoensis in 1995 was discovered in the Andes of Chile. This monkey was really tiny with an estimated weight of less than 600 grams but this humble animal could be very important to understand the evolution of the brains of monkeys and humans. The problem is typical in the field of paleontology: the well-preserved fossil skulls of ancient relatives of humans are scarce so it’s difficult to reconstruct above all the early stages of their brain evolution.
The fossil of Chilecebus carrascoensis is very well preserved and this made it possible to carry out an accurate investigation by creating a 3D reproduction after subjecting it to a high resolution CT scan in a collaboration between the American Museum of Natural History, the Chinese Academy of Sciences and the University of California, Santa Barbara. Modern technologies can be crucial in this research, for example to study this extinct monkey’s brain characteristics. In this case the result offered some surprises.
In modern primates there’s a negative correlation between the brain’s visual and olfactory centers so primates with an acute sight generally have a poor sense of smell and vice versa. Chilecebus carrascoensis had a small olfactory center but it wasn’t compensated by a developed visual system. This suggests that during the evolution of primates these two centers were not strictly connected as previously thought.
Other interesting discoveries concern the characteristics related to the optic nerve, which suggest that Chilecebus carrascoensis was a diurnal monkey. The brain shows the presence of at least seven pairs of sulci in a far simpler pattern than that of modern humans and monkeys but surprisingly complex for a primate that lived 20 million years ago.
The researchers estimated the encephalization quotient (EQ) of Chilecebus carrascoensis in 0.79. This measure provides an estimate of the size of a species’ brain compared to its body’s and is high in primates. Today’s monkeys have an EQ between 0.86 and 3.39 but the value is much higher in hominins with a peak of 13.64 in modern humans.
The analysis of Chilecebus carrascoensis’ brain characteristics provides new points of reference for the study of encephalization in primates. There was the idea that the growth of brain size was linear but this and other recent research show that in the various primate families that growth was independent and in some cases there was a decrease.
This research contains only a hint to the genetic characteristics linked to the nervous system in connection to the changes occurred in the various primates. The data available suggest that in humans there were different evolutionary pressures compared to monkeys. The understanding of mutations and natural selection that led hominins to a brain growth higher than other primates also passes through the study of a monkey that lived 20 million years ago.
