Introduction

It is now generally accepted that life arose on the Earth early in its history. The sequence of events started with the synthesis of simple organic compounds by various processes. These simple organic compounds reacted to form polymers, which in turn reacted to form structures of greater and greater complexity until one was formed that could be called living. This is a relatively new idea, first expressed clearly by Oparin (1938) with contributions by Haldane (1929), Urey (1952), and Bernal (1951). It is sometimes referred to as either the Oparin–Haldane or the Heterotrophic Hypothesis (Horowitz 1945). Older ideas that are no longer regarded seriously include the seeding of the Earth from another planet (panspermia), the origin of life at the present time from decaying organic material (spontaneous generation), and the origin of an organism early in the Earth's history by an extremely improbable event. This last process assumed that the primitive Earth was essentially the same as at present, except possibly for the absence of molecular oxygen, and that the first organism would have to have been autotrophic. That is, it would have to have been photosynthetic, using light energy to synthesize all its organic compounds from CO2 and H2O.

One of Oparin's major contributions was to propose that the first organisms were heterotrophic, that is, organisms that utilized prebiotically produced organic compounds available in the environment. They still had to build proteins, nucleic acids, and so forth, within their cells, but they did not have to synthesize de novo the amino acids, purines, pyrimidines, and sugars that make up these biopolymers.