Humans, apes, monkeys, and also tarsiers and lemurs are all considered primates. Primates is an order within the class Mammalia. All mammals are warm-blooded vertebrates with a coat of hair and which nurse their young from mammary glands.
But primates have additional characteristics that separate them from all other mammals. We will look at some of the important ones. Primates have large brains in relation to their body size.
The average human brain of about 1,300 grams is not the largest brain in the animal kingdom, but it is the largest in relation to body mass. Brain size is not the... only factor in intelligence, however.
The density of neurons, or nerve cells, within the cerebral cortex is important, as is the speed at which nerve impulses travel along those neurons. In intelligence, humans rank highest in the animal kingdom, followed by the apes, whales, and elephants. Next come the monkeys, followed by all other animals.
Primates not only... have relatively large brains, but more of their brain power is devoted to vision than in other animals. Notice the flattened face of the primate with both eyes in the same plane and facing forward to provide excellent stereoscopic or 3D vision.
Compare this with other mammals that have an elongated snout and more of their brain devoted to the sense of smell. The lemur is considered one of the most primitive of the primates because of its long snout and keen sense of smell. It also has six mammary glands rather than only two, as other primates have. Primates have a long gestation period and a long developmental period following birth.
The human baby is in the womb for nine months, the mountain gorilla for eight and a half months, and the orangutan a little more than eight months. The black bear, on the other hand, is in the uterus for seven months, and the Florida panther only three months. Development after birth is also slowed for primates. The human baby will not walk for almost a year, but a wildebeest can stand within 7 minutes of birth. The wildebeest will reach sexual maturity in 4 years or less, as opposed to the human which reaches adolescence in the early teens.
With a large brain but few instincts, the primate has much to learn before it is equipped to survive on its own. Primates have five digits on the hands and feet, with flattened nails at the ends of the digits. Old world monkeys and apes have a true opposable thumb.
New world monkeys have grasping hands, where the thumb is either non-opposable or only partly so. A true opposable thumb is a huge advantage that allows a primate to manipulate objects. The human foot is specialized for walking rather than grasping and has no opposable digit. In the primates, especially in the apes, there is a tendency toward holding the torso in an upright position and for bipedalism.
Most of the time, however, apes rely on knuckle walking. Bipedalism is fully developed in only the human. Finally, the shoulder joints of primates allow for rotation in all directions.
This is different than in most mammals, whose legs move primarily in one plane. The tremendous mobility of the shoulders and arms, as well as the dexterity of the hands, and especially the large size of the brain, have given primates distinct advantages in escaping from predators and providing for their own survival.