Behavioral Human vestigiality

goose bumps example of vestigial human reaction stress.

humans bear vestigial behaviors , reflexes. example, formation of goose bumps in humans under stress vestigial reflex; possible function in human evolutionary ancestors raise body s hair, making ancestor appear larger , scaring off predators. raising hair used trap layer of air, keeping animal warm. due diminished amount of hair in humans, reflex formation of goose bumps when cold vestigial.

the palmar grasp reflex supported vestigial behavior in human infants. when placing finger or object palm of infant, securely grasp it. grasp found rather strong. infants—37% according 1932 study—are able support own weight rod, although there no way can cling mother. grasp evident in feet too. when baby sitting down, prehensile feet assume curled-in posture, similar observed in adult chimp. ancestral primate have had sufficient body hair infant cling unlike modern humans, allowing mother escape danger, such climbing tree in presence of predator without having occupy hands holding baby.

it has been proposed hiccup evolutionary remnant of earlier amphibian respiration. amphibians such tadpoles gulp air , water across gills via rather simple motor reflex akin mammalian hiccuping. motor pathways enable hiccuping form during fetal development, before motor pathways enable normal lung ventilation form. thus, according recapitulation theory, hiccup evolutionarily antecedent modern lung respiration. additionally, point out hiccups , amphibian gulping inhibited elevated co2 , may stopped gabab receptor agonists, illustrating possible shared physiology , evolutionary heritage. these proposals may explain why premature infants spend 2.5% of time hiccuping, possibly gulping amphibians, lungs not yet formed. fetal intrauterine hiccups of 2 types. physiological type occurs prior 28 weeks after conception , tend last 5 ten minutes. these hiccups part of fetal development , associated myelination of phrenic nerve, controls thoracic diaphragm. phylogeny hypothesis explains how hiccup reflex might have evolved, , if there not explanation, may explain hiccups evolutionary remnant, held-over our amphibious ancestors.

this hypothesis has been questioned because of existence of afferent loop of reflex, fact not explain reason glottic closure, , because short contraction of hiccup unlikely have significant strengthening effect on slow-twitch muscles of respiration.