The Mercator projection—which transposes the globe onto a cylinder or flat surface—misrepresents the area closer to the poles, expanding it. Relative to North America and Europe, the Congo is far larger than it appears: at 905,355 square miles, it is 3.37 times the size of Texas, the eleventh-largest country on earth and the second largest in Africa, after Algeria. As of 2005, nearly 60 percent of it was forestland.
Though little of it was logged under Mobutu because of lack of transportation infrastructure, by the 1990s, 37 percent of the country’s forests that could be exploited commercially were officially designated as timber concessions. With the war now over, much of the region has become more reliably accessible for systematic exploitation, so deforestation, which has been occurring at a rate of about 1 percent a decade, is likely to speed up.
When trees are cut down and decay, and especially when they are burned, they release CO2 into the atmosphere. This carbon then absorbs solar radiation, warming the planet. Already, global deforestation emits more carbon dioxide than all of the transportation on earth—automobiles, airplanes, trains, and boats—combined, and nearly as much as transportation and industry together. Furthermore, each tree cut down has a double negative impact, not only releasing carbon but no longer assimilating it from the atmosphere. Through photosynthesis, trees create carbohydrates from CO2 and water, synthesizing the carbon molecules with water and releasing oxygen as a waste product. In the process, the world’s remaining tropical forests sequester 20 percent of global carbon emissions from fossil fuels, a number that decreases with logging and the clearing of land even as manmade carbon emissions rise steadily.
So dramatically have humans transformed the earth that in the early 1980s the American scientist Eugene F. Stoermer proposed the name Anthropocene for our current geologic epoch. Zoologists Guy Cowlishaw and Robin Dunbar write: “Not since the demise of the dinosaurs 65 million years ago has this planet witnessed changes to the structure and dynamics of its biological communities as dramatic as those that have occurred over recent millennia, and especially in the past four hundred years.” Humans have devastated millions of square miles of habitat, and since 1600, eighty-nine of the planet’s approximately five thousand mammal species have gone extinct, with 169 others critically endangered. More recently, agricultural and industrial revolutions have reshaped the world, changing the composition of the soil, water, and air, and the estimated current rate of extinction in rainforests alone, for all organisms—insects, plants, bacteria, and fungi—is 27,000 a year. Despite the severity of our impact, the entire 250,000 years of human history hardly compares to the damage we have done in the last fifty years, and given our current rate of expansion, hundreds, if not thousands, more animal species are expected to die off within the century.
In a way, the asteroid strike that most likely ended the dinosaurs’ rule 65.5 million years ago and our current age are bookends, containing a long, largely continuous span of evolution and diversification of life that created humans, bonobos, and the rainforests as we currently know them. After the asteroid’s collision, dust and ash filled the atmosphere, blocking sunlight and disrupting the food chain by killing off photosynthesizing organisms. When herbivorous dinosaurs could no longer graze, the carnivores that preyed on them also died, eliminating all top predators. The only creatures that endured were those that could subsist on insects and worms, which themselves bred in the carrion and detritus. One of the traits that has made us so destructive to our environment allowed our small, rodentlike ancestors to survive: they could eat just about anything.
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
