Death in the family tree.

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 last Ice Age 10,000 years ago, the human population might have reached 10 million. As consummate hunter-gatherers, we had become the most widespread primate - the only one to colonize both hemispheres. But there was still at least one other group of large primates that outnumbered us: the adaptable baboons of Africa, whose natural densities and range suggest a prehistoric population on the order of 20 to 40 million.

The total number of humans probably remained in the tens of millions for several more millennia, until agriculture had become a major social force. But by the time of Christ, humans had probably become the most numerous primate species, when our population passed 250 million, on its way to 1 billion by the early 1800s. Our numbers likely eclipsed all other primate species combined by 1930, when the human population climbed past 2 billion.

Since then, the demographic gap between ourselves and the 232 other known primate species has widened even more dramatically. As our population has soared to nearly 6 billion, other primate populations have dropped sharply, and the continued survival of many species is now in doubt. According to the World Conservation Union (IUCN), primates are now the most imperiled major order of mammals. Nearly half of all primates are already threatened with extinction, and the IUCN considers another one-fifth "near-threatened." In general terms, the reasons for the declines are no mystery: they all relate, directly or indirectly, to human actions. But we will need to understand the relationship between ourselves and our closest relatives in detail, if we are to secure the safe passage of primates - and the rest of our planet's biodiversity - across the human demographic explosion.

The Homeless

In general, primates are an adaptable bunch. Some, like the hamadryas baboon of the Arabian peninsula, forage in deserts, while others, like Tibetan macaques, thrive on cold mountain slopes. But most species retain a strong affinity for the habitat in which primates first evolved: the verdant belt of tropical forest that hugs the Earth's equator. The fate of these forests will therefore largely determine the fate of most primates. And more and more of these forests are losing their ecological integrity, as they are logged, colonized, and cleared for agriculture. All told, forest loss and other forms of habitat degradation are a factor in the decline of nearly 90 percent of threatened primate species, making this the single biggest problem they face.

Not surprisingly, the highest concentrations of endangered primates occur in "hotspots" of tropical forest loss - the places where forest communities are disappearing most rapidly [ILLUSTRATION FOR MAP OMITTED]. In south and east Asia, the IUCN has classified a full 90 percent of all primate species as threatened or near-threatened. In Indonesia and Malaysia, orangutans - the most arboreal of the great apes and, therefore, the most dependent on trees - have lost over 80 percent of their forests in just 20 years. ("Orangutan" is Malay for "man of the forest" - see sidebar, page 18.) Vietnam's Tonkin snub-nosed monkey may now be the rarest primate in the world. Hunting and the loss of nearly 90 percent of its lowland rainforest habitat since 1950 have reduced it to a handful of populations totaling fewer than 200 individuals, all of them living in forest patches outside the country's national parks and nature reserves.

In Madagascar, another hotspot, the odds of extinction are rising not just for individual species, but for an entire evolutionary lineage millions of years in the making. Madagascar's long geologic isolation allowed evolution to take a separate course there: instead of monkeys or apes, the island's primates consist entirely of lemurs. (Lemurs look a little like a cross between a monkey and a raccoon.) Of all the primates alive today, lemurs are thought to be the most "ancient" - the group that most closely resembles the earliest primates. All of Madagascar's primates are endemic to Madagascar and to the nearby Comoros Islands (that is, they are found nowhere else). Since the arrival of humans roughly 1,000 years ago, Madagascar has lost at least 80 percent of its old growth forest. Even before the first Europeans set foot on the island in 1500, at least 15 primate species had gone extinct - including the giant Megaladopsis, which was as large as a female gorilla. Of the surviving 30 or so species, fully two-thirds are threatened with extinction, primarily because of forest loss. One of the highest primate conservation priorities in Madagascar today is the aye-aye, a bat-eared, bug-eyed nocturnal creature that fills the ecological role of a woodpecker, extracting grubs from decaying wood with its long, crooked middle finger. It was once thought to be on the threshold of extinction, but scientists now believe several thousand aye-aye may remain in widely scattered locations.

Even Madagascar's forests are in better shape than the forest along the coast of southeastern Brazil, where only vestiges remain of a rich biome that once blanketed nearly a million square kilometers. Centuries of colonization and agriculture have reduced Brazil's unique Atlantic forest to little more than a few tattered patches; only 1 to 5 percent of the original forest still stands. Collectively, the Atlantic forest monkeys form the world's most endangered primate assemblage: 11 of the forest's 12 endemic species are right on the edge of extinction. For example, the 12- to 15-kilogram muriqui, the largest of all New World primates, has an estimated population of only around 500. The Atlantic forest is also home to the tiny lion tamarins, four closely related species whose striking manes and buoyant mannerisms seem so incongruous in the face of their dwindling numbers; their populations range from 250 to 2,500.

But lemurs, tamarins, and other endemics are not the only primates threatened by habitat loss. Even the generally more adaptable widespread species are suffering. The red colobus monkey, for example, is distributed across the entire equatorial waist of Africa, from Senegal to eastern Kenya. Yet according to the IUCN's Primate Specialist Group, it is declining everywhere, and nine of its 14 recognized subspecies are threatened with extinction. One of the most endangered subspecies occurs only along the lower Tana River in eastern Kenya, where a large-scale hydroelectric project financed by international aid agencies is threatening the riverine forests upon which the colobus depends.

In Japan, the macaques find themselves in a similar plight. These monkeys are at home in both the warm temperate forests of southern Japan and the snow-bound northern mountains, where some troops are famous for congregating in natural hot springs to ward off the winter cold. In recent decades, however, Japanese macaques have steadily lost habitat to urban development, agricultural expansion, and the spread of monoculture Japanese cedar plantations. Deprived of natural food supplies, the macaques have turned to raiding orchards and fields, earning the wrath of local farmers. Of the roughly 50,000 macaques remaining in Japan, about 5,000 are captured or killed each year in what some researchers have called a "civil war" between wild monkeys desperate for food and farmers determined to protect their produce. Japan has a land area smaller than California's but its population (126 million) is more than four times as large. Unless the country forges a stronger consensus on the need to restore its native forests, the math doesn't look good for the macaques.

The Hunted

Even where primates still find ample habitat, they often face heavy hunting pressure - a serious threat to at least one in every three threatened species. Primates are usually hunted for the cooking pot: we are the primate clan's leading predator and, except for chimpanzees, the only primate that routinely hunts other primates for food. While some cultures have a long history of subsistence hunting, the expansion of many rural populations is radically increasing the ecological pressures that the hunting creates. Ecologist Carlos Peres has documented how over a one-and-a-half year period, a single family of Brazilian rubbertappers in rural Amazonia killed 380 large primates - wooly monkeys, spider monkeys, and howler monkeys. With that kind of hunting effort, it doesn't take all that many people to empty a forest.

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Who would feel any great surprise at hearing that . . . one of the fossil monkeys was few in number compared with one of the now living monkeys? And yet in this comparative rarity we should have the plainest evidence of less favourable conditions for their existence. To admit that species generally become rare before they become extinct - to feel no surprise at the comparative rarity of one species with another, and yet...to marvel greatly when a species ceases to exist, appears to me much the same as to admit that sickness in the individual is the prelude to death - to feel no surprise at sickness - but when the sick man dies to wonder, and to believe that he died through violence.

Charles Darwin, Voyage of the Beagle