The English word “evolve” comes from a Latin word, used years before the familiar Darwinian connotation took over, meaning “unroll.” As individuals, we don’t evolve — it’s genes that evolve — but as our lives unroll, we can see and feel the influence of natural selection at every stage, from birth to death.
The signs of evolution are most easily discerned at key points. When we reproduce, we are exposed to selection; when we develop as a fetus and as a child, selection is ever-present, shaping us. Even when we fall in love and marry, the effect of selection can be seen. And it can be seen, of course, when we fall ill and as we get older. For it is by this mechanism that selection has the final word, over who lives and who dies.
By looking at these stages in the light of evolutionary explanation, we can understand features of our lives that are otherwise mysterious. Why is the womb constructed in a way that seems almost parasitic? Why are certain men more attractive? Why do our brains change and our emotions change when we are teenagers? Why do children bully each other? Why are some people obese? Why do our brains start to perform less efficiently when we get old?
And by comparing our genes with those of our closest relative, the chimpanzee, we can approach the central question of what it means to be human. Almost 99 percent of our genes are the same as those in chimps. But if we zoom in on those that are different, and pick out the ones that are most different, we can isolate the genes that are most important in making us human.
This is what Katherine Pollard and colleagues at the University of California, Santa Cruz, have now done. There are two genes humans have that have changed most in comparison to those of chimps, and it will come as no surprise to learn that they control brain development. All primates have relatively large brains compared to other animals, but the human brain is three times larger than that of chimps.
The key brain genes are 118 letters of DNA code long, and there are 18 differences in the “spelling” of the code between chimps and humans. That doesn’t sound like much, but between chimps and chickens, species far more distantly related than are chimps and humans, there are only two changes. Pollard says that the difference in the sequence between chimps and humans is dramatic.
“That is an incredible amount of change to have happened in a few million years,” she said.
Chimps and humans shared a common ancestor about 5 million years ago, so the changes in the gene sequence must have happened in that time.
“At this point, we can only speculate about this gene’s role in the evolution of the human brain, but it’s exciting to find a new gene involved in brain development, and it’s especially exciting for us because it validates our approach of letting evolution guide us and tell us what are the important parts of the human genome,” said co-author David Haussler, director of the Center for Biomolecular Science and Engineering at the University of California, Santa Cruz.
The sequence of DNA containing the two genes has been named HAR1 — human accelerated region 1. One of the genes has already been found to be active in the developing brain during weeks 7 to 9 of pregnancy.
“It’s a very exciting finding because it is expressed in cells that have a fundamental role in the design and development of the mammalian cortex,” said Haussler.
Pollard was understated in her appraisal of the discovery.
“It was very rewarding to find that this region that came out at the top of our list is potentially relevant to human evolution in an interesting way,” she said.
HAR1 now joins other genes that are known to have evolved recently. They include the gene, common in people of northern European descent but less common in East Asia, that allows adults to digest milk. Other recently evolved genes include those for skin pigmentation.
Clearly, by identifying these genes we can discover what has been important in our evolution. Understanding this is not just an academic exercise. It illuminates our common ancestry and common humanity — something often forgotten, ignored or completely unappreciated by religious fundamentalists.
And now for a shameless plug. You can read more about how evolution has shaped us and is a process that is affecting us now, and throughout our lives, in my new book, the second volume of collected Natural Selections columns. It answers the questions posed at the beginning of this article, about why we develop and behave in the way we do.