The hominid line faced a crisis with the confrontation between men and women over procreation and food. One important contributor to the solution was the development of language. Now, there are as many theories explaining the rise of language as there are explanations for the fall of Rome, so we shouldn't be too dogmatic here. We know that language started developing around this time -- several million years ago -- but it's difficult to assign a single cause. Perhaps it was toolmaking, or coordination of the hunt, or any of a hundred other factors. My belief is that it was all of the above, that language started off humbly but proved to be so useful in so many fields of human endeavor that it just kept growing in importance and expressive power.
Language permitted people to ask questions like, "Will you still love me in the morning?" or "Is it really my child?" The big leap here was making explicit the notion of tense that had already been developed as part of the social relationships mental module. Hominids had already developed the concepts of past and future as part of their social relationships. Concepts already in use, like reciprocity and obligation, were built on top of the concepts of past and future. But language made those concepts explicit. In so doing, the basic deal between the sexes became more accountable in two ways. First, the deal could be made in front of witnesses who would presumably help enforce the deal in the future. This is how the institution of marriage arose. By exchanging vows in front of both families, the man and woman made cheating on the deal much harder to get away with.
Second, the application of language to the battle of the sexes allowed each party to apply lie detection techniques to the other party's statements. We tend to think of lying as a simple activity: one merely speaks false words, and the listener will be fooled. In practice, the mental mechanisms behind language creation aren't so easily subverted. The words we speak are not simply stamped out on some mindless production line -- they are wrung out of the brain like water from a sponge. To deliberately speak an untruth requires a controlled distortion of the entire cognitive process. Sure, it's possible, but it requires great effort and concentration, and can never be done perfectly. No matter how hard we try, a lie never emerges from our minds as smoothly and cleanly as the truth does. A skilled observer can often detect the effort of lying. The cues are subtle, and some people are better at it than others, but it can be done. Over the millenia, men and women have been locked in an arms race, each side improving its lie-telling skills even as it honed its lie-detecting talents.
There's always a catch, and in this case the catch was that language skills require huge amounts of brain matter. So once again, brains ballooned, skulls swelled, and childbirth became even more difficult. This was starting to look like a vicious circle. It is entirely conceivable that the hominid line could have been snuffed out by a failure to keep up with the ever-growing demands of that monster brain perched on top of that scrawny spinal cord. But luckily, the benefits of larger brains were always one step ahead of the costs, and the hominid brain expanded like a tick who's struck an artery.
But lurking inside those developing linguistic skills lay something else that was to have profound significance for humankind.