AP BIOLOGY

Detailed explanation-1: -The common ancestors of all primates evolved an opposable thumb that helped them grasp branches. As the grasping hand evolved, claws disappeared. Today, most primates instead have flat fingernails and larger fingertip pads, which help them to hold on.

Detailed explanation-2: -Primate derived traits include opposable thumb and big toe, prehensile hands and feet, nails instead of claws on the digits, ability to sit for extended periods of time in an upright position without using the upper limbs for balance, reliance on vision, and reduced sense of smell.

Detailed explanation-3: -Primates with fully opposable thumbs include the Great apes (humans, chimpanzees, gorillas, and orangutans) and Old World monkeys (those native to Asia and Africa) such as baboons and Colobus monkeys. A fourth group of monkeys have comparatively long opposable thumbs.

Detailed explanation-4: -Harrison, who is student at Michigan State University College of Human Medicine, explains that opposable thumbs evolved about 2.6 million years ago when humans began using stone tools more frequently.