Evolution__3rd_Edition

546 PART 5 / Macroevolution
Changes in hands, ...
. . . feet, ...
. . . brains, ...
. . . jaws, ...
. . . locomotion, ...
. . . social behavior, ...
ively opposable thumbs, as compared with other mammals, but a fully opposable
thumb is (with minor exceptions) confined to the great apes a orang-utans, gorillas,
chimpanzees, and humans. A fully opposable thumb means that you can touch the
front tip of all four digits with your thumb. We can do it, but a cat or dog (for example)
cannot.
In human evolution, the opposable hallux has been lost as our feet evolved for
bipedality. The opposable thumb has been retained and modified. In our ancestors, it
enabled a “power grip” used in gripping branches. We can still do the power grip, but
changes in the hand bones allow us to use a “precision grip” not seen in other species.
We use the precision grip in handling fine tools.
The big changes in human evolution may have occurred after our ancestors moved
from forests to more savannah-type habitats. In fossils, the big changes can be understood
in three categories. A fourth category concerns changes in social behavior, which
is less easily studied in fossils.
1. Brain enlargement. Modern chimpanzees have brains of about 350–400 cm 3 , and
our ape ancestors 5 million years ago probably had brains of about the same size.
Modern human brains are about 1,350 cm 3 in size.
2. Changes to the jaw and teeth. Chimpanzees, and our ape ancestors, are more prognathic
than us, with their jaws sticking out more from their faces. During human
evolution, the jaw shrunk back into the face, giving us flat faces. The jaw of our ape
ancestors, and chimpanzees, has a semicircular shape (if viewed from above or
below). In us, the semicircle has been pushed back to a shape more like a rectangle.
Our teeth also got smaller, particularly the canine teeth, and our molars have
evolved into grinding millstones.
3. Bipedality. The evolution of upright locomotion, on two legs, has resulted in
changes throughout our bodies. Adaptations for bipedality are particularly clear
in the anatomy of fossil feet and leg bones; but they can also be seen in the back
vertebrae, the length of our arms, and the position of our skulls on our backbones.
Changes in these three categories are not independent. For instance, the themes of
brain enlargement and bipedality combine in the evolution of human birth. Birth is
relatively uncomplicated in chimpanzees, but our big brains and pelvic size, which is
constrained by bipedality, have made birth more problematic in human beings. Brain
size and gestation length are correlated across primates as a whole, and simple extrapolation
from the general primate relationship suggests that humans might be expected
to be born after 18 rather than 9 months. It may be that we are born relatively early
because birth at a later fetal age would be impossible; 9 months is the latest possible
point before the brain grows too big. Human babies are relatively undeveloped, compared
with newborn chimps, in their motor development and other brain skills. Thus,
newborn humans are relatively dependent on their mothers, and the intensity of
parental care in humans is part of the other main trend of human evolution, which is
not easily visible in fossils.
4. Changes in social and cultural behavior. The main way we differ from other apes is in
our social and cultural lives. This development can be followed only indirectly in
fossils. Sexual dimorphism, for example, is probably related to the breeding system.
In apes other than humans, males weigh about twice as much as females on average
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