SAN FRANCISCO, Jan. 29 -- Researchers have found evidence that indicates the human genome has been deteriorating since the lineage split from that of the chimpanzees some 6 million years ago. While natural selection eliminates the most harmful genetic mutations, the less severe ones remain, some of them forever fixed as part of the human inheritance, the study authors say. What has made Homo sapiens withstand extinction, they say, is the ability to adapt to their environment and their intelligence to overcome most of the unfavorable hands that Nature deals them. 'The number of harmful mutations that arise in each generation has been measured, and it is surprisingly high,' said James Crow of the Department of Genetics at the University of Wisconsin in Madison. 'This supports one theory of why evolution favors sexual reproduction, but the consequences for human health are unclear.' Delving into humanity's distant past, two British investigators set out to separate fact from suspicion in the long-standing hypothesis that humans suffer un unusually high rate of deleterious genetic mutations over the generations. What they found may surprise you. 'Deleterious mutations are occuring continuously. There are two fates for deleterious mutations; they are either eliminated by natural selection, or they become fixed in the population,' Adam Eyre-Walker of the University of Sussex told United Press International. 'Our paper presents two main results: We have shown that the rate at which deleterious mutations occur, which are subsequently eliminated by selection, has been very high since humans diverged from chimpanzees.
It is also high in other hominids (i.e. chimps and gorillas),' he said. 'We have also inferred that humans and other hominids have accumulated many slightly deleterious mutations; that is, many mildly deleterious mutations have become fixed.' Eyre-Walker and study co-author Peter Keightley of the University of Edinburgh estimate 38 percent of the mutations occurring in human generations over the past 6 million years are harmful in so far as they reduce fitness. 'This value is so high (even despite the conservative assumptions made) that if the effects of these mutations reinforced one another in a multiplicative way, it is hard to see how a species such as Homo sapiens, which has a low reproductive rate, could have avoided extinction,' the authors write in the British journal Nature. One possibility is that natural selection has eradicated the most serious of Nature's 'mistakes' in bunches by weeding out individuals with a host of mutations, they say. In an accompanying News and Views article, Crow notes the results also indicate that selection against deleterious mutations is quite weak. 'This is possibly because Homo sapiens have a low effective population size: that is, only a small proportion of a given generation passes its genes on to the next generation, a consequence that enhances random sampling effects at the expense of selection,' he said. The effect may have been that a large number of slightly deleterious mutations have become fixed in the population, Keightley said. If such mutations are accumulating today, they could in the long run leave a mark on man's wellbeing as Nature's efforts at selection are thwarted by humans' higher living standards and, paradoxically, better health care. 'In interpreting our data, several important provisions should be kept in mind,' Keightley told UPI. 'First, we infer there has been an accumulation of slightly deleterious mutations in our genome, but over a very long timescale -- since we split from chimpanzees 6-7 million years ago. We can say nothing about what is going on at present,' he said. 'Second, we do not know what the effects of the mutations are. The effects could be very small and the decline in fitness could therefore be negligible. Third, there may be advantageous compensatory mutations occurring which would ameliorate the effect of deleterious mutations which have been fixed.' So what's humanity's long-term outlook? 'We are of the opinion that ecological factors such as over- exploitation of finite resources and environmental degradation will have a much more profound effect on our survival prospects than any possible genetic degradation,' Eyre-Walker told UPI. ---
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