Biology Reference
In-Depth Information
When the glacial cycle started in the Northern Hemisphere, a brief 2.6 million years ago, trap-
ping vast quantities of the planet's moisture in polar ice, many African rivers, the Nile among them,
ceased to reach the ocean, and the tropics withered. The Congo basin was one of the few areas to
retain some of its rainforest. Though great apes likely died off elsewhere on the continent, small
groups of them could have survived here.
As early as six million years ago, in the long cooling period leading up to our current ice age,
our earliest ancestors became increasingly bipedal, spending more time on the ground. However,
the severe changes caused by glaciation coincided with a rapid spurt in evolution that may have
given rise to modern humans. The situation could have been similar to that of the earlier apes, only
more extreme: the drying eliminated trees and created a hostile environment with few resources.
Paleontologist and anatomist Kevin Hunt argues that as trees became much smaller and branches
thinner, early hominids found that foraging by climbing was increasingly difficult; instead, they
often stood on the ground and reached up for the fruit. Supporting this theory is the observation
that today, among chimpanzees, tool use, carrying food, confrontational display, and looking over
obstacles account for only 1 to 2 percent of bipedal behavior, whereas feeding, most often from the
low branches of small trees, accounts for 85 percent of it. Again, if the environmental conditions
were harsh, a bottleneck might have occurred, many of the early hominids dying off, leaving only
those who were adapted to the new circumstances and capable of using bipedalism to their advant-
age, not only for foraging but also for hunting. Those human ancestors who could walk upright
also used less energy during travel and freed up their hands to carry food, thereby nourishing them-
selves even when they couldn't forage successfully. They could also move between distant patches
of forest more readily, and, as a result, provision more offspring.
In
Catching Fire
, Richard Wrangham offers an analogy to evoke the close bond that humans
share with these early hominids. He gives the example of an australopithecine, an ancestor of mod-
ern humans that lived three million years ago, approximately halfway along the evolutionary path
from our last common ancestor with chimpanzees and bonobos:
Imagine going to a sporting event with sixty thousand seats around the stadium. You arrive
early with your grandmother, and the two of you take the first seats. Next to your grand-
mother sits her grandmother, your great-great-grandmother. Next to her is your great-great-
great-great-grandmother. The stadium fills with the ghosts of preceding grandmothers. An
hour later the seat next to you is occupied by the last to sit down, the ancestor of you all.
She nudges your elbow, and you turn to find a strange nonhuman face.
As for all of the grandmothers going back to the last common ancestor of humans, chimpanzees,
and bonobos, it is conceivable that they could be housed in the United States's largest stadium,
that of the University of Michigan, which hosted a record 114,804 attendees in 2011. In Richard
Dawkins's essay, “Gaps in the Mind,” he describes a similar thought experiment and writes that, in
the lineage leading back to the last common ancestor, “you would nowhere find any sharp discon-
tinuity. Daughters would resemble mothers just as much (or as little) as they always do.”
Recent research suggests that there were numerous species of early humans, often overlapping
or being rendered extinct as they spread out.
Homo sapiens
—modern humans—originated in Africa
sometime before two hundred thousand years ago, then moved into Europe, killing off or absorbing
the Neanderthals, adept dwellers of cold climates whose cranial cavity, despite our image of them
as brutes, was in fact larger than our own. The Neanderthals left a significant trace of their DNA
