Читать книгу Of Bonobos and Men - Deni Ellis Bechard - Страница 13
ОглавлениеAs we were about to climb the steps to the CAA jet that would take us to Mbandaka, a police officer and Dr. Nicolas Mwanza Ndunda, BCI’s scientific director, ran from the N’Djili Airport terminal to give a package and contracts to Sally. Mwanza is a tall, jovial-looking man in his sixties, with a paunch and a small mustache. Sweat beaded on his forehead, and the sun had us squinting, the heat off the tarmac so palpable I could feel it in my muscles.
The other passengers hurried past us to claim seats as Sally read the papers, a subcontract for a grant that would employ staff from the Congolese Ministry of Scientific Research. She signed and handed them to Mwanza, and the officer began to lift his hand in an imploring gesture. She gave him the equivalent of five dollars in Congolese francs for letting Mwanza meet us.
After we took our places, not a seat remained in the narrow Fokker jet, the last six rows of which were loaded with bags and cardboard boxes heavily sealed with brown tape. It was hot inside, the passengers sweating, though we cooled down once we were in motion.
We left Kinshasa, crossing inland away from the Pool Malebo, heading east over Bandundu Province. Below us, forested rivers scored savannah plateaus, giving the landscape the look of interlocking puzzle pieces. Équateur was just above, bordering Congo-Brazzaville to the west, the Central African Republic to the north, and Orientale Province to the east.
Équateur is known for being the most heavily forested province in the DRC, and forty minutes into our trip, as we neared Mbandaka, I stared out the jet’s window at the rainforest curving against the horizon. A distant plume of smoke rose from the endless rippled green, calling to mind a war photograph I saw years ago: a burning ship far away on the uniform ocean.
Before this trip, I’d studied the DRC on a map. Its lopsided bulk, in its place at the center of Africa, looked—just as the hackneyed metaphor says—like a heart. But maps don’t do the Congo justice. 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 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 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, 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 the carbon emissions of its cities, and the lesson in planetary history also serves to remind us of how carbon dioxide can transform the earth, and how the forests that we’re cutting down are essential for sequestering it.
For years, studies of ice cores from glaciers have revealed that the current level of carbon dioxide is the highest the planet has known in the last eight hundred thousand years. New research, however, suggests that the last time the atmosphere held this much carbon dioxide was fifteen million years ago, when, according to the scientist Aradhna Tripati, a professor at the University of California, Los Angeles, “global temperatures were 5 to 10 degrees Fahrenheit higher than they are today, the sea level was approximately 75 to 120 feet higher than today, there was no permanent sea ice cap in the Arctic and very little ice on Antarctica and Greenland.” Historically, global temperatures largely correlate to atmospheric carbon levels, and though temperatures are at their highest level in four thousand years, they are expected to rise at an unusually rapid rate over the next century, one too fast to allow most creatures to adapt. Some scientists have suggested that we are crossing into unknown territory, over a tipping point, where carbon emissions will create a domino effect, transforming the planet at an exponential rate. And yet our impact is increasing, a day in Kinshasa enough to make me understand the urgency of human need and hunger. The DRC’s population—already the fourth largest in Africa after those of Nigeria, Egypt, and Ethiopia—is set to double to 140 million within twenty years. A glance from the airplane window sufficed to remind me of how isolated and unknown our few remaining rainforests are, how they can disappear without our knowing, and how much of a challenge it will be for humanity to work together to save them.
