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These two female rhesus macaques (Macaca mulatta)—mother on the left with her daughter on the right—are on the island of Cayo Santiago, off the northeastern coast of Puerto Rico. Rhesus macaque monkeys live in groups of multiple males and females. Their hands have long fingers with nails and a divergent thumb, which they use for grasping food in trees and for social interactions, such as grooming. The hands of humans and rhesus macaques look very similar. That is because we share an evolutionary history that can be traced back to a time when our ancestors lived in trees.