http://www.abc.net.au/science/slab/tittat/story.htm
Long before humans started playing games, natural selection discovered the fundamentals of game theory and shaped animal societies according to its rules. Within species, individuals adopt alternative competing strategies with frequencies that reflect the success of each strategy. Evolutionary Stable Strategies occur when alternative competing strategies are at equilibrium. Competition within species has generated many Evolutionary Stable Strategies with colourful titles like: Bourgeois, Scrounger, Sneaky, Satellite, Transvestite, and Sex-change. However, co-operation within and between species has generated only one Evolutionary Stable Strategy., TIT FOR TAT.
The importance of TIT FOR TAT to the evolution of co-operative behaviour was discovered in a very unusual way, through a worldwide computer competition to find the winning strategy for the well known paradox 'The Prisoner's Dilemma'. In 1981 TIT FOR TAT won that competition, and ever since then it has grown in stature to where it now dominates our thinking about the evolution of co-operative behaviour in animal and human societies.