Continuing our series of Big Science Questions, Mark Buchanan asks if we are in a post-evolutionary age, and Michael Ruse looks at our role in change.
It is tempting to suppose that in this modern era - "postmodern" even, some might say - the human species has grown up and left its slightly embarrassing evolutionary history far behind. So what if we share 98 per cent of our DNA with the chimpanzee, our obvious evolutionary cousin. With modern science and technology, haven't we entered into an altogether "new" phase of history? Doesn't that 98 per cent point to the distant past, while that special 2 per cent - the part that accounts for that big human brain - points boldly to the future? Hasn't the blade of Darwinian evolution been dulled by our immense capacity to learn and adapt?
Between 1700 and 1900, life expectancy in Britain soared from 17 to 52 years, mostly because of better nutrition and hygiene, cleaner air and drinking water. That was just the prelude to the feats of modern medical science. In the 20th century, the death rate from infectious diseases in the US fell by a factor of many thousands, and in 1977, small pox was eradicated worldwide. We have artificial limbs, transfusions, pacemakers and heart transplants, and we may soon have artificial tissues and organs. Scientists have mapped out the human genome, as well as those of the microbes that would threaten us. As Barry Bloom of the Harvard School of Public Health noted recently: "Every gene of every human pathogen can be displayed in the computer screen of every student and researcher."
This will multiply our adaptive skills still further.
From the biological perspective, organisms evolve by natural selection:
"fitter" individuals tend to live longer and have more offspring, thereby sending more of their genes into the next generation. Meanwhile, the genes of the less fit tend to get weeded out. But what if medical science gets in the way? Science has largely rendered a harsh world benign, strengthened the naturally weak, and "artificially" levelled the playing field of human reproduction. But does this really mean that we have stopped evolving?
The answer is no - although the details depend on what we mean by "we". In the past decade, researchers have discovered that Tibetan villagers living more than 4,000m above sea level are genetically adapted to do so. At least one extra gene helps their blood cells bind more oxygen and appears to boost their reproductive fitness under low-oxygen conditions. Within the past 10,000 years or so, these people have evolved genetically to be adapted to their environment. Many must have passed by the way because they could not hack it.
This would not be surprising. Over the past 25 years, 1,173 people have scaled Mount Everest. Some used extra oxygen, others relied on that available in the free air. Many died during the subsequent descent, but those who used supplementary oxygen were nearly three times more likely to survive than those without. It is easy to imagine that the Tibetans had to evolve over many generations to stay where they were.
But most of us are not Tibetan. If "we" means readers of The THES , then "we" are also by and large not living in the third world, where about 25 per cent of all deaths result from infectious diseases such as tuberculosis, pneumonia, Aids, malaria and measles. Most of the world's population remains at war with the environment, as their genetic defences battle against the genetically tuned weaponry of countless microbes. There can be no suspicion here that evolution has come to an end.
Only if "we" refers somewhat self-consciously to readers of The THES and others in the developed West does it become interesting to wonder if evolution has halted. Geneticist Steve Jones of University College London argues that modern medicine has largely taken natural selection out of the picture - because people live longer and die differently. In the US, more than 50 per cent of all deaths now result from heart disease and cancer, which tend to strike late in life and after the reproductive years. The percentage of Americans dying of heart disease has risen fourfold since 1900, mostly because people no longer succumb to infectious disease. This backs up Jones's argument that evolution might have stopped - or, if not, that it might at least be slowing.
Other scientists, however, suggest that we in the West are not only still evolving, but that we are doing so faster than before. Biologist Christopher Wills of the University of California at San Diego suggests that mankind is being subjected to new environmental pressures such as global warming and holes in the ozone layer. Moreover, he adds, the technology that has made possible mass international travel promotes immigration flows and the mixing of gene pools that for millennia have been far more isolated.
From any theoretical perspective, it seems, the answer to the question "are we still evolving" is "yes". As anthropologist Meredith Small of Cornell University argues: "Humans haven't really changed the rules of natural selection. We might think that because we have culture - and with it all kinds of medical interventions and technologies - that we are immune from natural selection, but nature proceeds as usual. Some people live and some people die, and some people pass on more genes than others."
By the definition of evolution, then, we are still evolving. But what is more interesting is the question of how we are evolving and, in particular, how our culture and our science affect this evolution. If you work with your hands, your skin toughens. This is not evolution, but adaptation - of the simplest kind. The bluehead wrasse, Thalassoma bifasciatium , is a coral reef fish that lives in large schools made up mostly of females. There are at any time only a few dominant males. If one of these is removed, one of the females will change sex and become a male, restoring the proper ratio. The genes of this fish remain the same - this is adaptation of a rather more sophisticated kind.
Human culture likewise is not stored in the genes - it resides in social structures and habits, in language and in the library, and it has to be taught by one generation to the next. Culture, science included, is a learned adaptation of our society and, strictly speaking, not a direct consequence of evolution (although it is a product of our larger brains). Some years ago, the historian E. H. Carr pointed out the misunderstanding that can result from "confusing biological inheritance, which is the source of evolution, with social acquisition, which is the source of progress in history". That acquisition can be in the form of things such as financial wealth or learning; in any case, it affects the future irreversibly, while leaving the genes untouched - at least for a time.
But culture does have an impact on biological evolution, and Wills may be right that the effect is occurring faster than ever. What are the myriad forces that make some people have children and others not? Disentangling the influences is very difficult, but these cultural effects have a far quicker impact on the genetics of the human population than do the fairly rare genetic defects that underlie many diseases. Small also points out that, at the national level, economic and technological development tend to lead to a marked fall in birthrate. The regions with the highest birth rates are now Latin America, Africa and Asia. These populations are contributing most to the gene pool of the human future. "Culture may not seem a 'natural' force," Small says, "but because it is part of our environment, it is just as natural as disease, weather or food resources."
In the distant past, evolutionary forces favoured those with greater mental capacity - those who could devise better tools and make better decisions. Wills argues that for humans, there is little reason to suspect that this has changed. On the other hand, it could be that higher intellectual capacity tends to be linked to an awareness of problems such as overpopulation, and could lead to a fall in the average number of offspring of the highly intelligent. If so, then evolution would be working in the opposite direction.
Where is the future leading us? The final bastion of biological evolution is the gamete - the sexual cell. No mutation or other manipulation of the physical organism gets carried on to its offspring unless encoded in the DNA of the gamete. How long will this remain true? When will some variant of gene therapy create the means - and the well-conceived desire - to conscious manipulation of the DNA that is the gene pool? When this happens, and it is only a matter of time, the distinction between biological and cultural inheritance will be over for the human species. Even then, of course, we will still be evolving.