Saturday, 8 March 2014

Human Evolution is Speeding Up

Human Evolutionary Change 100 Times Higher in Past 5,000 Years

A popular theme on creationist websites is that no-one has ever seen evolution occur. We'll, it's certainly true that no-one has ever seen a creationist parody version of evolution occur, but then they wouldn't do, would they. The parody was designed to be ludicrous; that's the point of parody.

For a successful snake-oil salesman, the trick is to fool your audience into believing the parody is the real thing and that must be the simplest thing in the world to achieve, given that the target audience is not only largely ignorant of science but proudly so and is eager to be fooled because what they crave most is confirmation, no matter how spurious.

An ancillary theme to this is endless debate about if and why human evolution has stopped. Even some serious scientists will wade in with explanations about why human evolution is no longer happening because we've removed all the natural selection mechanisms by isolating ourselves from our environment and even keep people with genetic defects alive long enough to have children when they would have died in childhood just a few years ago. The latter argument is often used either to try to justify eugenics or to smear those who accept the evidence for evolution as secret eugenicists, in some sort of attempt to argue from consequences. Even if it were true, abusing evolutionary theory in pursuit of a social or political agenda does not render the theory wrong anymore so than the undesirability of nuclear war renders atomic theory wrong or the harm which comes from stepping off a high building renders the theory of gravity wrong.

But, for the real meaning of the word 'evolution', not only do we see evolution occurring almost everywhere we look in the natural world but we can see examples of it happening quite recently in lactase persistence, altitude adaptation in Han Chinese on the Tibetan Plateau, and in kuru resistance in a New Guinea tribe.

And as recently as 2007 it was found that human evolution has not stopped or even slowed down, but for the last 5,000 years it seems to have been continuing at a rate some 100 times faster than previously and may even be accelerating and that we are genetically more different to people living 40,000 years ago than they were to Neanderthals.

Genomic surveys in humans identify a large amount of recent positive selection. Using the 3.9-million HapMap SNP dataset, we found that selection has accelerated greatly during the last 40,000 years. We tested the null hypothesis that the observed age distribution of recent positively selected linkage blocks is consistent with a constant rate of adaptive substitution during human evolution. We show that a constant rate high enough to explain the number of recently selected variants would predict (i) site heterozygosity at least 10-fold lower than is observed in humans, (ii) a strong relationship of heterozygosity and local recombination rate, which is not observed in humans, (iii) an implausibly high number of adaptive substitutions between humans and chimpanzees, and (iv) nearly 100 times the observed number of high-frequency linkage disequilibrium blocks. Larger populations generate more new selected mutations, and we show the consistency of the observed data with the historical pattern of human population growth. We consider human demographic growth to be linked with past changes in human cultures and ecologies. Both processes have contributed to the extraordinarily rapid recent genetic evolution of our species.

John Hawks, Eric T. Wang, Gregory Cochran, Henry C. Harpending, and Robert K. Moyzis;
Recent acceleration of human adaptive evolution; Proc Natl Acad Sci U S A. Dec 26, 2007; 104(52): 20753–20758. doi:10.1073/pnas.0707650104

The reasons for this high rate of change seems to be two-fold: very rapid population growth and simultaneous movement into new environments both physical and cultural.

Normally, as a population evolves into a new environment, natural selection ensures it evolves rapidly towards perfection for living in that environment but, as perfection is approached, the opportunities for a mutation to arise which gives greater fitness are reduced, so species tend quickly to reach a stable state of evolution so long as the environment remains stable.

Another factor which can affect the incidence of new mutations, and so the opportunity for an advantageous one arising, is population size. Larger populations improve the number of one in a million mutations or combinations of positive mutation in the presence of others in the same individual happening in each generation, both improving the opportunity for genetic drift or natural selection concentrating this up the improbability gradient towards fixation.

Over the period in question, humans developed agriculture which had four major consequences for our evolution:

  • Population increased rapidly.
  • Populations became settled and crowded into large settlements, so facilitating the spread of parasites.

  • Trade increased contact with other human groups and cultures, so facilitating gene flow throughout the entire range as well as the flow of parasites.

  • Domestication of animals placed us in close proximity to their diseases and parasites, increasing the chance of parasites crossing the species barrier.

One example of how this closer association with parasites, especially those we probably got from domestic animals, can cause evolution, is the incidence of the CCR5 gene which is present in 10% of Europeans and gives a measure of protection against AIDS/HIV. This is thought to be the result of evolved resistance to smallpox which we probably got when the virus causing cowpox in cattle crossed the species barrier into humans.

An example of how this rapidly evolved resistance to diseases gave Europeans an advantage is how it helped them become the dominant world power and culture. As Jared Diamond explained in Guns, Germs & Steel, as they came into contact with new people their parasites, to which they were at least partially resistant, acted as a kind of invisible biological fifth column, devastating and weakening the opposition with killer diseases to which they had no resistance but which in Europe were usually fairly mild childhood diseases like measles. It was the fact that we had lived for longer with more domestic animals than almost any other peoples which gave us and our genes this evolved biological advantage and an invisible weapon of mass destruction to wage biological genocide on our behalf.

It is sometimes claimed that the pace of human evolution should have slowed as cultural adaptation supplanted genetic adaptation. The high empirical number of recent adaptive variants would seem sufficient to refute this claim. It is important to note that the peak ages of new selected variants in our data do not reflect the highest intensity of selection, but merely our ability to detect selection. Due to the recent acceleration, many more new adaptive mutations should exist than have yet been ascertained, occurring at a faster and faster rate during historic times. Adaptive alleles with frequencies under 22% should then greatly outnumber those at higher frequencies. To the extent that new adaptive alleles continued to reflect demographic growth, the Neolithic and later periods would have experienced a rate of adaptive evolution more than 100 times higher than characterized most of human evolution. Cultural changes have reduced mortality rates, but variance in reproduction has continued to fuel genetic change. In our view, the rapid cultural evolution during the Late Pleistocene created vastly more opportunities for further genetic change, not fewer, as new avenues emerged for communication, social interactions, and creativity.

John Hawks, Eric T. Wang, Gregory Cochran, Henry C. Harpending, and Robert K. Moyzis;
Recent acceleration of human adaptive evolution; Proc Natl Acad Sci U S A. Dec 26, 2007; 104(52): 20753–20758. doi:10.1073/pnas.0707650104

So, not only is it wrong to claim that no-one has seen evolution or that here is no evidence for it, and not only is it wrong to claim that human evolution has slowed right down or stopped, but the claims run counter to the evidence. Evolution can be seen to be happening and the rate at which it is happening in humans has been increasing at least over the time scale at which this can be measured with any degree of confidence.

Perhaps more interestingly is that this rapid rate of evolution corresponds with what we know of human cultural and social development and population growth, illustrating neatly how different strands of evidence come together to reinforce the concept of evolution.

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1 comment :

  1. Read more about the book "The 10,000 Year Explosion: How Civilization Accelerated Human Evolution", written by anthropologists Gregory Cochran and Henry Harpending, here:,000_Year_Explosion .


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