After that cataclysm, the earth was a relatively quiet place, but over the next ten million years, it heated up significantly, and mammals thrived, spreading across the globe, speciating to fill ecological niches left vacant by the dinosaurs. That the subsequent transformation of the rainforest likely shaped modern humans reveals how changes in the environment can shift our path, transforming us from one kind of creature to another, with radically different behavior.
The planet’s hot phase could have had a number of causes, from changing ocean currents to volcanic venting that released massive quantities of atmospheric carbon. Trees covered the earth nearly pole to pole, the Canadian Arctic and Greenland host to lush, closed-canopy forests, to alligators, tapirs, flying lemurs, hippolike mammals, and giant tortoises. Palm trees grew in Wyoming, where primates left some of their earliest fossil evidence. Though resembling squirrels in both size and appearance, they had the nails characteristic of primates rather than claws. With the planet so densely forested, they easily spread across Europe, Asia, and Africa.
By forty-eight million years ago, plant life had sequestered a great deal of atmospheric carbon in oil and coal deposits, and the planet cooled as a result of the continents’ drift away from the equator. Until then, the earth had been in a warm phase, without significant polar ice, alpine glaciers, or continental ice sheets for 250 million years. The clustering of landmasses in the single massive continent of Pangea had allowed the warm and cold ocean currents to mix, maintaining relatively stable temperatures. But as the continents separated, they isolated the oceans, causing greater concentrations of cold water and the buildup of sea ice, so that sometime between thirty and fifty million years ago, average ocean surface temperatures dropped by a staggering eighty-six degrees Fahrenheit.
Cool periods tend to be arid, the planet’s humidity trapped in ice, and the earth began to take on an appearance we would recognize. The interiors of continents dried out, and grass, which first appeared fifty-five to sixty million years ago, limited to the shores of lakes and rivers, evolved into hardier species. It eventually covered savannahs, which, though usually described as plains, are grassland with scattered, open-canopy woodlands. This dry habitat came to predominate in Africa and offered fewer sources of nutrients to primates, increasing competition and requiring more dynamic foraging. And as rainforests shrank to a band around the equator, primates, which had evolved into creatures that we might recognize as similar to monkeys, survived only in Africa.
Several theories exist for the monkey-ape split, 24.5 to 29 million years ago. It may have resulted from feeding patterns that evolved in part due to competition between primate groups in contracting ecosystems. One strong theory holds that when some monkey species evolved from eating only ripe fruits to being able to digest even those that are unripe—thereby increasing their own numbers and limiting the food supply for all other tree-dwellers—a few competing primates adapted to survive. The earliest ape—our first ancestor after the split—most likely resembled the gibbons, the so-called lesser apes, of which sixteen species survive in Southeast Asia. They are the most monkeylike ape and the fastest, most agile arboreal primate. With an average body weight of fifteen pounds, they swing hand over hand and leap through the trees rather than climb with all fours like monkeys. Such abilities no doubt allowed their ancestors to snatch hard-to-get food on small, peripheral branches, and thus to outcompete monkeys. Those among the first apes who had the longest reach would have been most successful, which would explain the remarkably long arms that gibbons sport today. Furthermore, brachiation (swinging from branches with the hands) would have favored the upright posture and the head shape and position that remain distinguishing traits of modern apes. Gibbons also lack tails, an appendage that helped monkeys balance on all fours in trees, but that might have been ill suited to brachiation and—in the case of the great apes—terrestrial foraging and travel.
Evolution, however, is unlikely to be so picture-perfect. Numerous factors are often at play, from the isolation of a few animals from a larger group to random DNA mutations that occasionally provide adaptive traits. When individuals colonize a new environment or live through a gradual climatic shift, those among them most capable of surviving these changes—and having the chance to produce surviving offspring—pass on their traits. In every group of individuals of any given species, there is variation. A high school classroom will have students with different heights, proportions, personalities, metabolic rates, immune systems, athletic abilities, and colors of skin, hair, and eyes. A hypothetical group of early apes is no different, and those with traits most suited to new circumstances will outbreed the others. When their successful offspring pair up, each new generation gets a double dose of survivor genes. If the change in the environment is particularly harsh and rapid owing to geological activity or new weather 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