Earth's 'Boring Billion' Years Blamed on Sulfur-Loving Microbes

A new study suggests microbes could have stalled the evolution of complex life.

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By Rachel Ehrenberg

Sulfur-loving microbes may have been the party poopers of middle Earth. New research suggests that if such microbes dominated the oceans until half a billion years ago, the organisms could have contributed to the static period known as the “boring billion,” scientists report online September 28 in the Proceedings of the National Academy of Sciences. Various feedback loops involving biota and the nutrients they cycle could have maintained this stasis, creating an environment low in oxygen and unfriendly to multicellular life. 

“If we really want to understand what’s happed in the history of Earth, we really have to understand this cross talk between the physical and biological processes,” says study coauthor Andrew Knoll of Harvard University. 

Scientists have long wondered how the Earth remained in the geochemical and evolutionary stagnation that began about 1.8 billion years ago. The “Great Oxidation Event” had already happened, so atmospheric oxygen was up, at least a little. Eukaryotes—with DNA sequestered in a membrane—had already evolved. But then everything got put on hold for about a billion years: Oxygen levels remained flat, and life remained simple. 

“We don’t have a full understanding of what the full biogeochemical system might have been doing at this time,” comments Mark Claire of the University of Washington in Seattle. “Something was throttling the oxygen levels.”

That something may have been sulfur-oxidizing microbes, which don’t release oxygen into the atmosphere. Knoll, David Johnston and their Harvard colleagues propose. Previous work suggests that the oceans in this era, known as the Proterozoic, were rich in sulfur. So the researchers argue that an abundance of microbes that can use different forms of sulfur affected biogeochemical cycling. These sulfur-using microbes may have set in motion feedback loops that locked Earth in this stasis, the researcher propose. 

Though typical photosynthesizers were still generating oxygen from water in the uppermost layer of the oceans, a toxic layer was forming beneath. There, green and purple bacteria and other microbes used sulfur, washed into oceans by the weathering of the continents, to photosynthesize. Since it is easier to pry electrons from sulfur than from water, sulfur-cycling microbes had an edge, Knoll says. But sulfur-based photosynthesis does not release oxygen, and sulfur recycled and remained in the system.

As the sulfur-based organisms died, their decomposition further robbed the water of oxygen. These processes may have led to a thick dead zone, similar to that seen in today’s Gulf of Mexico. 

Eventually, something shifted—perhaps iron from continental activity entered the oceans, the researcher speculate. Iron binds to forms of sulfur, pulling it out of the system as pyrite. This may have been a blow to sulfur users, allowing photosynthesis by oxygen-generators to increase. And pretty soon, by 600 million years ago, multicellular life arrives at the party. 

“This releases the stranglehold of the system,” Knoll says. “The minute you get rid of sulfide you change how the world works.” 

The model reminds physical scientists to pay attention to the contribution of biota to Earth’s geochemical cycling, Knoll notes. The team now plans to investigate the timing of eukaryotic fossils and the demise of the sulfurous seas as tests for the hypothesis. The project, Knoll says, “gave me new glasses for looking at Earth’s history.”