Thankfully, the story of human evolution has not ended yet

`The vast majority of newborns face the same appalling child mortality rates as our ancestors'

HUMAN EVOLUTION has taken a holiday, but what we don't know is whether it is a summer vacation or just a long weekend. So says the geneticist Steve Jones, who argues that we in the developed world are now so cosseted from the outside environment that we are no longer subjected to the vagaries of natural selection and the Darwinian struggle for survival. Thanks to modern medicine, central heating and the invention of the bicycle, our physical and mental development has been frozen in time. Take a look in the mirror, suggests Jones, because this is about as good as it gets.

The idea will strike a chord with many people. It is, after all, an attractive thought that we Westerners living in the technological comfort of the late 20th century are at the pinnacle of evolution. This is not, of course, what Jones, professor of genetics at University College London, is saying. But the reason why his commentsarouse such interest is that they pander to our vanity as much as they excite our consternation that evolution may have come to an end.

The very word "evolution" implies - wrongly - some progressive development. Evolution, however, is neither progressive nor regressive - it just happens. The change it brings about is blind and not the result of some designer constantly trying to achieve a higher goal. The fact that evolution can make organisms, including humans, better adapted is merely the result of the unintended selection of some offspring over others.

Inevitably any suggestion that we have entered an evolutionary dead end raises the prospect that we must now have achieved a rather special status from which there is no exit. We know that over a period of about two million years, when the first members of the human family split away from our ape-like ancestors, the volume of the human brain grew explosively in relation to body size. Anything that suggests that our cranial capacity has now reached its upper limit is bound to create collective consternation.

The essence of Jones's argument, which he first gave in a lecture more than five years ago, is that only a tiny fraction of babies - about 2 per cent - now die before reaching adolescence. Death in childhood is normally one of the strongest agents of natural selection; if an individual fails to reach sexual maturity and pass on his or her genes, then he or she is, in evolutionary terms, a genetic cul-de-sac. As a result of modern medicine, this powerful force for evolutionary change has been emasculated.

A problem with Jones's argument is that it applies only to a minority of babies born in the world today. The vast majority of newborns still have to run the gauntlet of the same appalling child mortality rates that confronted our ancestors. Jones carefully confines his argument to the fortunate babies of the developed world, but he would be the first to acknowledge that when it comes to evolution you cannot divide the gene pool into convenient segments; we all belong to the one family of humankind.

This is best illustrated by something raised by Jones when he first lectured on the issue in the early Nineties. He said that in 500 years' time he will be black. He meant that at some time in the next millennium, the professor of genetics at University College London, like just about everyone else in the country, would most likely possess the genes for dark skin. The reason, he said, is that the human gene pool for skin pigmentation is drifting inexorably darker owing to the differential reproduction rates of white and black people in the world. But how does he tie in the statement that human evolution has come to an end with the view that most people will, within several centuries, be dark-skinned?

Another difficulty with the argument that human evolution has ended as a result of falling child mortality is that it does not take into account the hidden mortality of the unborn. More fertilised eggs end up in failure than culminate in a successful live birth. Even in the comfortable West, there are forces at work that discriminate between successful pregnancies and unsuccessful ones.

Even the use of technology could in theory be aiding and abetting evolutionary change. Take intracytoplasmic sperm injection (ICSI), a procedure whereby a defective sperm is artificially injected into an unfertilised egg cell. In the natural world, men who produced defective sperm would leave fewer offspring, and as a result the genes for the sperm defect would dwindle to very low levels.

In the modern world, however, ICSI allows these men to have children, and their children are presumably at risk of inheriting the defective sperm genes of their fathers. Over many generations, these defective genes will increase in frequency (although by how much depends on how often ICSI is used). Inevitably this will result in a greater number of men in the future having to resort to ICSI if they want to become fathers. It could therefore be argued that evolution - albeit by artificial selection - is producing a race of infertile men who have to rely on technology to conceive.

But the biggest objection to the idea that human evolution has come to an end rests on the question of time-scales. Jones tacitly admits this when he suggests that the holiday taken by human evolution may last a weekend or a summer. Evolution can be considered as "ending" for any one species if you choose a suitably short period of time - which can, in reality, be thousands of years long.

We humans can just about manage time-scales of a few decades or so, but we become overwhelmed by the time-scales on which evolution works.

There is, for example, a creature that lives on the ocean floor, called a horseshoe crab, which has not evolved perceptibly for tens of millions of years. Has it come to an evolutionary dead end, or is it just that it is "perfectly" adapted to an environment that doesn't change from one millennium to the next?

Our environment, although it is cushioned by technology, is likely to undergo radical changes in the near future as a result of pollution, overpopulation, global warming and the destruction of the ozone layer. Anyone who knows anything about evolution knows that a fast-changing environment means a fast-changing gene pool and the consequent acceleration in the rate of evolution.

There is, however, one alternative to the scenario of continuing evolution: if humans cannot adapt and survive, we may escape the forces of survival altogether and - like the dinosaurs before us - become extinct.

Then, it could be truly stated that human evolution had ended.