Answer
Humans belong to the order of primates. It is the same order to which chimpanzee, bonobos, and gorillas belong--tarsiers and lemurs also are part of this order. It is believed that these primates represent evolution from squirrel-sized insectivorous animals that took up life in the trees about 50 million years ago.
While life in the trees was faced with fewer predators, the arboreal habitat had its challenges. For example, one had to hold on securely, and be able to judge distances in order not to fall while jumping from branch to branch in pursuit of prey or to elude predators Under these and other selection pressures the new tree dwellers developed the following adaptations. : opposable thumbs that enabled secure grasping of branches and holding and manipulating small objects; stereoscopic vision that improved depth perception, and the judgment of distances.--this reduced the incidence of falling. The evolution of color vision facilitated discrimination between green and ripe fruits, and between young edible leaves and mature very green toxic ones. Also of great advantage was the development of a more flexible shoulder joint that enabled the owners to move their prehensile forelimbs in all directions,including overhead.
The development of bipedalism came much later, and occurred in some primates that dropped down to the ground in the African Savannahs as the forests receded because of increasing desertification. The vegetation of the savannahs was tall grass, and the new terrestrial primates had to stand on their hind legs to see ahead while travelling, or to look out for predators. In time the primates adapted to partial bipedalism as their skeletons and muscles changed to accomodate the practice of walking on two hind legs while sometimes simultaneously carrying objects or young with the modified forelimbs . It should also be noted that while bipedalism was very advantageous for the ground-living primates, they had little use for a prehensile tail--or any tail. Thus over many millions of years the tail disappeared, leaving only the vestigial coccyx to remind of its former existence.
Work Step by Step
Primates also developed large brains as their skeletons and muscles became adapted for bipedalism. A larger brain meant a larger skull. This presented a problem since primates had to be born through a utero-cervical passage with very tight spacing. To facilitate this difficult passage the human brain could not develop to full size or birth would be impossible. The evolutionary compromise was to have humans fetuses be born in a relatively early stage of embryonic development, and to have them complete
the rest of development to independence, post-nataly. As a consequence, primate offspring are relatively very dependent at birth and need extensive care-- including being carried by mothers-- for some time after birth. Bipedalism, prehensile forelimbs and flexible mobile shoulders help mothers to take care of dependent young. Large brains confer better memory useful in food finding and enabling tighter social organization for greater group support and defence.
An knowledge of the outlines of primate/human evolution makes it easier for us to understand several human and primate feature such as early birth of fetuses, human body hair, human bipedalism, and forward facing eyes. It also helps to account for such apparently useless vestigial features as the appendix, the coccyx, the auricularis muscle and wisdom teeth.
