Females of the ape-like predecessors of Homo sapiens almost certainly exhibited overt ovulation. Over the ensuing 5-6 million years, this has completely changed; modern males have little idea of when a female is ovulating since she provides no visual clue. Why has this change occurred?
The hominid group that discovered bipedalism found important new uses for the forelimbs that had been released from locomotive duties; fighting, gesturing, making tools, building lairs etc. Size of brain became strongly selected as the ability to handle higher levels of intentionality became more relevant to survival and procreation. The size of the birth-canal became a bottle-neck as the head of the hominid baby became bigger to accommodate the expanding brain. Since upright walking was compromised by widening the hip structure, the optimal solution was to opt for earlier birth and therefore a very immature baby. Females then found it impossible to nourish themselves and their highly dependent baby without the assistance of a male. The problem was twofold: firstly, how to get the father to help with the nurture of the child and secondly, how to get the father to stay to protect the female and her child from predators and other males in the longer term.
Attracting a multitude of sexual encounters during ovulation increased the chance of becoming pregnant but resulted in no particular male knowing whether or not he was the father. Therefore no male assistance with child-rearing. Overt ovulation had to go. But what are the advantages of covert ovulation? Males are attracted to copulate by (a) the genetically mediated desire to pass on their genes (b) hormonally-driven sexual desire and (c) enjoying the companionship and comforts provided by the female together with its status enhancing effect. Covert ovulation would mean that (a) could be moderated and copulation not confined to a relatively short period. Consequently, the relative importance of (b) and (c) would be increased. Furthermore the father would be more inclined to pair-bond and stay on to protect his female and her/ his offspring in the longer term.
Female emancipation, the availability of child-care, the welfare state and the rule-of-law mean that - these days - our new mother no longer needs a male. What next I wonder? Back to overt ovulation? Suggestions please!