The World Without Us

Olduvai Gorge and the other fossil hominid sites, together comprising a crescent that runs south from Ethiopia and parallels the continent’s eastern shore, have confirmed beyond much doubt that we are all Africans. The dust we breathe here, blown by zephyrs that leave a coating of gray tuff powder on Olduvai’s sisals and acacias, contains calcified specks of the very DNA that we carry. From this place, humans radiated across continents and around a planet. Eventually, coming full circle, we returned, so estranged from our origins that we enslaved blood cousins who stayed behind to maintain our birthright.

Animal bones in these places—some from hippo, rhino, horse, and elephant species that became extinct as we multiplied; many of them honed by our ancestors into pointed tools and weapons—help us know how the world was just before we emerged from the rest of Mammalia. What they don’t show, however, is what might have impelled us to do so. But at Lake Tanganyika, there are some clues. They lead back to the ice.

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The lake is fed by many streams that pour off the mile-high Rift escarpment. At one time, these dropped through gallery rain forest. Then came miombo woodland. Today, most of the escarpment has no trees at all. Its slopes have been cleared to plant cassava, with fields so steep that farmers are known to roll off them.

An exception is at Gombe Stream, on Lake Tanganyika’s eastern Tan-zanian coast, the site where primatologist Jane Goodall, a Leakey assistant at Olduvai Gorge, has studied chimpanzees since 1960. Her field study, the longest anywhere of how a species behaves in the wild, is headquartered in a camp reachable only by boat. The national park that surrounds it is Tanzania’s smallest—only 52 square miles. When Goodall first arrived, the surrounding hills were covered in jungle. Where it opened into woodland and savanna, lions and cape buffalo lived. Today, the park is surrounded on three sides by cassava fields, oil palm plantations, hill settlements, and, up and down the lakeshore, several villages of more than 5,000 inhabitants. The famous chimpanzee population teeters precariously around 90.

Although chimps are the most intensely studied primates at Gombe, its rain forest is also home to many olive baboons and several monkey species: vervet, red colobus, red-tailed, and blue. During 2005, a Ph.D. candidate at New York University’s Center for the Study of Human Origins named Kate Detwiler spent several months investigating an odd phenomenon involving the last two.

Red-tailed monkeys have small black faces, white-spotted noses, white cheeks, and vivid chestnut tails. Blue monkeys have bluish coats and triangular, nearly naked faces, with impressive jutting eyebrows. With different coloring, body size and vocalizations, no one would confuse blue and red-tailed monkeys in the field. Yet in Gombe they now apparently mistake one another, because they have begun to interbreed. So far, Detwiler has confirmed that although the two species have different numbers of chromosomes, at least some of the offspring of these liaisons—whether between blue males and red-tailed females or vice versa—are fertile. From the forest floor, she scrapes their feces, in which fragments of intestinal lining attest to a mix of DNA resulting in a new hybrid.

Only she thinks it’s something more. Genetics indicate that at some point 3 million to 5 million years ago, two populations of a species that was the common ancestor to these two monkeys became separated. Adjusting to distinct environments, they gradually diverged from each other. Through a similar situation involving finch populations that became isolated on various Galápagos islands, Charles Darwin first deduced how evolution works. In that case, 13 different finch species emerged in response to locally available food, their bills variously adapted to cracking seeds, eating insects, extracting cactus pulp, or even sucking the blood of seabirds.

In Gombe, the opposite has apparently occurred. At some point, as new forest filled the barrier that once divided these two species, they found themselves sharing a niche. But then they became marooned together, as the forest surrounding Gombe National Park gave way to cassava croplands. “As the number of available mates of their own species dwindled,” Detwiler figures, “these animals have been driven to desperate—or creative—survival measures.”

Her thesis is that hybridization between two species can be an evolutionary force, just like natural selection is within one. “Maybe at first the mixed offspring isn’t as fit as either parent,” she says. “But for whatever reason—constrained habitat, or low numbers—the experiment keeps getting repeated, until eventually a hybrid as viable as its parent emerges. Or, maybe even with advantage over the parents, because the habitat has changed.”