Biology Reference
In-Depth Information
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 plan-
et'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 sur-
vived 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 dramatic-
ally reduced. They likely would have withdrawn to wet climates near mountains or died off, espe-
cially 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 signi-
ficantly 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 re-
sembled chimpanzees or bonobos, or had a unique disposition from which its descendents dramat-
ically diverged.
Is a lesson in 65.5 million years of global history necessary to understand the planet's few re-
maining rainforests and the ways that apes now occupy them? If humans are the bookend, the driv-
ing 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
