Bob Trivers was the first person to describe the conditions under which reciprocal altruism should evolve. Trivers is one of the more well-known biologists in the history of evolutionary biology. His paper is:
Trivers, R. L. 1971. The evolution of reciprocal altruism. Quarterly Review of Biology 46:35-57.
A LOT of biological research on altruism was inspired by his paper.
He reflects on the paper 30 years later here:
Trivers, R. 2005. Reciprocal altruism: 30 years later. In C.P. van Schaik and P.M. Kappeler (eds.) Cooperation in Primates and Humans: Mechanisms and Evolution. Berlin: Springer-Verlag, pp 67-83.
He spends a lot more time talking about humans in the second paper.