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Modifications
for upright posture and human musculature
Human
muscle can be divided into striated (or skeletal), smooth,
and cardiac. With a very few exceptions, the arrangement
of smooth and cardiac muscle has undergone little modification
with the assumption of the upright posture. A discussion
of the changes in the striated, or somatic, muscles follows.
Evolutionary context
The
arrangement of striated muscle in modern humans conforms
to the basic pronograde (horizontal) quadrupedal vertebrate
and mammalian plan that was outlined in an earlier section.
The primates inherited the primitive quadrupedal stance
and locomotion, but since their appearance in the Cretaceous
Period, several groups have modified their locomotor system
to concentrate on the use of the arms for propulsion through
the trees. The most extreme expression of this skeletal
adaptation in living primates is seen in the modern gibbon
family. Their forelimbs are relatively elongated; they hold
their trunk erect; and for the short periods that they spend
on the ground they walk only on their hind limbs (in a bipedal
fashion).
Modern
humans are most closely related to the living great apes,
the orangutan, chimpanzee, and gorilla. Their most distant
relative in the group, the orangutan, has a locomotor system
that is adapted for moving among the vertical tree trunks
of the Asian rain forests. It grips these trunks equally
well with both fore and hind limbs and was at one time aptly
called quadrumanal, or “four-handed.”
There
is little direct fossil evidence about the common ancestor
of modern humans, chimpanzees, and gorillas, so inferences
about its habitat and locomotion must be made. The ancestor
was most likely a relatively generalized tree-dwelling animal
that could walk quadrupedally along branches as well as
climb between them. From such an ancestor, two locomotor
trends were apparently derived. In one, which led to the
gorillas and the chimpanzees, the forelimbs became elongated
so that when these modern animals come to the ground they
support their trunks by placing the knuckles of their outstretched
forelimbs on the ground. The second trend involved shortening
the trunk, relocating the shoulder blades, and, most importantly,
steadily increasing the emphasis on hind limb support and
truncal erectness. In other words, this trend saw the achievement
of an upright bipedal, or orthograde, posture instead of
a quadrupedal, or pronograde, one. The upright posture probably
was quite well established by 3,000,000 to 3,500,000 years
ago, as evidenced both by the form of the limb bones and
by the preserved footprints of early hominids found from
this time.
