What makes your brain different from a Neanderthal’s?

Scientists have discovered a glitch in our DNA that may have helped set the minds of our ancestors apart from those of Neanderthals and other extinct relatives. The mutation, which arose in the past few hundred thousand years, spurs the development of more neurons in the part of the brain that we use for our most complex forms of thought, according to a new study published in Science on Thursday.

“What we found is one gene that certainly contributes to making us human,” said Wieland Huttner, a neuroscientist at the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden, Germany, and one of the authors of the study.

The human brain allows us to do things that other living species cannot, such as using full-blown language and making complicated plans for the future. For decades, scientists have been comparing the anatomy of our brain to that of other mammals to understand how those sophisticated faculties evolved. The most obvious feature of the human brain is its size — four times as large as that of chimpanzees, our closest living relatives. Our brain also has distinctive anatomical features. The region of the cortex just behind our eyes, known as the frontal lobe, is essential for some of our most complex thoughts. According to a study from 2018, the human frontal lobe has far more neurons than the same region in chimpanzees does.

But comparing humans with living apes has a serious shortcoming: Our most recent common ancestor with chimpanzees lived roughly seven million years ago. To fill in what happened in since then, scientists have had to resort to fossils of our more recent ancestors, known as hominins. Inspecting hominin skulls, paleoanthropologists have found that the brains of our ancestors dramatically increased in size starting about two million years ago. They reached the size of living humans by about 600,000 years ago. Neanderthals, among our closest extinct hominin relatives, had brains as big as ours.

But Neanderthal brains were elongated, whereas humans have a more spherical shape. Scientists can’t say what accounts for those differences. One possibility is that various regions of our ancestors’ brains changed size. In recent years, neuroscientists have begun investigating ancient brains with a new source of information: bits of DNA preserved inside hominin fossils. Geneticists have reconstructed entire genomes of Neanderthals as well as their eastern cousins, the Denisovans.

Scientists have zeroed in on potentially crucial differences between our genome and the genomes of Neanderthals and Denisovans. Human DNA contains about 19,000 genes. The proteins encoded by those genes are mostly identical to those of Neanderthals and Denisovans. But researchers have found 96 human-specific mutations that changed the structure of a protein. “This is really a tour de force,” said Laurent Nguyen, a neuroscientist at the University of Liège in Belgium who was not involved in the study. “It’s remarkable that such a small change has such a dramatic effect on the production of neurons.” “I don’t think it’s the end of the story,” he said. “I think more work is needed to understand what makes us human in terms of brain development.”