Dead Sea. Then it widened into the Red Sea, and is now branching down two parallel cleavages through the crust of Africa. Lake Tanganyika fills the Rift’s western fork for 420 miles, making it the longest lake in the world.
Nearly a mile from surface to bottom, around 10 million years old, it is also the world’s second-deepest and second-oldest, after Siberia’s Lake Baikal. That makes it extremely interesting to scientists who have been extracting core samples of the lake bed sediments. Just as annual snowfalls preserve a history of climate in glaciers, pollen grains from surrounding foliage settle in the depths of bodies of freshwater, neatly separated into readable layers by dark bands of rainy-season runoff and light seams of dry-season algal blooms. At ancient Lake Tanganyika, the cores reveal more than the identities of plants. They show how a jungle gradually turned to fire- tolerant, broad-leafed woodland known as miombo, which covers vast swathes of today’s Africa. Miombo is another man-made artifact, which developed as paleolithic humans discovered that by burning trees they could create grassland and open woodlands to attract and nurture antelope.
Mixed with thickening layers of charcoal, the pollens show the even greater deforestation that accompanied the dawning of the Iron Age, as humans learned first to smelt ore, then to fashion hoes for furrows. There they planted crops such as finger millet, whose signature also appears. Later arrivals, like beans and corn, produce either too few pollens or grains too large to drift far, but the spread of agriculture is evidenced by the increase of pollens from ferns that colonize disturbed land.
All this and more can be learned from mud recovered with 10 meters of steel pipe lowered on a cable and, aided by a vibrating motor, driven by the force of its own weight into the lake bed—and into 100,000 years of pollen layers. A next step, says University of Arizona paleolimnologist Andy Cohen, who heads a research project in Kigoma, Tanzania, on Lake Tanganyika’s eastern shore, is a drill rig capable of penetrating a 5-million- or even 10- million-year core.
Such a machine would be very expensive, on the order of a small oil-drilling barge. The lake is so deep that the drill could not be anchored, requiring thrusters linked to a global positioning system to constantly adjust its position above the hole. But it would be worth it, says Cohen, because this is Earth’s longest, richest climate archive.
“It’s long been assumed that climate is driven by advancing and retreating polar ice sheets. But there’s good reason to believe that circulation at the tropics is also involved. We know a lot about climate change at the poles, but not at the heat engine of the planet, where people live.” Coring it, Cohen says, would capture “ten times the climate history found in glaciers, and with far greater precision. There are probably a hundred different things we can analyze.”
Among them is the history of human evolution, because the core’s record would span the years during which primates took their first bipedal steps and proceeded through transcendent stages that led to hominids from
East of Lake Tanganyika in the African Rift’s parallel branch, another lake, shallower and saline, evaporated and reappeared various times over the past 2 million years. Today, it is grassland, hard-grazed by the cows and goats of Maasai herdsmen, overlying sandstone, clay, tuff, and ash atop a bed of volcanic basalt. A stream draining Tanzania’s volcanic highlands to the east gradually cut a gorge through those layers 100 meters deep. There, during the 20th century, archaeologists Louis and Mary Leakey discovered fossilized hominid skulls left 1.75 million years earlier. The gray rubble of Olduvai Gorge, now a semidesert bristling with sisal, eventually yielded hundreds of stone-flake tools and chopper cores made from the underlying basalt. Some of these have been dated to 2 million years ago.
In 1978, 25 miles southwest of Olduvai Gorge, Mary Leakey’s team found a trail of footprints frozen in wet ash. They were made by an australopithecine trio, likely parents and a child, walking or fleeing through the rainy aftermath of an eruption of the nearby Sadiman volcano. Their discovery pushed bipedal hominid existence back beyond 3.5 million years ago. From here and from related sites in Kenya and Ethiopia, a pattern emerges of the gestation of the human race. It is now known that we walked on two feet for hundreds of thousands of years before it occurred to us to strike one stone against another to create sharp-edged tools. From the remains of hominid teeth and other nearby fossils, we know we were omnivores, equipped with molars to crunch nuts—but also, as we advanced from finding stones shaped like axes to learning how to produce them, possessed of the means to efficiently kill and eat animals.
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
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 Galapagos 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
