Will We Keep Evolving?

By IAN TATTERSALL

We take it for granted that human beings are the pinnacle of the living world, that Homo sapiens is nature's most advanced and magnificently burnished product. And since numerous lines of evidence testify that our species is the result of a long evolutionary history, we tend to assume that in the future there will be more perfecting change along essentially the same lines.

Two million years ago, for example, our predecessors had brains barely half as large as ours today. So it would seem to follow that in another couple of million years, our brains will be twice again as large, housed in the huge globular heads familiar from innumerable sci-fi images. Conversely, our immediate forebears were robustly boned and, we think, more heavily muscled than we are today. What could be more natural than to conclude that supported by increasingly complex labor-saving technologies, our bodies will in future be frailer and shorn of such frivolities as the little toe?

Back in the 1930s, my predecessor at the American Museum of Natural History in New York City, Harry Shapiro, while sensibly warning of the "dangers of prophecy," wondered what humans might become a half-million years hence. His predictions included such features as a rounder skull, a smoothing of the area above the brows, a reduction in the size and number of teeth, and a shrinking of the face in general. Shapiro also predicted that we would get taller and even balder and that body hair would continue to diminish.

When Shapiro revisited the subject three decades later, his vision of the future was essentially unchanged except that he had become increasingly worried about the potential effects of technology. Many today share similar beliefs, assuming, for example, that lives spent in front of computers will rob humans of fully functional arms and legs or proper eyesight. Or that genetic engineering will lead to such calamities as a world populated by Bill Gates clones.

Well, not to worry. As seductive as such extrapolations may be, they overlook what we know about how evolution works. In particular, they buy into the idea that evolution consists of a sort of generation-by-generation fine-tuning in each population as time passes. Under the benign guidance of natural selection (the name we give to any and all factors that promote or inhibit successful reproduction by members of those populations), this process of gradual change inexorably leads to improvement in the species and ultimately to new species as those improvements accumulate.

Superficially persuasive as this view is, it ignores certain basic realities. It assumes, for instance, that organisms are little more than agglomerations of special-purpose mechanisms, each of which can be tracked independently of the "packages" of which they form part. We speak of the "evolution of upright walking" or the "evolution of the hand," often without realizing that legs and hands can only be parts of the story.

The reality is that natural selection can vote up or down only on entire organisms, warts and all. Individual organisms are mind-bogglingly complex and integrated mechanisms; they succeed or fail, economically and reproductively, as the sum of their parts.

It's the same with populations and species. Species are out there competing with others in a real world of limited resources. They cannot survive as disembodied attributes. What's more, the ecologies of which they form a part have an alarming tendency to change abruptly. If your habitat is covered by an ice sheet, it's pretty irrelevant how well you are adapted to the meadows and forests now buried beneath the ice.

Finally, we have to bear in mind how distinctive new species originate. We don't understand everything about how this happens, but we do know that in large interbreeding populations, it is extremely difficult, if not impossible, for new genetic variants to become established. If any meaningful innovations are to become fixed in a population or if a population is to become established as a new species, it is essential that the population be small. Large populations simply have too much genetic inertia.

You can guess where this argument is heading. During the Ice Ages, when our own species emerged, human populations were small and scattered and were continuously disrupted by climatic fluctuations. Conditions were ideal for genetic innovation. Today, however, the human population is 6 billion and mushrooming and increasingly densely distributed. At the same time, individual humans are incomparably more mobile than ever before. Efficient communication means that, for example, American males can advertise for wives in journals distributed halfway around the globe.

The upshot is that after a period of diversification, Homo sapiens is in a mode of reintegration, as witness the fact that the boundaries between the former geographical variants of our species are becoming increasingly blurred. If present trends continue, those boundaries will become blurrier still. Amid all this, the conditions for incorporating meaningful new innovations into human populations have all but disappeared--and with them the prospects for significant evolutionary change.

Of course, such predictions are based on the assumption that current conditions will prevail into the foreseeable future, and it is quite possible that this assumption is wrong. Anything that would serve to fragment the current huge human population might help re-establish the conditions necessary for future human change. Unfortunately, we would undoubtedly perceive such an event as a terrible disaster, since it would necessarily entail the disappearance of billions of human beings.

For example, an asteroid impact of the kind that finished off the dinosaurs might do the trick, as might the appearance of a super-virulent and highly contagious virus. More probable, perhaps, is a man-made catastrophe--a general environmental collapse provoked by overexploitation of the world's resources, say, or nuclear conflicts. The awful possibilities are limited only by the imagination.

This being the case, all we can do is hope that things stay pretty much the way they are--which is both good and bad. The bad news is that we will forever remain the willful, capricious, appalling creatures that we have been ever since our species emerged during the late Ice Ages. The good news is that we will remain the creative, interesting, caring, wonderful creatures that we have been for exactly as long. Over the long haul, human nature will almost certainly stay as idiosyncratic as it is today. It's unrealistic to hope that evolution will ride in on its white horse in the next 50--or even 500,000--years to improve the breed and save us from our follies. We shall have to learn to live with ourselves the way we are.

Ian Tattersall is a curator at the American Museum of Natural History. His latest book, Extinct Humans, is due out in June