Margulis, L. (1981) The endosymbiotic theory. In: Symbiosis in Cell Evolution: Life and Its Environment on the Early Earth. San Francisco: W.H. Freeman & Co., pp. 1-14.

Doolittle, W. Ford (1998) A paradigm gets shifty. Nature 392: 15-16. (This is a short news article that examines the hypothesis put forth in the next article.)

Martin, W. M. and M. Müller (1998) The hydrogen hypothesis for the first eukaryote. Nature 292: 37-41. (Read this for the hypothesis it proposes, and how that contrasts with the hypothesis proposed by Margulis. Do not be concerned with the complicated biochemistry that is also discussed. I will attempt to "simplify" that for us.)