Evolution of the human menopause.

Menopause is an evolutionary puzzle since an early end to reproduction seems contrary to maximising Darwinian fitness. Several theories have been proposed to explain why menopause might have evolved, all based on unusual aspects of the human life history. One theory is that menopause follows from the extreme altriciality of human babies, coupled with the difficulty in giving birth due to the large neonatal brain size and the growing risk of child-bearing at older ages. There may be little advantage for an older mother in running the increased risk of a further pregnancy when existing offspring depend critically on her survival. An alternative theory is that within kin groups menopause enhances fitness by producing post-reproductive grandmothers who can assist their adult daughters. Such theories need careful quantitative assessment to see whether the fitness benefits are sufficient to outweigh the costs, particularly in circumstances of relatively high background mortality typical of ancestral environments. We show that individual theories fail this test, but that a combined model incorporating both hypotheses can explain why menopause may have evolved.