Credit: © NPG

During the Great Oxidation Event about 2.4 billion years ago, the surface of the Earth tipped irrevocably into an oxygenated state, as free molecular oxygen began to accumulate in the oceans and atmosphere. But the first whiffs of oxygen began to appear at least 300 million years earlier, as organisms capable of producing the gas through photosynthesis evolved. As the Earth's chemistry changed, so too must have the microbes that lived on its surface. But the rock record leaves only hints of the ecosystem, primarily in the form of isotopic fractionation of the elements — including iron and sulphur — that presumably fuelled the bacteria.

To assess the evolution of these metabolisms, Lawrence David and Eric Alm of the Massachusetts Institute of Technology looked to the genetics of extant organisms. They re-examined existing gene families using a technique that accounts for both the evolution of new genes, and the transfer of genes between different species (Nature 469, 93–96; 2011). They identified a narrow window of genetic expansion between about 3.3 and 2.8 billion years ago, which they call the Archaean expansion.

During this period, almost 27% of the largest modern gene families arose. Perhaps unsurprisingly, given the geochemical upset at the time, most of the genes that evolved are associated with metabolism. Gene groups involved in the binding of iron, sulphur and oxygen were particularly expanded, as were others that used metabolites produced by respiration.

The earliest metabolic genes identified within the Archaean expansion were probably used in anaerobic respiration, and also in oxygen- and non-oxygen-producing photosynthesis. David and Alm suggest that these genes may have later contributed to aerobic respiration pathways, as well.

Intriguingly, the expansion of genes associated with molybdenum and copper utilization coincides with the increasing availability of these elements in the oceans, as predicted by geochemical models and observations. It seems that microbes were adaptable, and quickly able to evolve pathways to use these elements as they became available.