What makes mammals, including us, unique in the animal kingdom? One standout feature we have but which does not exist in non-mammals is the neocortex. Making approximately half the volume of the human brain, the neocortex is the seat of our intellect, able to handle the sophisticated computations and associations that power cognitive thinking.

How did the human (and more generally, mammalian) neocortex come about? Did it evolved from more ancient creatures like reptiles – therefore implying our brain is in some sense, a “lizard brain”, or did the human neocortex evolved through its own innovations?

To answer this question in a precise way requires powerful gene expression techniques. Essentially, scientists need to have to way to compare gene A in a lizard to the same gene in a mammal – an “apples to apples” type of comparison. Single-cell RNA sequencing is precisely this technique. In 2015, breakthroughs in single-cell RNA sequencing allowed scientists for the first time to study a large number of cells, paving the way for them to probe the evolutionary origin of the neocortex.

The answer to the “lizard brain hypothesis” came in 2022, in a landmark paper that was published in the journal Science. The work in that paper was led by Maria Antonietta Tosches at Columbia University. Using the single-cell RNA technique, Tosches and her colleagues showed that mammals evolved the neocortex independently as an entirely new brain region, without a trace of what came before it. In other words, ours is not lizard brain as was once thought. Not only that, this process of innovation isn’t limited to the creation of new parts. Related work carried out by Tosches and her team showed that what appears to be ancient brain regions have evolved and continue to evolve by getting rewired with new types of cells. Together, their findings have not only debunked the lizard brain hypothesis, they also cast exciting new light on the remarkable innovative ability of the mammalian brain.

Notes:
This post is adapted from a Quantamag article by Allison Whitten titled, “Gene Expression in Neurons Solves a Brain Evolution Puzzle” (14 Feb 2023). All images in this post are from that article.