The diversification of the first primates from other early mammals took place partly because the ancestors of the primates came to inhabit a unique environment --- the trees. These early primates developed many features to allow them to move quickly in this arboreal habitat. Early primates evolved the ability to direct and focus both eyes on objects, called binocular vision, which allowed them to accurately judge distances to branches and other objects. They also developed grasping hands and feet, with opposable digits --- thumbs and big toes. The fingertips were broader in these early primates, to apply greater grip strength to branches, and instead of claws primates developed wide nails. All of these features helped primates to succeed as arboreal specialists.
- Primate adaptations to arboreal environments include binocular vision, opposable digits, enlarged fingertips and nails.
Many primates today continue this arboreal existence, spending large proportions of their time off the ground and in the trees. Moving in an arboreal environment has the obvious risk of falling out of the tree if everything does not go exactly right. Yet primates are virtual trapeze artists, masters of the arboreal environment. Smaller monkeys and lemurs can rush headlong from branch to branch, seemingly mindless of any risk of falling, because of their unrivaled arboreal skills. These primates bridge immense gaps by leaping from one tree to another, limited only by the acceleration of gravity. Even large primates like chimpanzees and orangutans can rapidly scale trees and move effectively from one tree to another. These arboreal skills are made possible by a large suite of adaptations, many of which originated very early in primate evolution.
But even though all primates are climbers, some of them have developed more specialized adaptations to other kinds of movement, or locomotion. These specialized forms of movement are adapted to different kinds of environments. Some primates are excellent at moving terrestrially, on the ground. Others are good at climbing tall vertical trunks and branches, or at swinging from one branch to another. Species who use these special forms of locomotion have consequences in their skeletons and muscle configurations.
Vertical clinging and leaping
Many prosimians, including tarsiers and many lemurs, use a form of locomotion called vertical clinging and leaping. These kinds of primates live in habitats where they climb relatively small tree trunks or bamboo. Sometimes they leap between these vertical supports. Other times, especially for the larger lemurs like sifakas, they leap along the ground. Their legs are relatively long compared to their arms, so that terrestrial movement can be more effective by leaping than by running on all fours.
This form of locomotion gives their bodies a very distinctive shape --- especially tarsiers, who are masters of this pattern. Tarsier legs are longer than their bodies and forelimbs put together, and they especially have very long foot bones. The name for these bones are tarsals, giving these unique little primates their name.
Some monkeys and all apes are adapted more to hanging beneath branches than running atop them. This suspensory style of locomotion is essential for larger arboreal primates, which can move above only the largest branches. Hanging below branches enables large primates to climb into the forest canopy, often on smaller branches than could support them from above.
A skeletal adaptation to suspension includes relatively long arms with very mobile shoulder joints. In contrast to quadrupedal monkeys, whose arms are limited in their range of motion, apes can move their arms through nearly a complete 360 degree circle. Also, apes' trunks are relatively flat from front to back, with the shoulders mostly alongside rather than in front of the spine. Putting the arms at the side requires long and strong collarbones --- called clavicles --- that serve as a strut supporting the shoulder musculature. Also, apes have extremely long fingers, useful for hooking onto branches quickly. Their thumbs are quite small, as they do not usually grip with their thumbs onto branches while hanging from them.
- Brachiation is arm-over-arm swinging.
Sometimes apes swing arm over arm from one branch to the next, a locomotor pattern called brachiation. Gibbons and siamangs are a brachiation specialists, but all living hominoids have the skeletal anatomy to enable them to brachiate. Brachiation conserves the energy of forward movement by treating the body as a swinging pendulum. This is a very efficient way to travel for medium-sized primates, combining energetic conservation with speed, and working with gravity instead of against it.
- Quadrumanous locomotion uses all four hands and feet to move among branches.
Large-bodied hominoids brachate less than gibbons. Branches of sufficient size to support the entire weight of a large ape are rarely located close enough to brachiate between them. Also, the pendular motion is less efficient for larger primates, because their arms just cannot be as long in proportion to the size of their bodies. Instead, when in the trees, the living great apes grip branches with three or four of their hands and feet at once. This allows large apes to support their weight on relatively small branches in the canopy. The masters of this kind of locomotion are orangutans, who move through the forest canopy moving one hand or foot at a time to a new support branch. This kind of locomotion, as if an animal was using four hands equally, is called quadrumanous locomotion.
Knuckle-walking and fist-walking
- Knuckle-walking allows quadrupedal walking in chimpanzees and gorillas, whose hands are adapted to suspension.
Chimpanzees and gorillas, when they are on the ground, walk quadrupedally using the proximal finger joints of their hands instead of the palms of their hands. They use this style of locomotion, called knuckle-walking, because of their unique forelimbs. These apes have long forelimbs relative to their hindlimbs, and their hands are very long with strong tendons for curling the long fingers into a powerful hook. These hands are very well adapted to suspension in the trees, but they are not capable of being extended with the palms toward the ground, which is the way most other primates walk quadrupedally. So instead of walking palm-down, they use their knuckles.
A few skeletal features of living chimpanzees and gorillas appear to be adaptations to knuckle-walking. Because the arms are held in a locked position while supporting the body, unlike the somewhat flexed position usual in quadrupeds, the proximal end of the ulna is more U-shaped, built for supporting the humerus mainly from below. Likewise, the joints of the hands are more limited in motion, and relatively incapable of being extended backward at the wrist and knuckles. It is not clear whether these unique anatomical consequences of knuckle-walking evolved in parallel in chimpanzees and gorillas, or whether they represent homologies inherited from a knuckle-walking common ancestor of gorillas, chimpanzees, and humans.
Although many orangutans spend little time on the ground in their natural habitat, they can walk quadrupedally. But unlike chimpanzees and gorillas, orangutans do not walk on their knuckles when they are on the ground. Instead, they curl their hands inward, walking on the outside of their fists. This fist-walking accomplishes the same goal as knuckle-walking --- it allows walking on all fours without facing the palms of the hands toward the ground.