Biology Reference
In-Depth Information
tinent 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 concentra-
tions 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 Fahren-
heit.
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 res-
ulted from feeding patterns that evolved in part due to competition between primate groups in con-
tracting 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 adap-
ted to survive. The earliest ape—our first ancestor after the split—most likely resembled the gib-
bons, 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 distinguish-
ing 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 cli-
matic 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 spe-
cies, there is variation. A high school classroom will have students with different heights, propor-
tions, 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 gener-
ation 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 an-
imals 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 qual-
ities become more prominent and survivors begin to look different from their ancestors, whereas
