The evolution of the primate brain does not appear to have relied first on the autonomous growth of the frontal lobe, often associated with advanced cognitive functions.
A study published in Science more closely links the expansion of the neocortex to regions involved in vision. Fossils provide a decisive element here, as they make it possible to track this transformation across lineages that are now extinct.
The neocortex is the outer layer of the brain involved in numerous sensory, motor and cognitive functions. In primates, it occupies a significant proportion of the brain. Yet determining which areas drove its expansion over the course of evolution remains difficult, especially when only fossil skulls are available.
To overcome this limitation, the researchers used digital endocasts. These are three-dimensional reconstructions of the inner surface of the skull, which indirectly preserves certain characteristics of the brain’s shape. This method makes it possible to compare living and fossil species without access to brain tissue.
The team examined how different parts of the neocortex evolved in relation to one another. The result does not match the image of a frontal lobe that grew separately and alone supported the increase in cognitive abilities. The observed variations are more closely linked to visual regions.
This point is particularly important in primates. Their evolutionary history is marked by a strong dependence on vision, with eyes directed forward and advanced processing of visual information. Seeing in depth, locating objects or precisely guiding hand movements requires processing a great deal of information from the eyes.
The expansion of certain visual areas may therefore have driven broader reorganizations of the neocortex. The brain would not evolve as an assembly of completely independent modules. A major change in one sensory system can accompany changes in other regions connected to that system.
The frontal lobe nevertheless remains important. In living primates, this region is involved in planning, decision-making and behavioral control, among other functions. The study does not challenge these functions. Rather, it disputes the idea that its independent enlargement is enough to explain the overall evolution of the neocortex.
Fossils also require particular caution. An endocast primarily provides information about the shape and certain boundaries visible on the brain’s surface. It does not directly reveal the number of neurons, their connections or the precise functioning of each area. The authors can therefore test anatomical relationships at a broad scale, but not reconstruct the brain activity of an extinct animal.
The next step will be to compare these results with more fossils and anatomical data from living primates. New reconstructions may help clarify when vision-related changes emerged in the different branches of the primate evolutionary tree.