Evolution was chemically constrained.
The objective of this paper is to present a systems view of the major features of biological evolution based upon changes in internal chemistry and uses of cellular space, both of which it will be stated were dependent on the changing chemical environment. The account concerns the major developments from prokaryotes to eukaryotes, to multi-cellular organisms, to animals with nervous systems and a brain, and finally to human beings and their uses of chemical elements in space outside themselves. It will be stated that the changes were in an inevitable progression, and were not just due to blind chance, so that "random searching" by a coded system to give species had a fixed overall route. The chemical sequence is from a reducing to an ever-increasingly oxidizing environment, while organisms retained reduced chemicals. The process was furthered recently by human beings who have also increased the range of reduced products trapped on Earth in novel forms. All the developments are brought about from the nature of the chemicals which organisms accumulate using the environment and its changes. The relationship to the manner in which particular species (gene sequences) were coincidentally changed, the molecular view of evolution, is left for additional examination. There is a further issue in that the changes of the chemistry of the environment developed largely at equilibrium due to the relatively fast reactions there of the available inorganic chemicals. Inside cells, some of these same chemicals also came to equilibrium within compounds. All such equilibria reduced the variance (degrees of freedom) of the total environmental/biological system and its possible development. However, the more sophisticated organic chemistry, almost totally inside cells until humans evolved, is kinetically controlled and limited by the demands of cellular reduction necessary to produce essential chemicals and by the availability of certain elements and energy. Hence the variability of reductive cellular organic chemistry and its limitations in cells have to be considered separately. While as a whole they drive the oxidation of the environment, they also allow speciation within the major changes of organisms. Human beings have introduced recently new, virtually irreversible, inorganic and organic chemistry in the environment, much of it new modes of irreversible storage of reduced chemicals, and this is, we state, the last possible step of chemical evolution. We must attempt to evaluate its effect on organisms generally. It must be clear that all the changes and the original life forms are dependent upon energy as well as material capture and flow. We shall have to consider in which forms energy was available over the period of evolution, how it was usefully transformed, and the ways in which its sources changed.