With its often telltale stench of rotten eggs, sulfur would not top most people's list of favorite culinary seasonings, but plenty of bacteria adore it. What's more, their predictable preferences for certain flavors of sulfur can be preserved inside rocks for billions of years, revealing detailed stories about the earth's distant past--most recently, a scenario for why the planet didn't freeze when the sun was considerably cooler.

It all boils down to who's eating what. All organisms are finicky, and the sulfur-loving bacteria are no exception. They prefer sulfur 32, the element's lightest isotope, to heavier sulfur 34 when given the choice. In the modern world, they leave a waste product (usually recorded in the mineral pyrite) with a surplus of sulfur 32 that is as much as 7 percent greater than that of their food source, a sulfur compound called sulfate (today a common salt in seawater). But the surplus of sulfur 32 in pyrite that formed before 2.5 billion years ago rarely exceeds 1 percent. This discrepancy suggests to geochemists that back then, during a geologic eon known as the Archean, sulfate was scarce: with less food available, sulfur lovers couldn't afford to be as picky about the isotope they favored.

This article was originally published with the title Foiling a Faint Sun.

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