patterns, or if the competition with other animals is fierce, a bottleneck may occur: most of the individuals of the species die off, and the few who are left are likely to have adaptive traits. Even within a few generations, these adaptive qualities become more prominent and survivors begin to look different from their ancestors, whereas elsewhere, in other parts of Africa, where the environment is more stable, the species can remain relatively unchanged.
About twenty million years ago, not long after the arrival of apes on the primate scene, the next step in their evolution took place. From DNA studies, we know that the apes separated into two groups, the lesser apes and the great apes. A number of factors could have been at play. With diversifying monkey species dominating the canopy’s diminishing food sources, it is likely that the larger and less agile of the gibbonlike early apes began foraging in the ground cover. Even today, unlike monkeys, great apes have the ability to digest a number of fibrous plants.
Environmental changes and the contraction of forests also could have influenced great ape evolution, and the simplest way to imagine the transition to a more terrestrial existence would be to picture a single group of early apes. They live in Africa, in the trees, but in a landscape particularly vulnerable to climatic drying. Though they are somewhat versatile, descending to forage for further sustenance, they never wander far. As savannah begins replacing forest, they compete for limited fruit resources with monkeys and with other apes, and have to venture farther on land. Those who survive gradually begin to resemble the earliest common ancestor of today’s orangutans, gorillas, chimpanzees, bonobos, and humans.
Today, the least terrestrial great ape is the orangutan, which lives exclusively in Southeast Asia. Of the surviving great apes, its lineage was the first to split from the common ancestor of the African apes, fifteen to nineteen million years ago, and the only one to spread outside of Africa and survive. Of the great apes, they swing most easily—though far from displaying the agility of gibbons—and on the ground, they employ fist-walking, a likely precursor to knuckle-walking, the signature technique of chimpanzees, gorillas, and bonobos, who, being far more terrestrial, evolved to have friction pads on their middle phalanges. As for the surviving African great apes—gorillas, humans, chimpanzees, and bonobos—the splits in their lineages occurred relatively close together. Nine to eleven million years ago, the gorilla ancestor separated from the common ancestor of chimpanzees, bonobos, and humans. Five to eight million years ago, the human ancestor bade the bonobo-chimpanzee line farewell. And 1.5 to 3 million years ago, bonobos and chimpanzees went their own ways.
However, all great apes—humans included—continue to share behavioral traits, and one that is essential for all of them is nest building. Whereas monkeys and gibbons rest in trees for short periods, with little protection, great apes weave branches together to create bowls that can accommodate an adult. The practice may have led to deeper sleep that promoted greater brain regeneration and neuron growth. This behavior would perhaps have provided them the advantage of waking rested and clear-minded, and could have catalyzed the evolution of ever larger-brained apes, who reaped the benefits so long as they remained as committed to nests as we are to our huts and townhouses.
With so many factors influencing evolution, the genealogy is far from resolved, and new discoveries in genetics and fossils frequently call various aspects of it into question. Though the anatomy of chimps, gorillas, and bonobos suggests that their ancestors, unlike those of orangutans, continued to adapt to ground conditions, they also retained the ability to climb, allowing them to get food and take refuge from predators. One theory 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,
