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• The foot. The human foot has an arch, and a big toe that
is in line with the others. In contrast, chimps do not have
arches, and their big toes are at an angle resembling that
of a thumb. The thumb-like big toe helps chimps to grasp
branches while climbing trees, something not necessary to
a bipedal animal walking on the ground.
The origin of human bipedalism was gradual. Several
other primates are partly bipedal. Gibbons and orangutans,
for example, are often upright as they climb and swing
through trees. This represents a division of labor between
arms (for climbing, gathering fruits, etc.) and legs. The
knuckle-walking of chimpanzees was mentioned earlier. The
common ancestor of chimpanzees and hominins was already
partly bipedal. The earliest members of the human lineage
were mostly bipedal, as indicated by the Laetoli footprints,
but had a projecting big toe that allowed these individuals to
retain at least a vestige of tree-climbing ability (see australopithecines).
The human lineage was fully bipedal by the
origin of the genus Homo (see Homo Habilis).
Bipedalism is much slower and less efficient than quadrupedalism.
Even relatively slow quadrupeds, such as
bears, can easily outrun even the most athletic humans.
The evolutionary advantages of bipedalism, therefore, must
have been tremendous and long-term. The problem is that
nobody is sure what the advantage or advantages might
have been. Many theories have been advanced. An early
theory was that, with hands no longer used for walking,
early humans could make tools. However, even the earliest
hominins, such as Sahelanthropus tchadensis, were bipedal,
more than three million years before the first stone tools
were manufactured.
The origin of bipedalism seems to have coincided with
the gradual drying of the climate of Africa, so that continuous
rain forest was replaced by savannas and scattered forests.
Hominins would have had to disperse between clusters
of trees in grasslands. This still does not explain bipedalism,
for quadrupedal animals would be able to do this.
Some scientists have suggested that upright posture
allowed hominins to look out over the grass and see predators
coming. This suggestion can hardly be taken seriously,
as the full suite of bipedal adaptations is clearly not necessary
for simply standing up once in a while. Others have suggested
that it allowed human ancestors to throw rocks at threatening
carnivores. Again, this cannot be taken seriously, since
lions could hardly be scared off by rocks; how much better it
would be to run away like an antelope, an option not open to
bipedal hominins.
The most likely advantage came from something that
the hominins were doing with their hands. But what were
these hominins doing with their hands that was so important
that it made up for the loss of speed, out in the open,
and a reduced tree-climbing ability? Some scientists suggest
that they may have been carrying food. If early humans were
scavengers of carcasses killed by lions and left by hyenas,
or stealers of leopard kills stashed in trees, they had to run
while carrying a hunk of meat. However, baboons are not
fully bipedal yet they can run and carry food at the same
time. Furthermore, the evidence that humans ate meat is
unclear until about two million years ago. Others suggest
that it allowed mothers (and possibly fathers) to carry their
infants and children. In other primates, infants cling to the
fur of the mother. Humans, being without significant fur
except on the head, would have to carry their kids (“Let
go of my hair! Okay, I’ll carry you!”). This explanation is
believable mostly because of the failure of all the others. A
more recent proposal combines the two previous explanations:
that bipedal characteristics were selected not so much
for walking as for running long distances while carrying
resources or children.
Three sisters and a brother living in Turkey are human
quadrupeds. Although they have fully human anatomy, they
prefer to walk on their feet and palms. This behavior has
been traced to a mutation on chromosome 17. Therefore the
evolution of bipedalism not only required anatomical changes
but also changes in the brain.
Bipedalism was apparently the first characteristic that
distinguished the hominin line from earlier primates. Its origin
remains unexplained.
Further Reading
Chen, Ingfei. “Born to run.” Discover, May 2006, 62–67.
Lieberman, Daniel, and Dennis Bramble. “Endurance running and
the evolution of Homo.” Nature 432 (2004): 345–352.
Summers, Adam. “Born to run.” Natural History, April 2005, 34–35.
birds, evolution of Birds are vertebrates that have feathers,
warm blood, and lay eggs externally. Birds are one of
the lineages of reptiles (see reptiles, evolution of); in fact,
they probably form a lineage from within the dinosaurs.
Because “reptile” is not a coherent group if mammals and
birds are excluded, and because “dinosaur” is not a coherent
group if birds are excluded, many scientists include birds and
mammals with the reptiles and include birds with the dinosaurs
(see cladistics). There are almost twice as many bird
species (more than 9,000) as mammal species.
The single most recognizable feature of birds is feathers.
Feathers are complex and lightweight structures consisting
of a central shaft with a vane consisting of barbs and
hooks. Flight feathers are very specialized, with the shaft
off-center in a manner that permits aerodynamic efficiency.
Other kinds of feathers are less complex. Down feathers, for
example, are small and function mostly in holding in body
heat. Since simpler feather structures are ineffective for
flight, the evolution of feathers probably began with simple
feathers that held in body heat, from which more complex
flight feathers evolved (see adaptation). Feathers also repel
water, especially when birds preen them, applying waterrepellent
materials from glands.
Birds have many other features that adapt them to flight.
In fact, the entire body of the bird appears adapted to flight,
especially by the reduction of weight:
• Skeletal features. The bones are partially hollow, which
retains most of their strength while greatly reducing their
weight. Because flight requires enormous muscular energy,
bird flight muscles are enormous relative to the rest of the