The steel tycoon and philanthropist Andrew Carnegie, who was wealthier in his day than Bill Gates, Sam Walton, and Warren Buffett combined, amassed his fortune through brash business practices and the labor of thousands who toiled in his mills, but he actually owed everything to the work of ancient microbes.

Refined steel consists mostly of iron, with a little carbon thrown in for strength and toughness. Carnegie’s steel, like almost all the steel ever produced in the world, was made with ore from Precambrian iron formations—stratified rock sequences, blood red and dull gray, that are typically mined in giant open pits. (One of the biggest, the so-called Grand Canyon of the North, is in Bob Dylan’s hometown of Hibbing, Minnesota.) Whereas most rock types are timeless, in the sense that they come into being today in the same way that they have for aeons, iron formations are, in a manner of speaking, extinct. They provide an insider’s account of a pivotal transition in Earth’s history, a time of geochemical upheaval, between about 2.5 billion and 1.8 billion years ago, that is known as the Great Oxygenation Event. Prior to that planet-changing revolution, Earth’s atmosphere was made up largely of carbon dioxide, water vapor, and nitrogen. Afterward, thanks to the industrious activity of photosynthesizing microorganisms such as cyanobacteria—whose modern descendants are often called, with less than due respect, pond scum—the air was full of free oxygen.

The ancient iron formations share many characteristics with modern limestones, which suggests that they accumulated in a marine environment. In today’s oceans, iron is in such short supply that it is a limiting nutrient—an essential element whose scarcity holds biological productivity in check. (A controversial climate-engineering scheme is even based on this fact; the idea is that if the oceans were fertilized with iron powder, plankton would bloom enthusiastically and then die, sinking to the ocean floor and sequestering large amounts of carbon there without, fingers crossed, wreaking havoc on the rest of the marine biosphere.) The primordial oceans, by contrast, must have been awash with iron. The richness of the rock formations—imagine all the steel in cars, cutlery, airplanes, buildings, bridges, and railroads—attests to that.

It was oxygen, a disruptive newcomer to the aquatic economy, that permanently altered the rules about what could and could not be present in seawater. Before the Great Oxygenation Event, iron was able to dissolve in the oceans, commingling invisibly with sodium, chlorine, and other ions. But, when oxygen began to make inroads, even in concentrations far lower than present levels, it found the iron atoms, bonded itself to them, and pulled them to the seafloor. Oxygen, and the microbes that manufactured it, purged the oceans of iron literally by rusting it out. Red was the color of the revolution.

The iron formations precipitated from seawater, but where did the dissolved iron come from in the first place? Geologists have generally pointed to deep-sea volcanic vents like the modern Mid-Atlantic Ridge. Here, incandescent magma from the mantle meets cold seawater, and metallic brines gush forth from the rock chimneys known as black smokers. But a paper published in July in the Proceedings of the National Academy of Sciences suggests an alternative source. Up to half of the iron in iron formations, a group of geochemists at the University of Wisconsin argues, may have come originally from land. According to their theory, iron from continental rocks, which had become newly reactive in the oxidizing atmosphere, was carried by rivers to the ocean, where it was released into seawater by non-photosynthesizing microorganisms—the comrades of the cyanobacteria that were transforming the air.

The study examines the iron formations of the Hamersley Basin, in western Australia. Like most elements, iron comes in a range of flavors, known as isotopes, which vary in mass and stability. Iron from the mantle, released at black smokers, has a predictable ratio of iron-56 (full fat) to iron-54 (lite), but in the Hamersley rocks the ratio is skewed; the iron is, on average, lower-fat than expected. According to the Wisconsin group, this is evidence of biological activity, since iron-54 is easier for living things to metabolize than iron-56. In particular, they suggest that the isotope ratios are the signature of microbes that colonized the shores of the primordial ocean and found a way to make an opportunistic living from the incoming river sediments. These single-celled entrepreneurs might have gained energy by stripping oxygen from the land-derived iron—an electron-transfer reaction that anticipated the first battery by two billion years. The bacterial middlemen then discarded the unwanted iron, disproportionately enriched in the lighter isotope, into the water, where it was again hunted down by oxygen and rusted out. In other words, much of the iron in iron formations may come pre-owned.

This conclusion is not accepted by everyone in the surprisingly vigorous and contentious field of Precambrian biogeochemistry. Skeptics point out that iron isotopes can be sorted by mass through non-biological processes, as when bodies of water with different chemistries mix, and that the iron-54 signature in the Australian rocks could just as easily be due to the removal of heavy iron as to the addition of light, in the same way that the average income of a neighborhood could be reduced by a few rich people moving away rather than an influx of lower-income families. Whether or not a second group of microbial workers was involved in the manufacture of iron formations, though, the fact that the question can be posed at all reveals how far geology has come in learning the dialects of even very old rocks that formed on an Earth we would find alien.

In some strange way, Andrew Carnegie was not unlike the cyanobacterial minions to which he was unknowingly indebted. Through ruthless capitalism, and inexorable Darwinism, both concentrated huge stockpiles in their time and unilaterally changed the rules for their contemporaries. Yet both ultimately left behind egalitarian legacies of shared riches—libraries, Pell grants, public broadcasting, and free oxygen for all.