Biosynthetic pathways, gene replacement and the antiquity of life

The appearance of oxygen in the Earth's atmosphere, a by-product of oxygenic photosynthesis invented by primitive cyanobacteria, stands as one of the major events in the history of life on Earth. While independent lines of geological data suggest that oxygen first began to accumulate in the atmosphere c. 2.2 billion years ago, a growing body of biomarker data purports to push this date back fully 500 million years, based on the presumption that an oxygen-dependent biochemistry was functional at this time. Here, we present a cautionary tale in the extension of modern biochemistry into Archean biota, identifying a suite of examples of evolutionary convergence where an enzyme catalysing a highly specific, O-2-requiring reaction has an oxygen-independent counterpart, able to carry out the same reaction under anoxic conditions. The anaerobic enzyme has almost certainly been replaced in many reactions by the more efficient and irreversible aerobic version that uses O-2. We suggest that the unambiguous interpretation of Archean biomarkers demands a rigorous understanding of modern biochemistry and its extensibility into ancient organisms.