Denounced by the academic left, our main backers are in fact "selfish gene" Darwinians who translate everything into the language of "costs and benefits", "investments", "trade-offs" and strategies for "profit-maximising". Theirs is the language of free-market capitalism - and it works. They wouldn't consider solidarity and strike action as factors in human evolution unless forced to by the logic of the case.

So what is our theory? It sets out to explain the establishment of moral rules concerning sex. Accepting Darwinian premises, we may treat the male, on a biological level, as the "leisured sex". His best strategy for immortalising his genes is to get someone pregnant, leaving her with all the baby-producing work. Then, without wasting time, he should impregnate yet another female. In the case of human evolution, as brains were growing larger and babies becoming burdensome, the childcare work left to females was increasing. To make ends meet, mothers had no choice but to resist a dynamic in which males were left unburdened and free to philander. This resistance lasted some 400,000 years, culminating eventually in revolutionary change.

At that point, human language and culture were born. How then did female strategies succeed in changing male behaviour? First came concealment of ovulation combined with continuous sexual receptivity. Deprived of the vital time-saving information, males were now forced to stay with their partners in order to get them pregnant. But with ovulation concealed, menstruation became especially salient as a signal. Indicating imminent fertility, menstruants now attracted special male attention. To non-menstruating local females - those already pregnant or breast-feeding - this posed a threat. How could they prevent their mates from abandoning them in favour of such evidently impregnateable rivals?

There was a solution. Whenever someone began menstruating, the others had to prevent her from being "privatised" by some dominant male. Sisters and more distant kin rallied round her, bonding with her and presenting a united front, even to the point of sharing the basic signal. If anyone was bleeding, all had to "bleed" together, even if it meant "painting up" with animal blood, red ochre or other cosmetics. The final step was to use the accumulated solidarity not only to attract males, but - whenever meat supplies were low - to send them away periodically. To this end, the sisters would stage a menstrual ritual, signalling "no sex" for a while. To defend themselves against possible harassment, sons and brothers were drawn in. Only when the hunt had proved successful would the "sex strike" be called off. The human revolution was consummated when such performances were staged on a monthly basis, each menstrual ritual beginning at dark moon and ending with a communal feast at full moon. In this way, with women in possession of their bodies, sexual morality was established, laying the basis for the rest of culture, including language.

Archaeological evidence for the mining of red ochre and the manufacture of cosmetics suggests a time and place for this event: sub-Saharan Africa some 130,000 years ago. But what other theories are there? How do our "selfish gene" Darwinian colleagues think language and culture emerged? In fact, they freely admit to being puzzled. Assumptions about free competition just don't seem to work. They are turning to our theory because it speaks their own "industrial relations" language, it's the only theory currently on offer, and its predictions seem to be borne out.

What scientists most require of a model is testability. We can offer this. If our theory was correct, we wouldn't expect a hunter-gatherer culture anywhere in the world where marital sex was thought good for hunting luck. We'd expect pre-hunt ritual celibacy. Likewise, menstruation should be a focus of immense ritual attention and linked, symbolically, to the blood of animals. We might expect the earliest archaeological evidence for "art" to consist of red colouring materials for use in body-painting. Cave art should celebrate the ritual potency of women and associated game animals rather than, say, marital or family life. Religions should focus on shared blood as the basic symbol of communion with things sacred. Such detailed predictions render us vulnerable to falsification. We could, in principle, be proved wrong. Very possibly we are wrong. But it is this testability which distinguishes science from political ideology.