An101 chapter 3: the beginnings of human culture. Primate order: the group of mammals that includes lemurs, lorises, tarsiers, monkeys, apes, and humans. Adaptation: a process that organisms undergo to achieve a beneficial adjustment to an available environment, plus the results of that process. Natural selection: the evolutionary mechanism by which individuals with characteristics best suited to a particular environment survive and reproduce with greater frequency than those without them. Diet available to arboreal primates requires unspecialized teeth. Over time, there is a trend toward economy, with fewer and smaller teeth doing more work. Number of cusps on molars increases: sense organs. Decreased dependency on a sense of smell. Corresponding increase in brain size in the visual area. More acute sense of touch: the primate brain. Opening of the skull for the spinal cord shifts forward toward the skull"s base, accommodating upright posture. Arms at side rather than the front part of the body.