ponds and in the small puddles on beaches. From macro-molecules, pre biological "cells" arose. These were complex units that exhibited many of the characteristics of biological cells including the formation of a selective membrane, internal movement resembling cytoplasmic streaming, orderly arrangement of internal molecules, growth in size and complexity, budding, and aggregation in clusters. Within the pre biological cells, the intense concentration of proteins and nucleic acids increased the probability that these chemicals would engage in reactions that exerted a selective and regulatory influence on the pre biological cells. Where the creation of new pre biological cells by physical fragmentation left off and the reproduction of new biological cells via nucleic acids began, is impossible to say. No abrupt transition between pre biological cells and biological cells ever occurred; instead, the transition happened extremely gradually.
The earliest biological cells were heterotrophs that fed on organic compounds much as pre biological cells had done. These early cells probably made use of ATP as their energy source since adenine and, hence, ATP formed readily under the abiotic conditions of the early earth. As the supply of organic nutrients, such as ATP, declined and competition between organism increased, selection favored the organisms that were superior at obtaining or processing food or that could produce the needed nutrients from raw materials via enzymatic activity. Yet, if life had remained purely heterotrophic all nutrients, including ATP, would have been eventually used up because the organisms' metabolism released into the atmosphere, a gas from which the
