In a recent op-ed piece for the New York Times, British geneticist J. S. Jones asked a provocative question: Has the human species stopped evolving? Are the glory days, when it seemed we were always coming up with something new--opposable thumbs, bigger brains, upright posture-- gone forever? Or will we continue to evolve?
Jones doesn’t think we will. He rightly points out that we’ll never become creatures with either X-ray vision or computers for brains. There is no evolutionary pressure for us to do so--our technology already provides doctors and airport inspectors with X-ray vision, and it may not be long before computers are hooked up to the human brain. In fact, he suggests, the strong pressure applied by natural selection during the evolution of our species has all but vanished. Natural selection once ensured that only the fittest survived and reproduced. Now almost everyone in the industrialized Western world can grow up to have children.
But this does not mean that natural selection no longer operates in humans. True, it may have slackened slightly in the lucky groups Jones talks about, but at most this easing has lasted for less than a hundred years. This is almost no time at all when you realize that the human species has existed for somewhere between 50,000 and 100,000 years and that our prehuman ancestors were evolving for millions of years before that.
Even as we bask smugly in the comforts of Western civilization, natural selection is subtly at work. Consider the evolutionary consequences of the shrinking ozone layer that protects Earth from shortwave ultraviolet light. The Environmental Protection Agency predicts an increase of 12 million malignant melanomas over the next 50 years in the United States alone, many striking people in their reproductive years--a problem that former Interior Secretary Donald Hodel reportedly thought we could fix by simply wearing hats and sun block. In the absence of a better solution, some of us are taking his advice. But hundreds of millions of people on the planet have no access to sun block and must depend for protection on the melanin pigment in their skin. Of course, the amount of melanin in people’s skin varies, leaving some far more vulnerable to UV harm than others. Those with pale, freckled complexions are already 4 to 20 times more likely to develop melanomas, a risk bound to increase as the UV flux increases. Unless we patch the ozone shield, this new kind of selection could weed out the fairest of us all.
Jones also argues that the human species is nearing the end of its capacity for change. He uses as an example the length of the average human life, which increased encouragingly over the last century as a result of improved sanitation, better nutrition, and immunization. Lately, though, this increase seems to have reached a limit. If we can’t be improved any further, he says, then evolution must really have come to a stop for us.
There are two things wrong with this argument. First, it implies that evolution must result in improvement. Species evolve in response to whatever environment they encounter--if short lives were advantageous, shorter life spans would be selected for. Second, Jones confuses changes in our life spans that can be traced to alterations in the environment, such as better hygiene, with changes owing to our genes. It is not true that we are running out of the genetic potential to increase (or decrease) our life span. No despots have ever set out to select for increased or decreased longevity in the populations they control. If they did, the experiment would doubtless work. We already know it works with mice, some strains of which have been bred to have nearly twice the normal mouse life span and others only half. In this regard, humans are no different from the pigs, cattle, and wheat that our farmers subject to artificial selection. As a species we harbor ample amounts of genetic variation on which selection, either natural or artificial, can act.
This variation comes ultimately from mutations in our genes, the stretches of DNA that code for proteins. At a minimum, each child carries among its 100,000 genes two new mutations that can have a detectable, albeit possibly subtle, effect. Since there are more than 5 billion people on Earth, that means there are at least 10 billion new mutant genes flooding into the total human gene pool every generation--adding to the billions and billions of mutant genes that are already there. Many have no immediate consequence, some are catastrophic, and a tiny fraction are beneficial.
Luckily for our species, natural selection is continually sorting out this flood, retaining good mutations, removing bad ones. Because most have slight effects, the selection process is largely invisible. If you carry a gene that reduces by 1 percent the likelihood of your having children, you are unlikely to notice its effects. Even so, this weak selection is enough to reduce the frequency of the gene in the population over time. Indeed, once the gene is rare enough, it might be eliminated entirely if all its carriers fail by chance to reproduce. Conversely, if a new mutant gene increases by 1 percent the likelihood that you will have children, chances are it will stealthily spread through the population. Evolution is going on invisibly all the time.
Jones is not alone, of course, in suggesting that evolution has come to a stop. Others have claimed that it stopped long ago. Cro-Magnon man, who lived in Europe 35,000 years ago, had a skeleton virtually indistinguishable from those of contemporary humans. Dress a Cro-Magnon man up in a power suit and give him a shave, paleontologists like to say, and he could blend into a crowd of Wall Street brokers.
Well, not quite. Cro-Magnon man was a hunter-gatherer, eating a diet very different from our own. He lived in a sparsely settled world where communicable diseases were rare, so his immune system had not evolved to fight them. Like present-day hunter-gatherers who live in remote rain forests and are suddenly flung into the twentieth century, he would be highly susceptible to disease and dietary problems. My guess is that without immunization and a careful diet he would be dead in a week.
Soon, as a result of the Human Genome Project, it will be possible to compare the genotypes of the planet’s last hunter-gatherers with those of people whose ancestors went through the agricultural revolution. We will then be able to see the genetic changes that took place as we adapted to farming and urbanization. I expect them to be profound. They will include genes that permit us to handle changed diets, genes that confer resistance to diseases that multiply in crowded towns, perhaps even genes contributing to behavior more suited to urban than hunter-gatherer existence.
For example, the Pima Indians of southern Arizona, who adopted agriculture relatively recently, appear to have genes that cause their cells to display few active insulin receptors. Among other things, insulin regulates the cells’ uptake of sugar, their principal fuel, from the bloodstream. So the cells of Pima Indians can only use up sugar slowly, and a small amount of fuel lasts a long time. Historically, during long periods of famine, this thrift enabled the Pima to eke out the emergency stores of sugar hoarded in their livers and released into their bloodstream.When food supplies were uncertain, that was an asset. But now that their world has changed to include a plentiful, modern high-sugar diet, it’s a liability. Their bodies are flooded with more sugar than their cells can use or their livers can store, leading to soaring sugar levels in their blood--and appallingly high rates of insulin-resistant diabetes.
In contrast, most of us are descendants of people who survived the agricultural revolution. We have adapted to abundant food, large amounts of sugar, and regular mealtimes by increasing the number of active insulin receptors on our cells. As a result, most of us do not develop diabetes. But if we were ever forced to go back to hunting and gathering, most of us would probably do very badly--only consider the agonies that we go through when we skip a meal.
So evolutionary changes of many different kinds are still going on in the human population. Will we ever control them? Probably not, despite the dawning of the age of gene therapy. At the National Institutes of Health, a team of researchers is attempting to cure a rare fatal genetic condition--severe combined immunodeficiency, or bubble boy disease-- that’s often caused by the lack of an enzyme called adenosine deaminase. The researchers took white blood cells from two girls with the disease, gave their cells the gene for making the missing enzyme, grew them to huge numbers, and transfused them back into the girls. The therapy appears to be working: the girls are no longer as susceptible to the many illnesses that used to threaten them, and their immune systems make normal levels of protective antibodies in response to vaccines (which they did not do before gene therapy).
In evolutionary terms, though, what will the effect of this therapy be? The useful genes in those modified cells can’t be passed to the next generation. So these girls will grow up normally and give their defective genes to their children, which would not happen if natural selection were free to operate. Luckily their future husbands will almost certainly contribute the normal form of the gene to their children, masking the effects of the abnormal one, so the girls’ children won’t develop the disease. But the girls’ mutant genes will continue to survive in their descendants, creating the possibility that many generations later their descendants could marry other people with the mutant genes. If they do, bubble babies will be born who would not have been if natural selection had been allowed to operate unchecked. Overall, though, the defective genes will have virtually no impact on the gene pool of our species, for they will simply join those billions of other mutant genes, new and old, that we are all collectively passing on to our children.
Finally, what about the attribute that most clearly separates us from the other animals, our mental ability. Has that stopped evolving? It’s generally agreed that, among the forces that led to the immense sophistication of the human brain, the most powerful was a kind of feedback loop between the growing complexity of our ancestors’ physical and social environment and the ability of our ancestors to adapt to it. To see how it might have worked, let’s suppose that 50,000 years ago the invention of a new hunting technique or the discovery of a new food source allowed the number of tribes in an area to proliferate, thus creating more opportunities for social interactions--and eventually a rise in the complexity of the language required to deal with such interactions.This situation might have selected for a number of genes, including genes that increase the size of Wernicke’s area, the brain region chiefly responsible for language comprehension. People with such genes would have been better able to profit from the new social activity, perhaps gaining mates, wealth, and power that allowed them to leave more offspring. And a few of the people with these genes might in turn have made society and language still more complex, driving another cycle of the feedback loop.
Now in order for this feedback loop to operate, there must be a great deal of variation from person to person in genes that affect the brain’s capabilities. If there were not, if everyone had exactly the same set of genes controlling the brain’s development, there would be no genetic differences among people on which natural selection could act--and evolution really would come to a stop!
Data showing that such variation exists have been gathered by Thomas Bouchard and his colleagues at the University of Minnesota. They tracked down 56 pairs of identical twins who had been separated at birth and raised apart and gave them a battery of tests that measured IQ, social attitudes, and personality traits. As expected, test scores revealed a great deal of variation within this group of 112 people. But when scores for each pair of twins were scrutinized, the pair often showed an unusual similarity. Because the twins had been raised in different environments, these resemblances had to be due to their shared genes.
Since twinning can happen to any fertilized egg, Bouchard’s twins can be considered a random sample drawn from the nation’s fertilized eggs.What’s true for them is also true for the U.S. population at large. Bouchard calculates that genetic differences contribute between 40 and 70 percent of the total variation in the mental characteristics he tested for.
But why, you may ask, has the enormous increase in complexity of our recent technological environment not had a measurable physical impact on our brains? (Our industrialized brains, after all, are not demonstrably different from those of contemporary hunter-gatherers.) Simply because there has not been time; none of us need go back many generations in our family trees before we find that most of our forebears were peasants leading simple lives quite free of fax machines and the IRS. The rate at which we are changing our environment now has outstripped even the fastest biological evolution. We basically still have peasant or hunter-gatherer brains in a high-tech world.
The ineluctable laws of evolution will continue to operate, probably even more strongly, in the overcrowded, ecologically damaged world of the future. And if things get really bad, the evolutionary consequences could be extreme. Any survivors of a nuclear holocaust or an ecological catastrophe are likely to be a small and highly selected subset of today’s population. Further, we cannot forecast what long-term effects such a disaster would have on the gene-environment feedback loop. If, for example, destruction were so widespread that people could not form viable social groups, the evolution of our descendants would inevitably be driven in the direction of brutishness.
No, human evolution has not stopped, and indeed cannot be stopped. What we must do is change the way we interact with each other and with our planet, so that the evolutionary forces that continue to shape us will not drive us to premature extinction, or rob us of the essence of our humanity.
