Geneticists have linked the differences in the physical appearance of humans and chimpanzees to the loss of certain genes in the five million years since we shared a common ancestor with the chimps.
One loss prevented men from gaining penile barbs possessed by chimpanzees, while another enlarged some regions of the human brain.
The study, published in the March 10 issue of Nature, identified molecular events particularly likely to produce significant regulatory changes in humans.
The researchers confirmed 510 deletions in humans, which are otherwise highly conserved between chimpanzees and other mammals. They report: “One deletion removes a sensory vibrissae and penile spine enhancer from the human androgen receptor (AR) gene, a molecular change correlated with [the] anatomical loss of androgen-dependent sensory vibrissae and penile spines in the human lineage.”
“Another deletion removes a forebrain subventricular zone enhancer near the tumor suppressor gene growth arrest and DNA-damage-inducible, gamma (GADD45G), a loss correlated with expansion of specific brain regions in humans.”
Only two percent of our DNA consists of genes that code for proteins. The rest, previously known as “junk DNA,” helps control and regulate gene activity. It is this regulation that brings about physiological complexity.
Development geneticist Gill Bejenaro of Stanford University and his team compared the genomes of chimpanzees and macaque monkeys, which share a 20-million-year-old ancestor.
They identified regions that had not changed in chimpanzees, and compared these to corresponding parts of the human genome. They found over 500 mutations, known as deletions, lost in humans but present in chimps.
Two deletions were particularly interesting: one near a male hormone-signaling gene and another near a neural development gene. Modifying those genes in mice suggested possible outcomes of the loss: eliminating penile spines and improving the growth of the cerebral cortex.
Bigger brains are obviously an advantage, but it is unclear why the penile spines were weeded out by evolution. Penile spines are barb-like structures found in many mammals. Scientists are uncertain what their true role is, and they may play different roles in different species.
Tiny, hair-like projections found in male chimps and cats can trigger female ovulation, but there is evidence that they can cause damage to the female too. They also increase sensitivity and remove existing sperm, giving males a reproductive advantage. Bejerano suspects the spines contribute to monogamy.
There is also the suggestion that they might have evolved to remove “mating plugs” — material that some male species deposit in the female genital tract to block attempts by other males to fertilize the same female.
David Kingsley of Stanford University, one of the authors of the study, says that it has been proposed these structures can help remove the mating plugs left by other males, indicating that in some mammals with multi-male mating systems there is a race going on to fertilize females.
The researchers believe the loss of these spines may be related to changes in human courtship. According to them, the loss of penile spines would result in less sensitivity and longer copulation, and may be associated with stronger relationships in humans and greater paternal care for human offspring.
According to the study, deletions of tissue-specific enhancers may accompany both loss and gain traits in the human lineage, and provide specific examples of the kinds of regulatory alterations and inactivation events long thought to have an important role in human evolutionary divergence.
