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The Wikipedia page on the Estrous cycle says:

Humans have menstrual cycles instead of estrous cycles. They shed their endometrium instead of reabsorbing it. Unlike animals with estrous cycles, human females are sexually receptive throughout their cycles.

The page on the menstrual cycles says:

The cycle occurs only in fertile female humans and other female primates

Do we know how many million years ago that switch happened and our female ancestors got sexually receptive throughout their cycles?

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  • $\begingroup$ See also How is menstruation not suicide in the wild?. Although it doesn't speak to your question of when, I discussed the theorized adaptive benefits of the cycle in an answer there. $\endgroup$
    – Susan
    Commented Dec 25, 2014 at 20:12
  • $\begingroup$ Emera et al. argue that menstruation is not directly adaptive but a side effect of adaptations pertaining to the maternal-fetal conflict. $\endgroup$
    – augurar
    Commented Dec 28, 2014 at 1:11

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Menstruation occurs only in most of the primates, and a handful of other mammals. It is tempting to speculate that place the switch to menstrual cycles evolved right after the split between non-menstruating primates (tarsiers and their relatives) and menstruating relatives (almost all others), which is about 63 million years ago.

There are other menstruating animals, so I am not sure if nature reinvented menstruation whenever social conditions required unproductive sex, or menses are in fact a more archaic trait.

I doubt we have material evidence about all these speculations.

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  • $\begingroup$ Any sources to add? $\endgroup$
    – AliceD ♦
    Commented Mar 1, 2017 at 15:02

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