The human brain is among nature’s most impressively complex creations, although the evolutionary processes that gave rise to this cerebral masterpiece remain shrouded in mystery. However, by virtually reconstructing the brains of some of the earliest primates, scientists have now managed to catch a glimpse of some of the original prototypes and blueprints for the human brain. Interestingly, their work reveals that primates’ brains developed certain key structures before they began to expand in size, rather than the other way round.
Publishing their findings in the Journal of Human Evolution, the researchers describe how they used X-ray computed tomography to create virtual models of the brains that would once have been housed by the well-preserved skulls of a number of adapiforms. Similar to lemurs, adapiforms were among the earliest primates, existing around 50 million years ago and appearing shortly after the first known primate ancestors, known as plesiadapiforms.
Using skulls belonging to three different species of adapiform, the study authors were able to observe how their brains compared to those of their predecessors as well as modern primates.
Among their major findings was that while adapiform brains were in fact very similar to those of plesiadapiforms in terms of size, they showed some structural differences that provide some key clues as to how the brains of modern primates formed.
For instance, adapiform brains display a larger neocortex – the part of the brain responsible for sight, and smaller olfactory bulb – which processes smell – than plesiadapiform brains. As such, it is likely that they relied more on vision than smell than their predecessors did.
Given that modern primate brains are characterized by a large neocortex, this finding could represent a key piece of the evolutionary puzzle by revealing how the brains of our ancestors began to resemble our own. It also helps to clarify the long-standing mystery regarding the timeline of the primate brain, suggesting that many of its key characteristics appeared while it was still relatively small, only later to be followed by an expansion in size.
This would appear to put paid to the theory that primate brains first became large and then became specialized.
"While it's true humans and other modern primates have very large brains, that story started down at the base of our group," explained study author Jonathan Bloch in a statement. "As our study shows, the earliest primates actually had relatively small brains. So they didn't start out with large brains and maintain them."