Since our genetic makeup has changed very little in the last 150,000 years, I will make the key assumption that the main features of modern men's and women's reproductive life histories do not differ substantially from those present at the outset of our species. There can be no doubt that culture can affect sexual behaviors, but the features I will be referring to are more basic. For example, I assume that the average length of a contemporary woman's menstrual cycle and that of a current man's obsession with sex are both innate traits that ancestral humans exhibited. ...

  I will hypothesize that the male's behavior evolved soon afterward in response to the female's lead. In fact, I will argue that the history of our species could be written from the perspective that males have spent the last 150,000 years trying to regain the power they so emphatically lost to females when we differentiated away from Homo erectus. By examining the habits of modern human males and females, we can infer the many changes that emerged when the new, improved Homo sapiens female debuted in Nature's garden.

  The catalogue begins with the absence in Eve's daughters of some sort of signal that would inform a male that they were ovulating. Unlike the vast majority of other females, the one belonging to the human line does not advertise her ovulatory burst. With very few exceptions, other species' females have a distinct period of sexual receptivity during which they experience a powerful instinctual drive to mate. To the males of her species, a female emanates a distinctive "green light," whether olfactory, visual, auditory, gestural, or some combination thereof. These episodic heights of female sexual desire are exquisitely timed to coincide with her ovulation. Previously uninterested males are alerted by her attention-grabbing signals.

  Estrus, as this upsurge is called in female primates, promotes harmony between the sexes. When both male and female are equally excited about mating, it is likely that they will have an amicable and mutually rewarding encounter. Obviously, a considerable benefit accrues to the species if mating occurs in synchrony with ovulation. Sperm meets ovum, and conception occurs. Eve's daughters, however, lack this most basic sexual semaphore, having replaced it with concealed ovulation. Human ovulation is so cryptic that most women remain unaware when, precisely, their eggs have departed from their ovaries.

Further obscuring the timing of her ovulation, the human female acquired the potential to engage in sex, if she desired, 365 days of the year, during pregnancy, lactation, menstruation, and even after menopause. ...
  The innovations distinguishing the human female from other mammalian females mentioned thus far pale when compared with her most spectacular new feature. She became the first species who possessed the willpower to refuse consistently to engage in sex around the time she was ovulating. For that matter, she was the first animal of either sex , of any species, capable of deciding to remain celibate if she so desired.

  This resolve is the heart of Response W. This is the gift Natural Selection bestowed upon her for having to endure Factor X, high maternal mortality and painful childbirth. It is something that had heretofore never existed in the animal kingdom. Philosophers call it Free Will. And herein lies the crux of relations between the sexes. African Eve and her daughters developed the determination to choose consciously a course of action that overrode the instinctual circuits that drive every other species' females to copulate when they ovulate. Females of some other species may be able to choose which male among multiple suitors upon which they wish to confer their favors; an occasional female of any species may decide not to mate with anyone or at any time. But the human species was the first in which all the females evolved the capacity to decide consciously to refuse to mate during any one ovulation or all the time.

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