As humans, we know we are pretty unique: We are the most dominant species on Earth and our big brains are something worth boasting about. After all, they’re around three times larger than those of our closest relatives, chimpanzees, despite the fact that our DNA is 99% similar. So to what do we owe this prominent feature? This question has been plaguing biologists for some time, but it seems we may be slowly starting to piece together this genetic puzzle.
Just last week, scientists revealed that they had identified a stretch of DNA which, when inserted into mice, made their brains 12% larger than those with the same sequence from chimps. Now, scientists have found a uniquely human gene that not only increased the size of a key brain region in mice, but also bestowed this area with the distinctive morphology observed in primate brains. These findings suggest that this particular gene could have been a major player in the emergence of our unique mental abilities, which is an exciting discovery.
The region of the brain scientists homed in on for their study was the neocortex, which—as the name suggests—is the newest addition to our brain. In humans, this portion of the cerebral cortex is crammed with around 100 billion cells and is the center of higher cognitive function. Since its emergence in a mammalian ancestor some 250 million years ago, it has mushroomed and elaborated in primates, but to a significantly greater extent in humans. For rodents, however, its growth has been relatively static in comparison.
To find out what could be the driving force behind this boost in neocortex size, scientists from the Max Planck Institute began to investigate which genes are switched on, or expressed, during the development of the cortex. To do this, they first separated out the different types of cortical stem cells in mouse embryos and donated human fetal tissue. These stem cells go on to produce the nerve cells, or neurons, that make up the cortex. The simplest way to distinguish them, they found, was to give each cell a fluorescent tag, which allowed them to follow its fate.
After examining which genes were highly expressed, they found various active genes that were present in the human cells, but not the developing mouse. One in particular, ARHGAP11B, stood out as being very highly expressed in the human brain stem cells. Further digging revealed that while this gene is not present in chimps, our ancient relatives had it, such as Neandertals and Denisovans, suggesting it is uniquely human. Interestingly, prior research found that this gene arose when a different gene partially copied itself. Because chimps are not endowed with this gene, they propose that this duplication event must have occurred after humans and chimps split.
Since this gene seemed an ideal candidate as a driver of brain cell growth, scientists inserted it into developing mice and observed what happened. Not only did the number of cortex stem cells dramatically increase, resulting in an increase in neocortex size, but this region also developed characteristic folds which are observed in primates, but not usually mice.
Although it seems this gene may have been a key player in driving neocortex expansion, there must be other factors contributing to our unique mental abilities as well.