Final answer:
Primates may have developed nails instead of claws as an adaptation for precise grasping and tactile sensation in both arboreal environments and predatory behaviors, as per the visual predation hypothesis proposed by Matt Cartmill.
Step-by-step explanation:
Primatologists have theorized various reasons why primates have evolved to have nails instead of claws. One hypothesis stands out: the visual predation hypothesis, proposed by Matt Cartmill, suggests that primate features such as forward-facing eyes, the presence of nails rather than claws, and grasping hands and feet are adaptions not necessarily specific to living in an arboreal environment, but also potentially to succeeding as predators hunting insects and small prey in shrubbery and the lower canopy. This contrasts with the arboreal theory, which posits that primates' opposable thumbs and big toes, stereoscopic vision, and a tendency towards brachiation are adaptations to life in the trees. Each of these traits supports climbing and swinging through trees, which are essential for arboreal animals. Nails, in this context, possibly evolved from claws to assist with the delicate manipulation required during predation and more precise movements in a three-dimensional arboreal habitat. They are part of a suite of adaptations that include a rotating shoulder joint, a big toe widely separated from other toes, and enlarged brain size, all of which suit the complex environment primates have adapted to occupy. Nails also contribute to enhanced tactile senses in the fingertips, important for both arboreal maneuvering and the precision required of a visual predator.