Primates are a mammalian order placed as first of all mammalian orders by Carl Linnaeus (1707-1778), the Swedish naturalist who laid down the system for classifying living organisms, because it is the order to which man belongs. In addition to man, other members of the order include the apes (gorillas, orangutans, chimpanzees and the gibbons), old world and new world monkeys, lemurs, tarsiers and lorises. There are certain characteristics which are common amongst primates including nails or claws on fingers and toes, possession of a collar-bone, bone enclosed eye sockets, opposible thumbs or big toes, mammary glands on the chest, etc. Taken individually, none of these characteristics is peculiar to primates, but, when taken together, they form a pattern of attributes peculiar to the members of the order.
Of all primates, the apes are the ones that most resemble man, structurally, physiologically and psychologically, so it will not come as a surprise that in the study of man's evolutionary history, the apes have played a very prominent role. Yet, although the essential similarities between man and apes are so striking, e.g. all are tailless, all can stand erect, all have manipulatable hands and almost equally dextrous feet, all show a wide variety of facial expressions, etc., there exist some clear differences which sets man clearly apart from apes.
The first and, arguably, the most important difference between man on the one side and the apes on the other is the brain, or, rather, the difference in the size of the brain. Compared to all other animals, primates or otherwise, man is quite simply brain heavy. Although both man and the apes have relatively large and convoluted brains, man's is larger and more convoluted. The human brain, which averages about 1.5 kilograms, is more than twice the size of an average gorilla brain which weighs a mere 600 grams (somewhat less than half the brain weight that is considered to be the minimum required for rational human behavior). A gorilla can easily weigh twice the weight of the average human, and have a brain almost four times the size of the brain of a chimp or an orangutan, which weighs just about 400 grams.
Impressive as the above figures are, what is even more striking is the proportional weight of the brain to overall body weight. In gorillas, the brain constitutes only about 0.2 percent of overall body weight, in orang-utans it is about 0.5 percent and in chimps it is about 0.8 percent. By comparison, the human brain constitutes about three percent of the overall body weight of man, a not inconsiderable difference. Given the relative bigger size of the chimp's brain, perhaps it is not surprising that chimps have displayed the best ability of the great apes to learn, from a human point of view, that is.
The second major difference between man and apes is that apes are predominantly arboreal, i.e. tree dwelling, in their habits and are essentially four handed, the soles of the feet being in-turned and able to serve as a second pair of hands. Man, on the other hand, has left any arboreal phase, if he ever had any, far back in evolutionary history and has developed flat soled feet better suited for his almost perpetually upright habits and has become almost entirely two-handed. There is a possibility that far back in evolutionary time, the common ancestor of apes and man was a rock climbing primate, whose descendants split, on one hand taking to the trees and leading to the arboreal apes, and on the other, taking to the plains and leading to terrestrial man. This view is supported somewhat by the mountain gorilla, the least arboreal of all the great apes, whose feet tend more towards the human norm than the feet of the lowland gorilla or the other great apes. But, such a separation, if it did occur, must have taken place several millions of years ago.
Human and ape embryos share many similarities, starting off as single microscopic cells and growing to maturity giving the observer a quick study in evolution as the foetus passes through different stages reminiscent of invertebrate, lower vertebrate, general mammalian and, finally, primate form. Only in the final stages of gestation can the foetuses be distinguished and the similarities persist after birth, e.g. the baby's powerful grasp, which is useful for holding on to mum when she is flitting through the trees which is no longer of use to the human species and the in-turned soles of the feet which flatten out in the human as he/she grows older but remains just so in the apes. But as the youngsters grow older, the differences becomes more and more pronounced. Man retains many immature features into adulthood, a tendency called foetalization, features such as the large brain case, small jaw and teeth, vertical face, etc., but apes, as they grow older, lose these immature features which become submerged in the increase of the facial parts and of the bony outgrowths that are necessary to support the powerful muscles that are vital for the arboreal lifestyle of ape primates.