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Evolution

Evolution refers to changes in the heritable traits of successive generations of living populations due to natural or sexual selection. Evolution can lead to entirely new species or alter the representation of traits within a species. A basic assumption of evolutionary theory is that both physical and psychological traits will be selected over time if they enhance an individual’s ability to survive and successfully reproduce. Natural selection refers to the selection of traits that enhance an individual’s chances of surviving to reproductive age; sexual selection refers to traits that enhance one’s attractiveness to mates and therefore increase reproductive success. Sometimes natural selection and sexual selection are at odds, as in the case of the peacock’s tail. Elaborate plumage is metabolically costly and places a peacock in danger of attracting predators, but it evolved nonetheless because it is useful for attracting mates. This entry describes key aspects of human life-span development that have been understood through the application of evolutionary reasoning.

Evolutionary explanations are widely accepted for various enigmatic aspects of human development, including the extreme immaturity of the newborn at birth and the extended period of infants’ dependence on adults, phenomena such as infant–parent attachment and pair bonds between adult sexual partners, individual differences in the timing of puberty and the occurrence of menopause, as well as seemingly maladaptive psychological phenomena such as phobias and other forms of psychopathology. Applying evolutionary reasoning to human development involves examining similar behaviors in closely related species and in diverse cultures, especially nonindustrialized cultures. If a behavior is prevalent across a wide range of cultures and in related species, it is assumed to be at least in part genetically determined and may have adaptive significance. If a behavior appears among some members of a species and not others, it may be an adaptation to a specific environment. Caution is called for when considering human traits as adaptations, however, because morphological or behavioral traits can also arise through means other than natural or sexual selection.

The Infant’s Capacities and Immaturity at Birth

The human newborn’s behavioral repertoire at birth has both adaptive value and reflects our evolutionary history. Newborns are equipped with reflexive behaviors that include rooting (i.e., turning toward a nipple when stroked on the cheek), sucking, smiling, grasping, and crying. These seemingly unrelated behaviors all serve to promote proximity to a caregiver, which in ancestral human environments would have greatly increased the likelihood of an infant’s survival. The Moro reflex, which is present at birth and disappears within a few months, is believed to be a vestige of our evolution from the great apes: Newborn infants experiencing a sudden loss of support (i.e., given the sensation of being dropped) will spread their arms wide, quickly bring them together in a tight grasp, and then cry. This reflexive response to falling would be extremely useful among our close relatives, the great apes, who carry infants on their backs while climbing trees and foraging for food.

Compared with infants of other mammalian species, human infants are extremely immature at birth. They are unable to stand, crawl, walk, regulate their own temperature or feed on their own. Natural historians believe the extreme immaturity of human neonates may stem from a birthing crisis that occurred in human evolution. The obstetrical dilemma in human evolution refers to the competing selection pressures acting on two evolved human traits: bipedal locomotion (i.e., upright walking) and large brain size. On one hand, the evolution of bipedal locomotion was facilitated by narrowing of the bones in the human female’s pelvis. On the other hand, increasing intelligence spurred selection for larger and larger brain and head size. The extreme immaturity of human neonates is believed to be an evolved solution to the birthing crisis. By giving birth to premature babies whose brains were less fully developed, women were able to survive labor, and babies’ brains were able to continue to grow in size.

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