Biology Reference
In-Depth Information
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 di-
versifying 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 evol-
ution, 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 Afric-
an 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 gib-
bons—and on the ground, they employ fist-walking, a likely precursor to knuckle-walking, the sig-
nature 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 peri-
ods, with little protection, great apes weave branches together to create bowls that can accommod-
ate 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 discov-
eries 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, contin-
ued 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-chimpan-
zee 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
