yehhhhyy...2nd chapter na....matapos ko kaya ito...
Lord help me...
Humans come from a long line of ancestors, from reptile to mammal to ape, whose skeletons were built to carry their weight on all fours. Our ape ancestors probably evolved around 20 million years ago from small primates that carried themselves horizontally. Over the next several million years, some apes grew larger and began to use their arms to hold overhead branches and, perhaps, to reach for fruit. Then, six or seven million years ago, our ancestors stood up and began to move about on their hind legs. By the time the famous Lucy (Australo-pithecus afarensis) appeared inEast Africa 3.2 million years ago, they had adopted walking as their chief mode of getting around.
It was a radical shift. "Bipedalism is a unique and bizarre form of locomotion," says Craig Stanford, an anthropologist at theUniversityofSouthern California. "Of more than 250 species of primates, only one goes around on two legs." Stanford and many other scientists consider bipedalism the key defining feature of being human. "Some may think it's our big brain," Stanford says, "but the rapid expansion of the human brain didn't begin until less than two million years ago, millions of years after we got upright and began using tools. Bipedalism was the initial adaptation that paved the way for others."
Evolutionary biologists agree that shifts in behavior often drive changes in anatomy. Standing upright launched a cascade of anatomical alterations. The biomechanics of upright walking is so drastically different from quadrupedal locomotion that bones from the neck down had to change. The skull and spine were realigned, bringing the head and torso into a vertical line over the hips and feet. To support the body's weight and absorb the forces of upright locomotion, joints in limbs and the spine enlarged and the foot evolved an arch. As for the pelvis: It morphed from the ape's long, thin paddle into a wide, flat saddle shape, which thrust the weight of the trunk down through the legs and accommodated the attachment of large muscles. This improved the stability of the body and the efficiency of walking upright but severely constricted the birth canal.
All of these architectural changes, seen clearly in the fossil record, did not happen overnight. They came gradually, over many generations and over long periods of time, in small steps favored by natural selection.
