2 March 2015

Dr Meghna Kataria By

Appeared in BioNews 792

One of the genes behind the dramatic evolutionary enlargement of the human brain has been identified. By greatly increasing the number of cells in important brain regions, the gene in question might have helped humans develop the cognitive abilities that are unrivalled in the animal kingdom.

In the study, published in the journal Science, researchers report that the gene can also be found in the DNA of our extinct relatives, Neanderthals and Denisovans, but not chimpanzees, who otherwise share 98.5 percent of our DNA.

In particular, the gene - called ARHGAP11 - was found to greatly increase the number of stem cells specific to the neocortex - a brain area essential for reasoning, language and sensory perception. The discovery might help explain why the neocortex is three times larger in humans than in chimpanzees, containing 100 billion neurons.

'The neocortex is so interesting because that's the seat of cognitive abilities, which, in a way, make us human,' Marta Florio, a PhD student at the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden, Germany, and first author of the paper, told Live Science.

To understand what sets the human neocortex apart, the researchers first fluorescently dyed different subpopulations of stem cells found in the developing brains of mouse and human embryos. This allowed them to isolate and sort these subpopulations into different groups.

Next, they looked for genes that had only been switched on in a particular group - human stem cells. Out of the 56 human-specific genes found, only ARHGAP11B fulfilled the criteria of being tenfold more active in stem cells called the basal radial glia compared to other brain cells.

When this gene was introduced into mouse embryos at a critical period in their neural development, it led to an increased stem cell population. Half of the mice also developed the ridges and folds characteristic of the human brain, which correlate with the ability to house more neurons.

The researchers are now doing experiments to see how the human ARHGAP11B gene influences behaviour in adult mice.

'Will they learn better, or have better memories? That’s hard to say. But we should know that later this year,' study leader Professor Wieland Huttner told The Guardian.

Any predictions of a hyper-intelligent species of mice arising from these tests are likely to be proved false, however.

'What is unique about humans is not going to come down to one gene only,' Florio told The Guardian. 'Cognition is a complex thing. We don’t think a single gene makes us smarter than other animals.'

The discovery follows a similar but separate study where researchers identified a section of human DNA that controls the activity of brain-specific genes. When introduced into mice, the section led to a 12 percent increase in neocortical size. The sequence was found to be human-specific, but only subtly different from the version found in the chimpanzee genome.