June 4, 2018

Two research teams have uncovered the gene family that allowed us to evolve our impressively large brains

Compared to other organisms on the planet, our globs of gray matter are unusually big. That’s actually quite a fortunate turn of events since we owe them our success as a species today. But why we have such big brains is a different question altogether — one that two recently-published papers attempt to answer.

The two teams identified a genetic family christened NOTCH2NL as the likely catalyst for the evolution of our brains over the last 3 million years. The gene, which is located in a part of the genome previously linked to neurodevelopmental disorders, seems to play an important role in human cortex development by delaying stem cell specialization into neurons — in the long run, this results in more overall neurons being developed. The gene is also exclusive to humans (lacking even in our closest relatives), and is heavily expressed in stem cells in the cerebral cortex.

Moving up a notch

“Our brains got three times as big primarily through the expansion of certain functional areas of the cerebral cortex, and that has to be a fundamental substrate for us becoming human. There’s really no more exciting scientific question that I can think of than discovering and decoding the mysterious genetic changes that made us who we are,” says bioinformatician David Haussler, co-senior author of one of the papers.

Haussler’s team was busy comparing genes expressed during brain development in humans and macaques when they discovered that only human cells sported NOTCH2NL. Further testing revealed that orangutans also lacked this gene and that our closest living relatives — gorillas and chimps — only carry incomplete, inactive versions. After piecing together as much of its evolutionary history as they could, the team believes that it was borne out of a process known as gene conversion. They write that the process was likely applied to repair a non-functional version of NOTCH2NL, which itself first emerged as a partial duplication (i.e. a mutation) of the neurodevelopmental gene NOTCH2.

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