Approximately 390 million years ago during the Devonian period, marine life began to explore previously unoccupied depths. A recent study, conducted by researchers from Duke University, Washington University, NASA’s Virtual Planetary Research Institute, and Caltech, reveals that this underwater migration was spurred by a lasting increase in deep-sea oxygen levels, linked to the ground diffusion of woody plants. This rise in oxygen coincided with a time of notable diversification among jawed fish.

“While oxygen is recognized as essential for animal evolution, establishing its role in trends of animal diversification can be challenging,” remarks Dr. Michael Kipp, a researcher at Duke University.

“This study strongly supports the idea that oxygen has influenced the timing of early animal evolution, particularly concerning the emergence of jawed vertebrates in deep-sea environments.”

For years, scientists believed that deep-sea oxygenation was a singular event that occurred at the onset of the Paleozoic era, around 540 million years ago.

However, recent findings suggest that oxygenation takes place in stages, first making coastal regions more hospitable for respiratory organisms, followed by deeper waters.

Dr. Kipp and his team investigated the timing of these stages by examining sedimentary rocks formed beneath deep seawater.

They focused on selenium within the rocks, an element utilized to ascertain whether oxygen levels were high enough to support life in the ancient ocean.

In marine settings, selenium exists in various forms known as isotopes, which differ based on weight.

At oxygen levels conducive to animal life, the ratio of heavy to light selenium isotopes shows significant variation.

Conversely, at oxygen levels too low for most animals, the ratios remain relatively stable.

By analyzing selenium isotope ratios in marine sediments, researchers can deduce whether oxygen levels were adequate to sustain aquatic life.

The team collected 97 rock samples from around the globe, dating from 252 to 541 million years ago.

These samples were sourced from locations across five continents that were once situated along continental shelves millions of years ago, where the continental edge meets a steep drop-off underwater.

After processing the rocks through grinding, melting, and purifying selenium, the team examined the selenium isotope ratios in each sample.

Their findings reveal that two significant oxygenation events took place in deeper waters of the outer continental shelf, starting during the Mid Devonian, around 540 million years ago, and again between 393 and 382 million years ago during the Paleozoic’s Cambrian period.

For extended periods, oxygen levels plummeted, making survival challenging for most marine life.

“Our selenium data indicates that the second oxygenation event was permanent,” stated Kunmanee ‘Mac’ Bubphamanee, PhD candidate at the University of Washington.

“This event initiated in the mid-Devonian period and has persisted in our younger rock samples.”

This oxygenation event coincided with significant changes in ocean evolution and ecosystems, often referred to as the Paleozoic marine revolution.

Fossil evidence indicates that oxygen became a stable presence in deeper waters, allowing jawed fish known as Gnathostomes to invade and diversify in these environments.

These organisms grew larger, likely due to the supportive oxygen levels facilitating their growth.

The Devonian oxygenation event also correlated with the proliferation of woody plants.

“Our hypothesis posits that the increase in woody plants released more oxygen into the atmosphere, thereby elevating oxygen levels in deeper marine environments,” Dr. Kipp stated.

The cause behind the initial temporary oxygenation event during the Cambrian period remains more obscure.

“What is evident is that the subsequent drop in oxygen post-initial event constrained the spread and diversification of marine animals into deeper continental shelf environments,” Dr. Kipp explained.

“Today, marine oxygen levels are balanced with those in the atmosphere.”

“However, in specific zones, marine oxygen can plummet to undetectable levels.”

“Some of these areas arise from natural phenomena.”

“Still, they are frequently exacerbated by nutrient runoff from fertilizers, industrial activities that degrade plankton, and subsequent oxygen depletion as it decomposes.”

“This research clearly outlines the relationship between oxygen and marine life.”

“It’s a balance established around 400 million years ago, and it would be regrettable to disrupt it in the years to come.”

This study is set to be published this week in Proceedings of the National Academy of Sciences.

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Kunmanee Bubphamanee et al. 2025. Marine oxygenation in Mid Devonian allowed the expansion of animals into deeper water habitats. PNAS 122 (35): E2501342122; doi: 10.1073/pnas.2501342122