proposes that gorillas, chimpanzees, and bonobos are so terrestrial because their ancestors adapted to the savannah for millennia before finding their way to the remaining food-rich rainforests. And some genetic studies suggest that the human-chimpanzee split wasn’t clean, their ancestors having romped on occasion. As for DNA, we share between 98.6 percent and 98.7 percent of ours with bonobos and chimps, 98.25 percent with gorillas, and 96.6 percent with orangutans. There is a dearth of fossil evidence from between nine and fourteen million years ago, and much of what we know about the earliest days of our evolution comes from studies of living great apes and their DNA. In many ways, we build evolutionary history back from surviving species.
Given that the chimpanzee-bonobo ancestor and the human ancestor evolved from the same stock—the same common ancestor who was neither chimpanzee-bonobo nor human—it’s not surprising that there are some resemblances in social structures among the species. In fact, studies of chimpanzees and bonobos have shed light on the evolution of human behavior. Only a few decades ago, and especially after the World Wars, we humans strongly associated ourselves with the belligerence of chimpanzees, unable to deny our brutality. But over the last four decades, as we have become aware of bonobos, we’ve recognized a number of our other social traits in them, such as our proclivity for nonreproductive sex, our ability to construct largely nonviolent communities, and our practice of building peaceful coalitions.
But the greater mystery is how bonobos and chimpanzees, being so similar and having such a recent common ancestor, could have developed such divergent behaviors over a relatively short evolutionary period. Scientists have theorized that the Congo River formed at that time, between 1.5 and 3 million years ago, separating the common ancestor of bonobos and chimpanzees into two groups. While to the north the chimpanzees competed with gorillas for food, the bonobos lived in a lush enclave south of the river’s curve, where certain aggressive traits were less essential for their survival. This theory, however, doesn’t explain why there were no gorillas to the south of the river, and another argument exists for the evolutionary path of chimpanzees and bonobos, given that the Congo River may have formed millions of years earlier than once believed.
The bonobo-chimpanzee split roughly coincides with the beginning of our current glacial cycle 2.6 million years ago, which, relative to geologic time, rapidly transformed the planet and the great ape habitat. Though the earth had already been cooling for over forty-eight million years, the accumulation of polar ice sped up 5.3 million years ago, when the Isthmus of Panama joined North and South America, cutting off warm equatorial currents and cooling the Atlantic. Spreading ice reflected solar radiation into the atmosphere, preventing its absorption and starting a feedback loop that resulted in more rapid planetary cooling, and thus more ice. The term ice age is generally misused. Technically, it indicates a period during which substantial continental ice sheets exist in both the Northern and Southern Hemispheres. We have been in an ice age for nearly 2.6 million years, a time marked by interglacials, like our current warm period, and glacials, which most people erroneously refer to as ice ages. The glacials come in cycles of twenty thousand, forty thousand, and one hundred thousand years, mirroring shifts in the earth’s tilt and orbit around the sun.
With this forty-eight-million-year sketch of earth’s history since the planet began to cool during the Eocene, we can imagine a time-lapse film from space and see the movement of primates and forests to their current positions. First, we have a planet whose continents have nearly reached their present positions, though they are almost entirely green, forests fringing the poles. This coloring then melts away, the interiors of continents yellowing, flecked with green and outlined with it at the coasts, though a solid belt of forest still girds the planet’s middle. With the exception of Africa, the continents that host primates become inhospitable to them.
The most remarkable change in forest distribution occurs 2.6 million years ago, with the ice age. Ocean levels drop and continental shelves appear as the planet’s humidity gathers in ice more than two miles thick over much of the northern temperate zones. In places, glaciers stand nearly half the height of Everest, pressing the earth’s crust so deeply into the mantle that today parts of Northern Europe and Canada are still lifting back into place. If we continue our time-lapse film, the ice age would show white spreading from the poles, the green-yellow savannahs desiccating, and the planet’s rainforest belt withering to a few specks.
In Demonic Males: Apes and the Origins of Human Violence, Harvard zoologist Richard Wrangham and science writer Dale Peterson lay out one explanation for the divergence of bonobos and chimpanzees. They argue that even though tropical forests had been gradually retreating for millennia, the Congo basin rainforest, before the ice age, likely would have been much larger than now, allowing the common ancestor of chimpanzees and bonobos to circumvent the entire river system and cross over to the other side. But during the glacial maximum, the forest shrank, and survived only in the wettest pockets. Gorillas, who are vegetarians and sustain themselves on protein-rich shoots and buds, would have seen their food sources become scarce and their habitat dramatically reduced. They likely would have withdrawn to wet climates near mountains or died off, especially to the south of the river, where there was no mountainous terrain. The versatile chimpanzee-bonobo ancestor would have occupied more space and might, in certain areas, have lived largely in savannahs.
During the following interglacial, as ice caps melted and humidity returned to the equator, abundant rain carved new tributaries and enlarged existing rivers. Wrangham and Peterson explain that though the food sources optimal for gorillas would have reappeared in abundance throughout the basin, the gorillas would have struggled to return to all areas. Rivers would have hampered their travel, and despite the humid interglacial, the forests might not have returned to their previous size, no longer offering a clear path around the Congo’s elaborate river system.
Judging by the gorillas’ present habitat, it appears that they expanded only into the sections of the Congo rainforest currently inhabited by chimpanzees. The chimp-bonobo ancestors who lived in the same areas as gorillas faced limited resources and might have benefited by becoming significantly more competitive with one another for food, evolving toward chimpanzees. However, the chimp-bonobo ancestor across the river to the south, living without gorillas, had an easier time, benefiting from the diets of both chimpanzees and gorillas, as bonobos do today. With so many resources, it might have evolved to have increasingly less competition between individuals. Even now, chimps, just to the north of the river, rely much more on hunting. Of course, owing to the lack of fossil evidence, we can’t easily judge whether the chimp-bonobo ancestor more closely resembled chimpanzees or bonobos, or had a unique disposition from which its descendents dramatically diverged.
Is a lesson in 65.5 million years of global history necessary to understand the planet’s few remaining rainforests and the ways that apes now occupy them? If humans are the bookend, the driving force in a new mass extinction, it is clearly important to understand exactly what may be ending, and all that would be lost. The long, largely continuous evolution and expansion of species since the demise of dinosaurs appears, at least from our limited perspective, to be at a crucial juncture, with habitats being rapidly destroyed by humans. Given the exponential increase of human populations and industry, we must act quickly if we are to preserve remaining ecosystems at a time when few of us even understand their significance.
The story of this evolution changes how I see the forest—not as a natural resource or a feature of the landscape, but as a central factor in the story of our evolution. As it vanished, apes evolved and our ancestor separated from theirs. The only surviving members of their group took refuge in the equatorial forests that have existed in some form for millions of years, and they teach us more about the past and ourselves than fossils ever could. Sally Jewell Coxe often describes bonobos and chimpanzees as exemplifying the yin and yang of human nature, and their models shed light not only on how we can interact with each other, but on the ways an environment can cause us to change.
The plane banked and began to lose altitude, and I wondered how I would feel standing in virgin rainforest and seeing bonobos. As the last great ape that Westerners became aware of, they made us realize all that we didn’t know about ourselves and the forest itself. Increasingly, though, as BCI’s logo of a bonobo standing in a circle suggested, they represented the importance of coalitions to save that very forest. Today, Africa’s rainforests are barely absorbing