What makes human hands unique?
The human opposable thumb is longer, compared to finger length, than any other primate thumb. This long thumb and its ability to easily touch the other fingers allow humans to firmly grasp and manipulate objects of many different shapes. The human hand can grip with strength and with fine control, so it can throw a baseball or sign a name on the dotted line.
Where's the thumb?
When moving quickly through the trees, spider monkeys use their hands like hooks and swing from branch to branch. Spider monkeys have evolved an extremely small thumb bone—a full-sized thumb would hinder their swinging.
Like A Fifth Hand
The spider monkey's prehensile, or grasping, tail can support its entire body weight. Each monkey has a unique pattern of lines on its tail, like a fingerprint, which helps the tail to grip branches.
For most mammals, the bigger the species, the slower it grows and the longer it lives. Primates take this pattern to the extreme, with even longer lives and slower growth rates, both in the womb and after birth. Slow growth may have evolved because it gives young primates more time to learn complex social behaviors. And it may foster the development of another classic primate feature—an unusually large brain.
One fast monkey
Although primates grow slowly, they aren't slow in every way. Patas monkeys can run at speeds up to 34 miles an hour! The fastest human sprinter reaches only 27 miles an hour.
The Great Brains
Among mammals the general rule is: the bigger the body, the bigger the brain. But the brains of most primates are a lot larger than one would expect based on their body size. The average talapoin monkey (Miopithecus talapoin) weighs only one-tenth as much as an African porcupine (Hystrix cristata). Yet their brains are almost exactly the same size.
Primates are social animals. The mating habits of each primate species affect the structure of its societies. Humans, siamangs and a few other species form simple, two-parent families. In some other species one male will live with a "harem" of several females. Still other primate groups are organized by rank, with the higher-ranked males getting more chances to mate.
The song of the siamangs
Siamangs, a gibbon species, choose one mate of the opposite sex, and the parents live with their young just as typical humans do. The early morning calls between two parents reinforce the pair's claim to their territory—the part of the Sumatran rainforest where the family feeds. Some researchers say that these calls strengthen the family's bond.
Chimpanzees and bonobos, the two apes most closely related to humans, form very different societies. Chimps establish a ranked hierarchy of males, with each male fighting and intimidating others to maintain his position. In extreme cases one chimp may kill another. Among bonobos females establish the structure of society, and status seems much less important. Bonobos collaborate and share food, and when tensions do arise, they use sex and play instead of fighting to solve the problem.
Source: Hall of human origins: understanding our past