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.

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