* 12:48 25 January 2005
* NewScientist.com news service
* Will Knight
A lack of mates among human ancestors that lived million years ago has left modern humans more vulnerable to genetic disease, a new study suggests.
Researchers compared samples from the genomes of more than 1000 people with those of chimpanzees to see how much genetic mutation has occurred in the two species since they diverged from a common hominid ancestor, about six million years ago. They also made comparisons with another closely related pair of species, rats and mice.
They focused on portions of DNA close to protein-coding genes. These segments are thought to regulate the activation of these genes.
The researchers calculated that these stretches of human and chimp DNA contained approximately 140,000 non-advantageous mutations, higher than expected and well above the number of retained genetic mutations seen in rats and mice. The mutations occur naturally but make both chimps and humans more susceptible to diseases with a genetic basis, such as cancer.
The researchers believe the high rate of mutations is seen because the hominid ancestor to both species went through an evolutionary bottleneck, when its breeding population was limited to only about 10,000 individuals.
This meant that the process of pruning out damaging mutations via natural selection of the fittest mates was more difficult and slower. In contrast, rats and mice have descended from a much larger population, leaving them less susceptible to genetic diseases.
Adam Eyre-Walker, a member of the research team at the University of Sussex, UK, says the phenomenon is comparable to the genetic problems experienced by severely endangered species, in which inbreeding can accelerate extinction.
"The process happens in all species with fairly small populations," he told New Scientist. "But we've probably escaped our genetic fate by having a few advantageous mutations that have been functionally so successful," such as those that led to large brains and the development of language.
Eyre-Walker adds that humans have since halted the genetic deterioration by huge expansion of the global population, although selective pressures are less severe due to better healthcare.
Pinnacle of evolution
Martin Leecher, a member of the team based at the University of Bath, UK, adds that the results show the relative fragility of the human evolutionary line. "We're used to viewing us as the pinnacle of evolution," he says. "Seeing that rodents control their genes much more precisely is somewhat sobering."
Hendrik Poinar, a molecular evolutionary geneticist at McMaster University, Canada, says that the results are intriguing. But he adds that it is difficult to draw firm conclusions about the evolution of the human genome, as there may be other factors involved.
"It is hard to estimate the effect a bottleneck would have had on our populations in the past as we have poor knowledge of how large or small the populations were, and how they expanded and contracted," he told New Scientist.