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Pedal to the metal for human evolution

Mass migrations, the population explosion, and city living fuel rapid rate of genetic change

We can’t see it in our short lifetimes, but human evolution is going faster than ever before. Today, the rate of our evolution is over 100 times faster than it was in the depths of time, according to a paper published in PNAS called “Recent acceleration of human adaptive evolution.”

Dr. Harpending, an anthropologist at the University of Utah and co-author of the paper, says that the findings have come as a jolt to the scientific community.

“Everyone has been so surprised that no one has broken down the implications yet,” he said.

Our accelerated evolution is powered in part by population. Over the past 80 millennia, human population has increased steadily, and with it the pool of mutated genes we carry. Most mutations are not beneficial, leading to weaker or less “fit” individuals who don’t have as many offspring. But when a mutation is helpful, it gets passed on to the next generation, mixed into the greater population, and can become common. The more people, the more helpful mutations appear, and evolution moves faster – from a crawl to a creep.

Aside from population, our environment has caused accelerated evolution as well. When human beings first expanded out of Africa around 60,000 years ago, they were faced with new environments and new challenges. Habitats like tundra or mountainous regions changed the terms of human life, and therefore impacted the content of human DNA. For instance, whereas in Africa people who could brave the noonday sun survived, in Siberia, survival depended on weathering the cold. Such differences caused different mutations to be favoured, and different traits to evolve.

Yet perhaps the most radical new environment was the city, made possible by agriculture. With cities came high population densities, and with high densities came deadly epidemics. In turn, those who survived the epidemics became resistant to the disease, and this was sometimes conferred by new mutations. The paper in PNAS found that, of the 1,800 genes changed during the last 80,000 years, many were related to fighting off illness.

Dr. Bisson, Department Chair of Anthropology at McGill, says that while it’s true that there have been huge changes in where and how humans live, some might still debate whether the evolutionary acceleration is as recent as the paper suggests.

“Humans as biological entities have responded through evolution to particular ecological circumstances, which have indeed changed in the past 80,000 years…but some paleontologists might disagree and argue for a longer time span,” he said.

The paper’s results were obtained by a simple method called linkage disequilibrium. Human DNA doesn’t occur as a single long strand, but a set of shorter strands called chromosomes. Everybody has two sets of chromosomes: one from the father’s side and one from the mother’s side. When a sperm or egg is generated, chromosomes from both parents line up and swap small chunks of DNA. The swapped chunks may contain identical genes, or different versions of the same genes.

A new version of a gene created by a mutation has specific neighbouring genes. When it’s swapped, it gets new neighbours on the new chromosome. If the mutation is old, then enough swapping will have happened that the gene appears with all sorts of neighbours. If the mutation is new, then the gene will always be found with the same neighbors, because not much swapping has happened. By this method, young genes can be tracked down.

The investigators found far more young genes than they had expected to find, and concluded that the rate of human evolution is accelerating. Compared to our near evolutionary cousins, such as chimpanzees, we are evolving very rapidly. Where our fast evolution will take us is something that remains to be seen – in a hundred thousand years or so.