About 480 million years ago, a dazzling assortment of the variety of flavors of life emerged on Earth. There has long been speculation about what causes this abundant radiation of new species, from asteroid dust or tectonic activity, to a rise in oxygen levels in the atmosphere.
Now rocks from Maryland, USA, suggest an unlikely key player: prehistoric marine worms may have made an outsized contribution to the Great Ordovician Biodiversification Event.
“It’s really incredible to think how such small animals, which don’t even exist today, could change the course of evolutionary history in such a profound way,” says geobiologist Maya Gomes of Johns Hopkins University.
The US researchers found elevated levels of a mineral called pyrite in a specific sediment level at nine locations in Chesapeake Bay. Pyrite requires a constant supply of oxygen to form from sediment minerals, but it also reacts easily with oxygen, stealing it from the oceans and then the atmosphere.
But the more pyrite forms and is then trapped underground, the more oxygen concentrations can accumulate. It is a useful proxy to measure oxygen levels from long ago.
“It’s a bit like Goldilocks,” explains paleoclimatologist Kalev Hantsoo of Johns Hopkins. “The conditions have to be just right. You have to mix a little to introduce the oxygen into the sediment, but not so much that the oxygen destroys all the pyrite and there is no net buildup.”
The levels of pyrite in the sediment suggest that something was churning the ocean floor so much that it prevented this mineral from stealing back too much of the increasing oxygen levels.
Hantsoo and colleagues strongly suspect that these early oceanic sediment mixers were the worms that were burrowing, along with other creatures that interacted with the seafloor.
“We hypothesize that pyrite burial… increased during the prolonged onset of bioturbation,” the researchers explain in their paper.
Worms and other life below ground today still play a major role in bioturbation, physically mixing the upper layers of soil together, allowing moisture and oxygen exchange and supplying other important nutrients, including iron, sulfur and carbon dioxide, are circulated.
The researchers updated previous models of prehistoric oxygen levels with their measurements of bioturbation. The results suggest that oxygen levels remained stable for millions of years until they rose sharply during the Cambrian (beginning 538.8 million years ago) and Ordovician periods. These increases were larger than previous reconstructions suggested, but could not last indefinitely.
“This increase in pyrite burial efficiency would have been temporary due to the intensification of bioturbation, coupled with the increase [concentrations of O2]would have ultimately introduced sufficient oxidizing force into the sediment pile to suppress pyrite retention,” Hantsoo and team write.
It was not until the second detected eruption of pyrite burial, during the Ordovician, 485–445 million years ago, that Earth reached and subsequently maintained near-modern oxygen levels. This coincided with a 30 million year period of rapid evolutionary change that led to the creation of numerous new species.
“There has always been the question of how oxygen levels relate to the moments in history when evolutionary forces are increasing and you see greater diversity of life on Earth,” says Gomes.
“This work will allow us to explore the chemistry of early oceans and reinterpret parts of the geological record.”
It appears that these bursts of oxygen, aided by the worms’ excavations, contributed to the spectacular diversity of life on Earth.
This research was published in Geochimica and Cosmochimica Acta.