Primates were the latest mammalian order to evolve during the Cenozoic era is the Primates. This order shows a series of ascending levels from the little tree shrew (a mouselike creature) through the lemurs and tarsioids to the monkeys and anthropoid apes—a series in which the intervening gaps are partly filled by fossils. This does not mean that man includes types like existing tree-shrews, tarsiers and lemurs in his ancestry, for all present-day animals of these types have shown considerable specialization, which has led them away from the main line of evolution culminating in the human species.

The Primates have certain definite characteristics that are shared by all members of the group from shrew to man. Among these are the forward-looking eyes and relatively large brain (smell is becoming of less importance than vision), the flexible hands or forepaws with their five fingers, including the opposable thumb which makes manipulation possible. Then instead of claws we usually find nails, and on the underside of the fingers soft, fleshy pads.

But what is important about the Primates is that they have produced, as well as more specialized types, a line of development which has been remarkably unspecialized. The other mammalian orders are clearly distinguishable because they are hoofed or marked out in some other way. The fact that the Primates are the least specialized order makes the evolution of man possible. Even the Primates specialize to some degree— note, for example, the immense development of arms and chest in the larger anthropoids which do not run and leap along the high branches but swing from bough to bough. This degree of specialization rules out the possibility that the higher apes, such as the chimpanzee and gorilla, were the ancestors of man. For, once this degree of specialization has been reached, it cannot be reversed. Evolution in different directions can only proceed from an unspecialized type. It is from such a type that the anthropoid apes were evolved by a long process of specialization for swinging in the branches of trees, which we call brachiation. Man on the other hand evolved in an unspecialized direction from this common ancestor.

The prehensile (adapted for seizing or grasping) hand and well- developed vision go with an arborial habitat, The Primates took to the trees, but later some types came down again; and it was only from these (from some of them) that man could evolve.

That life produced many adaptations in sensory perception and the use of the hands, advances which led to others in the brain, which receives the information newly acquired from these sense organs, converting it into appropriate action. The emancipation of the forelimbs from the duty of always supporting the weight of the body would leave them free for exploring and manipulating the environment, thus adding to the creature's mental horizon.