Biochemical Innovations, Oxygen and Earth History

Abstract

It is conventional to view the origin and early evolution of life as having taken place beneath an atmosphere devoid of free oxygen. Of primary concern is the fact that experiments designed to produce simple prebiotic compounds as precursor “building-blocks” for life are generally successful only if free oxygen is lacking from the starting mixture of gases. Clearly, life could hardly have evolved in the absence of suitable starting materials. Secondly, organic compounds are thermodynamically susceptible to oxygen and, given enough time, will become oxidized, ultimately to carbon dioxide. A centerpiece of biochemical evidence commonly invoked to support early anoxic conditions is the fact that almost all of the metabolic pathways for respiratory electron transfer in aerobic organisms take place anaerobically. Only the last step in the process requires free oxygen. It is easy to argue that this last step was an evolutionary response to the development of oxygenic conditions and was a later addition to the less energy-efficient pathways found in primitive anaerobes. The assumption in arguments of this type is that the present-day descendants of these early, primitive organisms are using similar biochemical pathways today that they used billions of years ago. For paleobiologists the meagre early Precambrian fossil record is of minimal help and such a uniformitarian approach toward deciphering early evolution is, together with the protein and nucleic acid sequence studies, one of the few methodologies available to go on.