Biosynthetic pathways, gene replacement and the antiquity of life

摘要

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 thatoxygen 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 thistime. 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, O2-requiring reaction has an oxygen-independentcounterpart, 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 O2. We suggest that the unambiguous interpretation of Archean biomarkers demands a rigorous understanding of modern biochemistry and its extensibility into ancient organisms.