And so, several million years later, here we are: men are machines for making and firing billions of tiny, competitive, kamikaze sperm. Women, on the other hand, are the creators of a limited number of precious, embryo-nurturing eggs. Sperm are tiny, high-risk life-forms; they are expendable; they must put all their chips on one number; they must be bull-headed and blindly confident.
Eggs are scarce. Eggs don't want to move far from home; if they do, it's curtains for them. Sperms are buccaneers, adventurers. The last thing they want to do is stay at home. Does any of this sound familiar? Well, as Dawkins says, you just have to look at the relative sizes of sperm and eggs to reach the conclusion that “it is possible to interpret all the other differences between the sexes as stemming from this one basic difference.”
So how do we behave, we sperm-carriers? What does our status as sperm-shooters do to us?
I think we all know the answer to that question: being sperm-vectors makes men want to behave like Mick Jagger, Jack Nicholson, and Bill Clinton. Having millions of sperm puts certain men at an evolutionary advantage, yet this is something which, in a civilized society, we expect them not to cash in on. Still, as human biologist David Buss, author of The Evolution of Desire, would tell you, guys behave like guys because they are full of sperm; those who behave less like guys are less likely to pass their genes on.
When a man is away from his sexual partner, the number of sperm he produces dramatically increases. This is because, in evolutionary logic, his partner may have been unfaithful to him — she may be carrying another man's sperm inside her. So he automatically increases production, from 400 million sperm per ejaculation to as many as 700 million.
What does all this mean? It means that, since the Stone Age, males wanting to pass their genes on can do two things. Having a boundless supply of sperm, they can spread their seed far and wide, hoping that some of it will take root and flourish. Or they can settle down with a single mate and hang around, making sure their offspring is fully catered for. And then, when they have the chance, they can try to impregnate somebody else on the side.
Sadly for civilized society, this is an adaptive trait. It must be the case, because, if you're a guy, your genes are bound to be replicas of the genes of the most successful impregnators from the past. You are descended, in other words, not just from shaggers, but from the biggest shaggers in history.
Here's a good thing you can say about sperm: they may be the reason for the female orgasm. One theory is that having an orgasm is likely to keep a woman in a reclining position after having sex, thus minimizing “flowback.”
Another theory, as described by David Buss, is that “women's orgasm functions to suck up the sperm from the vagina into the cervical canal and uterus, increasing the probability of conception.”