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The question of whether modern humans are evolving has not gone away, though. Scientists recently added to a growing pile of papers that indicate human evolution is not only continuing, but may be accelerating at an unprecedented pace.

Where this will take us has become one of the most contentious questions in evolutionary biology. “We’ve been almost indoctrinated with this notion that human evolution stopped long ago,” says Henry Harpending, an anthropologist at the University of Utah. In an essay entitled ‘The Spice of Life’ published in 2000, the late evolutionary biologist Stephen Jay Gould reinforced the idea that thanks to improvements in medicine, shelter and, for many, plentiful food, humans had all but stepped off the evolutionary ladder.

“Natural selection has almost become irrelevant,” he wrote. “There’s been no biological change in humans in 40,000 years or 50,000 years. Everything we call culture and civilisation we’ve built with the same body and brain.”

Harpending’s group published details of a study that asked how much humans have evolved in the past 80,000 years, a period that includes the exodus of humanity from Africa. The answer, they concluded, was an awful lot.

They identified a rapid increase in evolution, as our ancestors adapted first to harsh latitudes with miserable climates, then to farming, which revolutionised the human diet. His group studied the DNA of four distinct groups around the planet: Japanese, Han Chinese, Europeans and Yoruba in Africa.

They found that nearly 2,000 genes, or 7 per cent of the genome, have been subjected to recent natural selection.

Evolution in its most basic sense is merely a shift in how common certain variants of genes are in a population. But evolution can occur through different processes.

Natural selection drives evolution if a gene improves an organism’s ability to pass on its DNA — usually by having children. But genes can also become more common if they cause traits or behaviours that are more attractive to the opposite sex.

One gene, called G6PD, is emerging in African populations.

Although it harms people by causing anaemia, it harms malaria more, and so confers a net benefit. Likewise a gene called CCR5-32, which improves resistance to HIV infection, is becoming more prevalent.

Harpending isn’t the only one to have found evidence of recent evolutionary change. Bruce Lahn, a geneticist at the University of Chicago in Illinois, has reported evolutionary changes in the brain, caused by two new genes involved in brain growth that emerged around 37,000 and 5,800 years ago.

But while other scientists agree that evolution continues, they are divided on the role played by natural selection — or whether it has a role at all.

In his book The Language of the Genes, the University College, London, geneticist Steve Jones argues that human evolution has slowed dramatically since we settled and developed farming.

The reason, he says, is that we have insulated ourselves from the upheavals we faced in the distant past which only the hardiest survived.

Another huge change is an averaging-out of the number of children people have.

Natural selection can only occur when people have different numbers of children, because if everyone has the same number, all genes are equal.

But despite natural selection losing much of its power over us, Jones concedes we are still evolving by other mechanisms. Modern life can in fact drive degenerative or reverse evolution, where genes that make us resistant to starvation, or to once life-threatening disease and infections, and even ones that give us good sight and hearing, lose their value and disappear.

As for the future, few scientists are willing to speculate on how humans will change, but there is agreement that mass transport will drive unprecedented mixing between previously isolated populations.

“People often think evolution means greater divergence, but now it’s going to become a grand homogenisation, a triumph of the average,” says Jones.

“The essence of human beings is their intelligence and at the present time people have an enormous range of different abilities.

My prediction is one of the ways we’ll evolve is to add genes that increase our range of abilities,” he says. — Guardian Newspapers Limited 2008

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