Microbiologists often say that if there is an energy-yielding reaction in nature, then there will be an organism that can make use of it. The relevance of that comment to the origin of life is that chemoheterotrophs evolved to various kinds of autotrophs. Some used sunlight, as various photosynthetic prokaryotes do today, bit others used such energy-yielding reactions as theoxidation of sulfur or iron. However, these latter reactions would imply the presence of a significant amount of gaseous oxygen in the environment. We have mentioned, in connection with Miller's work, that the early atmosphere was a reducing one. The change to an atmosphere such as today's probably is the result of the release, by photosynthetic organisms, of large amounts of oxygen as a metabolic by-product. But nevertheless, in this early nutritional evolution we need not think of evolution as following a single path from chemoheterotrophy to photoautotrophy. Along the way there were opportunities for chemoautotrophs, as well as photoheterotrophs, to emerge.
Biogenic macromolecules. Up to this point we have only discussed spontaneously occurring molecules (organic and inorganic) as nutritional building blocks and sources of energy
II. Nomenclature based upon ability to synthesize essential metabolites
A. AUTOTROPHY All essential metabolites are synthesized
1. Autotrophy sensu stricto
2. Ability to reduce oxidized inorganic nutrients
3. Mestrophy Inability to reduce one or more oxidized inorganic nutrients, i.e., need for one or more reduced inorganic nutrients
B. HETEROTROPHY Not all essential metabolites are synthesized i.e., need for exogenous supply of one or more essential metabolites (growth factors or vitamins)
C. HYPOTROPHY The self-reproducing units (bacteriophages, viruses, genes, and so on) multiply by reorganization of complex structures of the hostalong with sunlight or even heat. This means we have been ignoring another and by then extremely important source of molecules. These are the molecular products of organismic metabolism or biogenically produced compounds. If these compounds had not been utilized, the resources of the earth would have been eventually exhausted by living organisms. When, through natural selection, organisms began to utilize the products of other organisms, then recycling emerged on earth.
From this emergence of recycling, there appeared ecosystems, and new interrelationships of life on earth. In fact, organisms became interlocked in mutual dependence. Sometime at this point in evolutionary history, there were again many chemoheterotrophs. Organic compounds were again in an abundance, but now they were not formed spontaneously but as a result of biosynthesis, or they were formed biogenically. These secondary chemoheterotrophs were likely quite similar, nutritionally, to the prokaryotic or bacterial chemoheterotrophs of today. We shall call them chemoheterotrophs II to distinguish them from the earlier chemoheterotrophs I, which extend back (speculatively) to the origin of life. In between these two types of chemoheterotrophs, distinguished by the source of the organic compounds they use, there were the photoautotrophs and various combinations of chemotrophs, phototrophs, heterotrophs, and autotrophs.