The British geneticist Steve Jones has recently given a lecture at University College London entitled “Is human evolution over?” His answer to his own question is in the affirmative. I agree with Jones that human evolution has pretty much stopped, but for entirely different reasons.
His argument is that human evolution, at least in the western societies, has stopped or slowed down because very few older men in such societies reproduce. Sperm of older men carry many more mutations than those of younger men. Mutations provide the source of genetic variations on which natural selection works. Hence, no older fathers, no genetic mutations, no evolution.
Jones may be right; however, I think he is underestimating how long ago human evolution stopped. I think it stopped roughly 10,000 years ago, with the advent of agriculture.
Evolution takes many generations, and so the speed of evolution of a species is relative to how long it takes for individuals of the species to mature sexually and start reproducing (holding constant, for the moment, the other important determinant of the speed of evolution, the strength of selection pressure). Evolution happens faster for fast-maturing species and slower for slow-maturing species. Fruit flies are one of the fastest-maturing species in nature, and humans are one of the slowest. It takes only seven days for fruit flies to mature sexually under ideal conditions, whereas it takes 15 to 20 years for humans. It means that there can be more than 50 generations of fruit flies in one year, before a human baby can even begin to walk. There are more than a thousand generations of fruit flies in one human generation (20 years), for which human need more than 20,000 years. Evolution for fruit flies can happen pretty fast, which is precisely the reason why they are the favorite species for geneticists to study. Human evolution happens much, much more slowly. No human scientists can see it in action the way they can observe fruit fly evolution unfold in the lab.
Natural selection under most circumstances requires a stable, unchanging environment for many, many generations (once again, unless the selection pressure is enormously strong). For example, if the climate is very cold for centuries and millennia, then gradually individuals who have better resistance to cold will be favored by natural selection, and their neighbors who have less resistance to cold (who are more adapted to a hot climate) will die out before they can leave many children. This will happen generation after generation, until one day all humans have great resistance to cold. A new trait – resistance to cold – has now evolved and become part of universal human nature. But this trait could not have evolved if the climate was cold for one century (only five human generations, albeit 5,200 fruit fly generations) and then hot for another century, only to be cold again in the third century. Natural selection would not know who (with which traits) to select.
Since the advent of agriculture about 10,000 years ago and the birth of human civilization which soon followed, humans have not had a stable environment against which natural selection can operate. For example, a mere two centuries (10 generations) ago, the United States and the rest of the Western world were largely agrarian; most people were farmers. In the agrarian society, men achieved higher status by being the best farmers; those who possessed certain traits that made them good farmers had higher status and thus greater reproductive success than others who didn’t possess such traits.
Then, only a century later, the United States and Europe were predominantly industrial societies; most men made their living working for factories. Traits that make men good factory workers (or, better yet, factory owners) may or may not be the same as the traits that make them good farmers. Certain traits – such as intelligence, diligence, and sociability – probably remained important, but others – such as a feel for nature, the soil, and animals, and the ability to work outdoors or forecast weather – ceased to be important, and other traits – such as punctuality, the ability to follow instructions, a feel for machinery or mechanical aptitudes, and the ability to work indoors – suddenly became important.
Now, only one century later, we are in a post-industrial society, where most people work neither as farmers nor factory workers but in the service industry. Computers and other electronic devices become important, and an entirely new set of traits is necessary to be successful. Bill Gates and Sir Richard Branson (and other successful men of today) may not have made particularly successful farmers or factory workers. All of these dramatic changes happened within 10 generations, and there is no telling what the next century will bring and what traits will be necessary to be successful in the 21st century. We live in an unstable, ever-changing environment, and have done so for about 10,000 years.
For hundreds of thousands of years before that, our ancestors lived as hunter-gatherers on the African savanna and elsewhere, in a stable, unchanging environment to which natural selection could respond. That is why all humans today have traits that would have made them good hunter-gatherers in Africa – men’s greater spatiovisual skills, which allowed them to follow animals on a hunting trip for days and for miles without a map or a global positioning device and return home safely; and women’s greater object location memory, which allowed them to remember where fruit trees and bushes were and return there every season to harvest, once again without maps or permanent landmarks.
For the last 10,000 years or so, however, our environment has been changing too rapidly for evolution to catch up. Evolution cannot work against moving targets. That’s why humans have not evolved in any predictable direction since about 10,000 years ago.
I hasten to add that certain features of our environment have remained the same – we have always had to get along with other humans, and we have always had to find and keep our mates – so certain traits, like sociability or physical attractiveness, have always been favored by natural and sexual selection. But other features of our environment have changed too rapidly relative to our generation time, in a relatively random fashion – who could have predicted computers and the internet a century ago? – so we have not been able to adapt and evolve against the constantly moving target of the environment.