Evolution of the human menopause

Menopause is an evolutionary puzzle since an early end to reproduction seem s contrary to maximising Darwinian fitness. Several theories have been prop osed to explain why menopause might have evolved, all based on unusual aspe cts of the human life history. One theory is that menopause follows from th e extreme altriciality of human babies, coupled with the difficulty in givi ng 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 i n running the increased risk of a further pregnancy when existing offspring depend critically on her survival. An alternative theory is that within ki n groups menopause enhances fitness by producing post-reproductive grandmot hers who can assist their adult daughters. Such theories need careful quant itative assessment to see whether the fitness benefits are sufficient to ou tweigh the costs, particularly in circumstances of relatively high backgrou nd mortality typical of ancestral environments. We show that individual the ories fail this test, but that a combined model incorporating both hypothes es can explain why menopause may have evolved. BioEssays 23:282-287, 2001. (C) 2001 John Wiley & Sons, Inc.