Fisher hypothesizes that these differences stem from differing evolutionary forces governing mate choice. In prehistory (and today), a male was obliged to size up a potential female partner visually to ensure that she is healthy and age-appropriate to bear and rear their potential progeny.
Fisher hypothesizes that these differences are due to varying evolutionary pressures influencing mate selection. In prehistoric times (and today), a male's ability to assess a female's health and suitability for reproduction was crucial.