Geneticists have linked the physical appearance of humans to
patches of DNA lost in the five 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.
"We can know what makes us human, what makes us physically
different from other animals and why," said developmental
geneticist Gill Bejerano of Stanford University, an author of
the 10 March study in Nature.
Only two per cent 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
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 stretches of
DNA present in chimps but lost in humans.
Two deletions, one near a male hormone-signalling 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
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."
Citation: "Human-specific loss of regulatory DNA and the
evolution of human-specific traits." Cory Y. McLean, Philip L.
Reno, Alex A. Pollen, Abraham I. Bassan, Terence D. Capellini,
Catherine Guenther, Vahan B. Indjeian, Xinhong Lim, Douglas B.
Menke, Bruce T. Schaar, Aaron M. Wenger, Gill Bejerano and David M.
Kingsley. Nature, Vol. 471, No. 7337, pp 216-219. March 10, 2011.