Geneticists have linked the physical appearance of humans to patches of DNA lost in the 5 million years since we shared a common ancestor with chimpanzees. One loss prevented men from growing penile barbs, which chimps possess. Another enlarged some regions of our brain.
Only 2 percent of the DNA in our genome forms protein-coding genes. The rest, once called “junk DNA,” helps control and coordinate gene activity. Out of this regulatory coordination, physiological complexity emerges.
Bejerano’s team started by comparing the genomes of chimpanzees and macaque monkeys, which last shared a common ancestor 20 million years ago. They identified regions that hadn’t changed in chimps, then compared these to corresponding stretches of the human genome. They found more than 500 mutations known as deletions, or missing stretches of DNA, present in chimps but lost in humans.
Two deletions, one near a male hormone-signaling gene and another near a neural development gene, were especially intriguing. Tweaking those genes in mice suggested possible roles for the loss: eliminating penile spines and boosting cerebral cortex growth.
Bigger brains are an obvious advantage (“It probably helped us become the thinkers we are today,” Bejerano said), but it’s unclear why evolution weeded out the spines. These tiny, hair-like projections, found in male chimps and cats, can trigger female ovulation. They also increase sensitivity and remove existing sperm, ostensibly giving males a reproductive advantage. Bejerano suspects the spines are conducive to monogamy.
Could restoring the relevant regulatory DNA in humans resurrect penile spines? “I’m going to leave it to others to paint that picture and its consequences,” said evolutionary biologist Sean B. Carroll of the University of Wisconsin, who wasn’t involved in the study. “But my guess is that something would probably happen.”
More practically, the findings underscore the importance of regulatory changes to human evolution. “Regulation is a choreograph critical to shaping how organisms appear. This research is going to be a hot trail to follow,” said Carroll. “It’s not just about what genes you have, but how they’re used.”
