Blog

One of the most significant bursts of biodiversity on Earth—a 30-million-year period of explosive evolutionary change that spawned countless new species—can thank the humblest of creatures for a vital stage in life’s history: worms.

The digging and burial of prehistoric worms and other invertebrates on the ocean floor triggered a chain of events that released oxygen into the ocean and atmosphere and helped start what’s known as the Great Ordovician Biodiversification Event roughly 480 million years ago, according to new findings of Johns Hopkins University researchers published in the journal Geochimica et Cosmochimica Acta.

“It’s really amazing to think how such small animals, ones that don’t even exist today, could change the course of evolutionary history in such a profound way,” said senior author Maya Gomez, an assistant professor in the Department of Earth and Planetary Sciences. “With this work, we will be able to probe the chemistry of the early oceans and reinterpret parts of the geological record.”

To better understand how changes in oxygen levels affected large-scale evolutionary events, Gomez and her research team updated models that describe the timing and pace of oxygen increases over hundreds of millions of years.

They investigated the relationship between sediment mixing caused in part by burrowing worms with a mineral called pyrite, which plays a key role in oxygen accumulation. The more pyrite that forms and is buried under the mud, silt or sand, the more the oxygen levels rise.

The researchers measured pyrite from nine sites along the Maryland coastline in the Chesapeake Bay, which serves as a proxy for early oceanic conditions. Sites with even just a few centimeters of sediment mixing contained significantly more pyrite than those without mixing and those with deep mixing.

The findings challenge previous assumptions that the relationship between pyrite and sediment mixing remains the same across habitats and over time, Gomez said.

Conventional wisdom held that as animals scoured the ocean floor for sediment, the newly discovered pyrite would be exposed to and destroyed by the oxygen in the water, a process that would eventually prevent oxygen from accumulating in the atmosphere and ocean. Mixed sediments are seen as evidence that oxygen levels are kept stable.

The new data suggest that a small amount of sediment mixed in water with very low oxygen levels would expose the buried pyrite, sulfur and organic carbon to enough oxygen to start the formation of more pyrite.

“It’s kind of like Goldilocks. The conditions have to be right. You have to have some mixing to get the oxygen into the sediment, but not so much that the oxygen destroys all the pyrite and there’s no net accumulation,” said Kalev Hantsu, a postdoctoral fellow at Johns Hopkins and first author of the paper.

When the researchers applied this new relationship between pyrite and the depth of sediment mixing to existing models, they found that oxygen levels remained relatively flat for millions of years and then rose during the Paleozoic Era, with a sharp rise seen during the Ordovician Period .

The extra oxygen likely contributed to the Great Ordovician Biodiversification Event, when new species rapidly flourished, the researchers said.

“There’s always been this question of how oxygen levels relate to times in history when evolutionary forces are ramping up and you’re seeing a greater diversity of life on the planet,” Gomez said. “The Cambrian period also had a massive speciation event, but the new models allow us to rule out oxygen and focus on other things that may have driven evolution during that time.”