Breasts? Testicles? I'm not sure what you are thinking of here.
I call it the Percy Theory of the Three B's: brains, boobs, and boners. They drove human evolution by way of sexual selection. Boobs and boners are about sexual attraction, while brains are about increasing socialization abilities and the resulting improvement in sexual opportunities. Physical and social qualities determine who is desirable (dominant), and those best able to wend their way into the hearts of desirable partners contribute more of their genes to succeeding generations. The most successful are also the most dominant. Human brains, boobs and boners are all the result of a positive feedback relationship between physical and mental qualities and sexual selection.
I haven't had time to vet my theory in the scientific arena, but I'm sure it's correct.