How a tiny genetic typo grew our big brain

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Chimpanzees and humans shared a common ancestor around six million years ago. But where their brain has remained almost the same size, ours has ballooned. New research finds a genetic component that likely had a hand in this neuronal growth.

A typo in our genetic code a few million years ago may be behind our big, complex brain today.

Marta Florio at the Harvard Medical School in the US and colleagues in Germany suggest a single mutation in a gene only a million years after human and chimpanzee lineages split prompted more brain cell generation in our youngest years.

This ultimately led to the evolution of our species’ relatively large neocortex – the part of the brain responsible for higher cognitive functions such as language.

The study, published in Science Advances, “is the first to provide a genetic link to brain development that’s specific to humans, and not other species”, neuropsychiatrist and geneticist Chad Bousman at the University of Melbourne, Australia, who was not involved in the study, says.

We are, quite literally, brainier than our earliest Homo ancestors. Homo sapiens’ average cranial capacity is around 1,500 cubic centimetres while early humans, such as Homo habilis (the “handy man”) sported a paltry 600 cubic centimetres.

Homo neanderthalensis was the brainiest of all, boasting a whopping 1,600 cubic centimetres.

So how did hominins grow such big brains while it’s thought modern-day chimpanzees don’t have much bigger brains than our last common ancestor?

The answer, it seems, partially comes down to a small but significant genetic mutation.

Genes are segments of DNA that give instructions for building proteins, determining how they behave in the body. The letters that form DNA are called nucleotides, and each nucleotide is represented by a letter – either A, T, C or G (for the amino acids adenine, thymine, cytosine and guanine respectively).

When cells divide, our genetic code is copied too. But it’s not perfect; errors do get through.

Florio and her colleagues wanted to find the fortunate error that may be behind our big brains.

They examined the gene ARHGAP11B, found in Neanderthals and Denisovans (and us) but not chimpanzees. It amplifies progenitor cells that go on to become neurons in the developing brain.

Its predecessor, ARHGAP11A, is found in chimpanzees. So Florio’s team engineered the ARHGAP11B version to look more like the ARHGAP11A form – by swapping one nucleotide, C, to with another, G – and found the gene lost its neuroproliferative effect.

Bousman says the gene only would have played a role in the evolution of a large neocortex, which grew alongside the emergence of cognitive functions such as reasoning and language, rather the sole reason. 

“The study takes us a bit closer to learning what makes us human, but further work will need to be done for sure to really understand what’s going on,” he says.

And, he points out, nucleotide deletions in this particular part of the genome are associated with neuropsychiatric problems, such as schizophrenia.

“It fits, because those types of conditions are uniquely human, and it’s very rare to have a deletion in that part of the genome.

“With bigger brains come more problems other species don’t necessarily have.”

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