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Worms - uncredited heroes of evolution

The researchers examined the relationship between sediment mixing caused, in part, by burrowing worms with a mineral called pyrite, which plays a key role in accumulating oxygen. The more pyrite is formed and buried under the mud, sand or sand, the higher the oxygen levels

Red worms dig in the compost. Even the worms that exist today mix substances. Illustration: depositphotos.com
Red worms dig in the compost. Even the worms that exist today mix substances. Illustration: depositphotos.com

One of the most significant periods in the Earth's biological diversity - a 30 million year period of rapid evolutionary changes that gave birth to countless new species - probably occurred thanks to the humblest creature: the worms.

Burrowing by prehistoric worms and other vertebrates along the ocean floor triggered a chain of events that released oxygen into the ocean and atmosphere and helped drive the so-called Great Biodiversification Event of the Ordovician period, about 480 million years ago, according to new findings by Johns Hopkins University researchers published in The journal Geochimica et Cosmochimica Acta.

"It's truly amazing to think how such small animals, which don't even exist today, could have changed the course of evolutionary history in such a significant way," said senior author Maya Gomez, assistant professor in the Department of Earth and Planetary Sciences. "With this work, we can examine the chemistry of ancient 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 detailing the timing and rate of oxygen increases over hundreds of millions of years.

They examined the relationship between sediment mixing caused, in part, by burrowing worms with a mineral called pyrite, which plays a key role in accumulating oxygen. The more pyrite is formed and buried under the mud, sand or sand, the higher the oxygen levels.

The researchers measured pyrite from nine sites along the Chesapeake Bay coast in Maryland, which is a tool for determining ancient ocean conditions. Sites with even 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 in all habitats and over time," said Gomez.

The common assumption was that when animals stirred up the sediments by digging at the bottom of the ocean, pyrite that would have been exposed to oxygen in the water would have been immediately destroyed, a process that would have eventually prevented oxygen from accumulating in the atmosphere and ocean. Mixed sediments were taken as evidence that oxygen levels remained stable.

The new data suggest that a small amount of sediment mixing in water with very low oxygen levels would have exposed buried pyrite, sulfur and organic carbon to enough oxygen to drive pyrite formation.

The conditions have to be just right. "There needs to be some mixing to get the oxygen into the sediment, but not so much that the oxygen destroys all the pyrite and there is no net accumulation," said Caleb Hantsou, a doctoral student at Johns Hopkins University and lead 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 low and stable for millions of years and then increased during the Paleozoic, with a steep rise during the Ordovician.

The extra oxygen likely contributed to the great bioserification event during the Ordovician, when new species evolved rapidly, the researchers said.

"There's always been the question of how oxygen levels relate to moments in history when evolutionary forces become stronger until we see a greater diversity of life on Earth," Gomez said. "The Cambrian period also went through a mass extinction event, but the new models allow us to rule out oxygen and focus on other things that might be driving evolution at that time."

for the scientific article

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