Almost all primates have 10 fingers and 10 toes except for

The correct answer is - spider monkey.

One of the largest monkeys of the New World, the spider monkey, has four fingers and toes. This primate is an exception in the primate family, as the other members have five fingers and five toes.

The spider monkey is an arboreal primate. It has a very long tail that it is used as a fifth hand. The fingers and toes are very long, and they are perfectly adapted for the arboreal life. Probably the lifestyle of this primate, as well as the much more elongated fingers and toes, and the usage of its tail, has resulted in the reduction of the fifth fingers and toes.