In his first book, "On the Origin of Species," Charles Darwin deliberately stayed away from discussing the evolution of humans. He knew it would be a controversial topic, and he just did not have enough data at the time to make his argument. However, about a decade later, Darwin published a book dealing with just that subject called "The Descent of Man." As he suspected, this book began what has been a long-lasting debate and cast evolution in a controversial light.
In "The Descent of Man," Darwin examined special adaptations seen in many types of primates, including apes, lemurs, monkeys, and gorillas. They were very structurally similar to adaptations human have as well. With the limited technology in Darwin's time, the hypothesis was criticized by many religious leaders. Over the last century, many more fossils and DNA evidence have been discovered to lend support to the ideas that Darwin put forth as he studied various adaptations in primates.
All primates have five flexible digits at the end of their hands and feet. Early primates needed these digits to grasp tree branches where they lived. One of those five digits happens to stick out of the side of the hand or foot. This is known as having an opposable thumb (or opposable big toe if it is off of the foot). The earliest primates only used these opposable digits to grasp branches as they swung from tree to tree. Over time, primates began using their opposable thumbs to grasp other objects like weapons or tools.
Almost all animals with individual digits on their hands and feet have claws at the ends for digging, scratching, or even protection. Primates have a flatter, keratinized covering called a nail. These fingernails and toenails protect the fleshy and delicate beds at the end of the fingers and toes. These areas are sensitive to touch and allow primates to sense when they touch something with their fingertips. This helped with climbing the trees.
Ball and Socket Joints
All primates have shoulder and hip joints that are called ball and socket joints. As the name implies, a ball and socket joint has one bone in the pair with a rounded end like a ball and the other bone in the joint has a place where that ball fits in or a socket. This type of joint allows a 360-degree rotation of the limb. Again, this adaptation allowed primates to climb easily and quickly in treetops where they could find food.
Primates have eyes that are on the front of their heads. Many animals have eyes on the side of their heads for better peripheral vision, or on top of their heads to see when submerged in water. The advantage of having both eyes on the front of the head is that visual information comes from both eyes at the same time and the brain can put together a stereoscopic, or 3-D image. This gives the primate the ability to judge distance and have depth perception, allowing them to climb or leap higher in a tree without falling to their deaths when misjudging how far away the next branch may be.
Large Brain Size
Having stereoscopic vision may have contributed to the need to have a relatively large brain size. With all of the extrasensory information that needed to be processed, it follows that the brain would have to be larger to do all of the necessary work at the same time. Beyond just survival skills, a larger brain allows for greater intelligence and social skills. Primates are mostly all social organisms who live in families or groups and work together to make life easier. Subsequently, primates tend to have very long life spans, mature later in their lives, and take care of their young.