Someone asked me this last night. I hypothesized that it might be the result of a selective process – for instance,
if I, an aware human being, wish to deliberately conceive, I will have a better chance of doing so if I can gauge when I’m fertile by external cues, such as the phase of the moon. So a regular cycle about a month long will lead to greater reproductive success.
I have to admit that I am skeptical of my own hypothesis, but I have never heard or read anything about this. Does anyone wish to shed some light on the subject?