Biology Reference
In-Depth Information
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 sur-
prising 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 belli-
gerence 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 ac-
cumulation 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 mis-
used. 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 er-
roneously 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
