Death in the family tree.

• Author: Tuxill, John
• Position: Global decline of primates - Cover Story

As the human population continues its unprecedented expansion, the populations of more than half of the world's other primates continue their unprecedented decline. Will our demographic explosion amount to a death sentence for our closest relatives?

The little apes would hardly have dominated the patchwork of forest and savanna in which they lived. Two million years ago, east Africa was home not just to lions and leopards, but to saber-toothed cats, giant baboons, and wild pigs as big as buffalo. The apes must have invested a good deal of effort in just trying to stay out of the way. But if we could have watched them foraging in small bands, feeding on fruits and nuts or scavenging meat when they could, we would instantly have recognized several unusual abilities: a preference for walking upright, a high degree of cooperative behavior, and a penchant for using tools - sticks, stones, or bones - in their daily search for food. Those small, vulnerable hominids had embarked on a unique evolutionary experiment - a line of development that would one day confer on their descendants a power without precedent in the entire history of life.

Two million years is only a brief moment in evolutionary time, but the rift that has opened since then between ourselves and our fellow primates - modern apes, monkeys, lemurs, and lorises - is momentous on any scale. We humans share 98.4 percent of our gene pool with chimpanzees; only 1.6 percent of our genome is uniquely ours. But that seasoning of distinctly human DNA has, in a sense, catalyzed a reversal of our ecological role. We are no longer molded by the ecosystems in which we live - we mold them. Increasingly, however, we are learning that there is a price to pay for our ecological dominance - and no one is paying more heavily than our closest relatives.

The Demographic Gap

Consider the arithmetic of our success. When our species first emerged some 100,000 to 150,000 years ago, hominids (the group containing us, our direct ancestors, and their closest relatives) were still a minor branch on the primate family tree. Some primates - particularly little ones like the marmosets of South America - an number in the tens of millions. But for many thousands of years, the early human population probably totaled no more than 5 to 8 million widely-dispersed souls. As our ancestors moved into new regions and learned how to exploit natural resources more intensively, our numbers slowly expanded. By the end of the...