Human females are unusual among mammals in experiencing menopause, a nonvoluntary and irreversible cessation of fertility long before the senescence of other bodily systems and the end of average lifespan. Two main hypotheses have been proposed to explain the origin of menopause. Some evolutionary biologists and anthropologists suggest that menopause is an adaptation—the result of natural selection for a postreproductive lifespan to permit increased maternal investment in rearing offspring. At some point in human evolution, females who ceased reproducing before the end of their lives gained a genetic fitness advantage from dedicating their remaining reproductive effort to enhancing the reproductive success of their existing progeny. Others hypothesize that menopause is an epiphenomenon (by-product)—either the result of a physiological trade-off favoring efficient early reproduction or merely an artifact of increased lifespan and life expectancy.