How long is too long? A woman is fertile through age 40 (and useful to care for offspring for another 10-20 years), a man is fertile through age 80 (and a genet

How long is too long? A woman is fertile through age 40 (and useful to care for offspring for another 10-20 years), a man is fertile through age 80 (and a genetically fit male would have valuable sperm long after his fitness showed in his phenotype). Why would our bodies adapt for a 20-40 year life span?