A specific gene present in human stem cells, but not in the cells of other animals like mice (a laboratory standard) or chimps (our closest living evolutionary relative), appears to be responsible for the evolutionary increase in human brain size.
A team of scientists at the Max Planck Institute recently studied the gene, called ARHGAP11B, for clues to its purpose and origin. They hoped to explain how humans developed mental capacities suited for the expression of language and abstract thought while other species did not.
During crucial stages of human fetal brain development, scientists found the ARHGAP11B gene in high numbers. They also found the gene was very active, or "switched on," during these stages. The gene was absent in the brains of mice, but once ARHGAP11B was inserted, the rodents' brains grew in size and also grew what looked like larger neocortices:
"These amped-up brain regions contained loads of neurons and some even began forming the characteristic folds, or convolutions, found in the human brain, a geometry that packs a lot of dense brain tissue into a small amount of space."
By tracing back the evolutionary history of the gene, scientists discovered ARHGAP11B not only in modern-day humans, but also in Neanderthals and another branch of extinct humans called Denisovans. Researchers did not find the gene in chimpanzees.
While the gene may explain the expansion in volume of the human brain — a necessary, but not sufficient cause of intelligence — how our intelligence evolved is another story. Both humans and Neanderthals had large brains, said Dr. Wieland Huttner, a neurobiologist also at the Max Planck Institute, but humans' unique intelligence may have more to do with how brain cells form and prune neural networks over time.
So by now, you might think that humans were graced with the peak benefit that evolution has to offer. This isn't exactly the case, explains theoretical physicist and popular science author Michio Kaku. Actually, so-called intelligence may yet prove an evolutionary disadvantage if we are unable to cope with the existential threats facing our species:
"Some people think that intelligence is the crowning achievement of evolution. Well if that’s true there should be more intelligent creatures on the planet Earth. But to the best of our knowledge we’re the only ones. The dinosaurs were on the Earth for roughly 200 million years and ... not a single dinosaur became intelligent. We humans, modern humans, had been on the Earth for roughly a hundred thousand years. Only a tiny fraction of the 4.5 billion years that the Earth has been around. So you come to the rather astounding conclusion that intelligence is not really necessary, that Mother Nature has done perfectly well with non-intelligent creatures for millions of years and that we as intelligent creatures are the new kid on the block."
Read more at Scientific American.