Doubling Up

Our big brains may be the result of a doubled gene that lets brain cells migrate to new areas, says a new study.

SRGAP2 seems to have a lot to do with the human "inflate-o-brain."

The gene, known as SRGAP2, has been duplicated in our genomes at least twice in the four million years since our ancestors diverged from those of the other great apes. It codes for a certain protein that interferes with filopodia – tiny molecular structures that shape the growth of neurons in a developing brain. Researchers think that as SRGAP’s protein disrupted the “normal” growth of our ancestors’ filopodia, millions of their neurons migrated outward to thicken the cerebral cortex – the outer “rind” of the cerebrum where many of our most advanced cognitive functions are processed.

This SRGAP2 duplication isn’t just common – it’s universal: every human being alive today shares it. In fact, it’s one of 23 duplicated genes that are shared by every person’s genome, but aren’t shared with chimps, gorillas, and orangutans. Thus, geneticists think these 23 genes may be crucial parts of the instructions for building a full-fledged Homo sapiens.

A team led by Megan Dennis at the University of Washington examined SRGAP2 in more than 150 million people, and discovered that this duplication story has an interesting twist: it seems that about 3.4 million years ago, SRGAP2 was partially duplicated – and this partial duplication is missing in some people. But then, about 2.4 million years ago, a copy of that partial copy was created and added to chromosome 1. That copied copy is common to the genetic code of every human being alive today.

Dennis and her team studied the effects of this duplicated duplicate, and found that the version of SRGAP2 we all carry interferes with neurons’ ability to make filopodia. Since our great ape cousins don’t share this duplication, it seems reasonable to think that this filopodia “defect” played a major part in shaping the modern human brain.

This research is still in the pretty early stages (I’ve found plenty of popular-press reportage on it; but unless I’m missing something, there’s no peer-reviewed journal paper yet) but it still provides some exciting clues as to why our brains might be so different from those of our closest genetic relatives. I’m pretty interested to see what future studies on SRGAP2 will reveal about the structure of the human cerebral cortex – especially the prefrontal cortex (PFC). As I hear more news, I’ll keep you posted.

But for now, it’s off to kill some brain cells in the hope of making myself smarter. It’ll totally work – Science says so!

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