Modern times causing human evolution to accelerate
14 December 2007
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David Holzman
Human evolution is speeding up. Around 40,000 years ago our genes began to evolve much faster. By 5000 years ago they were evolving 30 to 40 times faster than ever before and it seems highly likely that we continue to evolve at this super speed today.
Our population explosion and rapidly changing lifestyles seem to be the drivers of this acceleration, the discovery of which contradicts the widely held notion that our technological and medical advances have removed most of the selection pressures acting upon us.
This stunning insight into humanity's development comes from a wide-ranging study of human gene variants gathered by the international HapMap project. Investigators led by John Hawks of the University of Wisconsin, Madison, studied 3.9 million simple differences in DNA called single nucleotide polymorphisms (SNPs, pronounced "snips") from 270 individuals, including people of Han Chinese, Japanese, Yoruban and northern European extraction. This revealed several pieces of evidence that provide clear support for the idea that human evolution is accelerating.
The first is that the genomes of the people within the study group contain a relatively high number of new genetic traits, marked in the genome by the presence of a relatively new SNP. Such SNPs are known to be linked to particular genes and affect their activity, and these mutations have been linked to significant changes in our lifestyle. For example, we evolved greater resistance to the cold as people migrated north out of Africa, and to infectious diseases such as smallpox, yellow fever and typhus, which became important killers when we began settled living. The advent of agriculture and changes to our diets also influenced our genome.
This high rate of mutation was caused by the explosive increase in the population - as more people are born, more mutations can be introduced into the gene pool. Darwin himself predicted that larger populations would evolve more quickly than smaller ones, something that has since been shown in insects and bacteria.
However, if humans had always evolved at the same fast pace, you would then expect to see relatively few SNPs surviving today, as selection would have weeded out most of the unfavourable genes they are linked to. "But when we look at the genome, we see that the variation is relatively high," says Hawks. In fact, the researchers managed to confirm that around 1800 genes, or roughly 7 per cent of the total in the human genome, have changed under the influence of natural selection within the past 50,000 years, a figure they first revealed in 2005 after conducting two similar but smaller genetic analyses. That is roughly the same proportion of genes that were altered in maize when humans domesticated it from its wild ancestors.
That high level of variation means our rate of evolution must have speeded up considerably, as there has not been time for many SNPs to be selected out. The researchers say this started around 40,000 years ago. Evolution then continued to accelerate until a peak, which they found occurred in Europeans and Yoruban Africans 5250 and 8000 years ago respectively (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0707650104). However, these dates are almost certainly artificial. The researchers believe it is likely that evolution has continued apace, but too little time has elapsed for more recent adaptive mutations to emerge in the study's sample.
The research also explains why there are just 40,000 or so differences in the number of adaptive SNPs seen between humans and chimps. If humans had always been evolving at a constant rate, instead of undergoing a recent acceleration in evolution, then this number would be in the millions.
“If humans always evolved at a constant rate, then the genetic differences between us and chimps would be far greater”
These findings flout the conventional wisdom that humans had reached their fully modern form in the mid-Palaeolithic, as stated in textbooks, says anthropologist Clark Larsen, of the Ohio State University in Columbus, who was not involved in the study.
"People have always thought that the force of selection had decreased, because it became easier to survive," says Hawks. In fact, he says, disease, population growth, sedentary lifestyles, and changes in diet, technology and social group size have greatly increased the forces of selection.
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From issue 2634 of New Scientist magazine, 14 December 2007, page 8-9
