Some 390 million years ago in the ancient ocean, marine animals began colonizing depths previously uninhabited. New research indicates this underwater migration occurred in response to a permanent increase in deep-ocean oxygen, driven by the aboveground spread of woody plants — precursors to Earth's first forests.
That rise in oxygen coincided with a period of remarkable diversification among fish with jaws — the ancestors of most vertebrates alive today. The finding suggests that oxygenation might have shaped evolutionary patterns among prehistoric species.
"It's known that oxygen is a necessary condition for animal evolution, but the extent to which it is the sufficient condition that can explain trends in animal diversification has been difficult to pin down," said co-lead author Michael Kipp, assistant professor of earth and climate sciences in the Duke University Nicholas School of the Environment. "This study gives a strong vote that oxygen dictated the timing of early animal evolution, at least for the appearance of jawed vertebrates in deep-ocean habitats."
For a time, researchers thought that deep-ocean oxygenation occurred once at the beginning of the Paleozoic Era, some 540 million years ago. But more recent studies have suggested that oxygenation occurred in phases, with nearshore waters first becoming livable to breathing organisms, followed by deeper environments.
Kipp and colleagues homed in on the timing of those phases by studying sedimentary rocks that formed under deep seawater. Specifically, they analyzed the rocks for selenium, an element that can be used to determine whether oxygen existed at life-sustaining levels in ancient seas.
In the marine environment, selenium occurs in different forms called isotopes that vary by weight. Where oxygen levels are high enough to support animal life, the ratio of heavy to light selenium isotopes varies widely. But at oxygen levels prohibitive to most animal life, that ratio is relatively consistent. By determining the ratio of selenium isotopes in marine sediments, researchers can infer whether oxygen levels were sufficient to support animals that breathe underwater.
Working with research repositories around the world, the team assembled 97 rock samples dating back 252 to 541 million years ago. The rocks had been excavated from areas across five continents that, hundreds of millions of years ago, were located along the outermost continental shelves — the edges of continents as they protrude underwater, just before giving way to steep drop-offs.
After a series of steps that entailed pulverizing the rocks, dissolving the resulting powder and purifying selenium, the team analyzed the ratio of selenium isotopes that occurred in each sample.
Their data indicated that two oxygenation events occurred in the deeper waters of the outer continental shelves: a transient episode around 540 million years ago, during a Paleozoic period known as the Cambrian, and an episode that began 393-382 million years ago, during an interval called the Middle Devonian, that has continued to this day. During the intervening millennia, oxygen dropped to levels inhospitable to most animals. The team published their findings in Proceedings of the National Academy of Sciences in August.
"The selenium data tell us that the second oxygenation event was permanent. It began in the Middle Devonian and persisted in our younger rock samples," said co-lead author Kunmanee "Mac" Bubphamanee, a Ph.D. candidate at the University of Washington.
That event coincided with numerous changes in oceanic evolution and ecosystems — what some researchers refer to as the "mid-Paleozoic marine revolution." As oxygen became a permanent feature in deeper settings, jawed fish, called gnathostomes, and other animals began invading and diversifying in such habitats, according to the fossil record. Animals also got bigger, perhaps because oxygen supported their growth.
The Middle Devonian oxygenation event also overlapped with the spread of plants with hard stems of wood.
"Our thinking is that, as these woody plants increased in number, they released more oxygen into the air, which led to more oxygen in deeper ocean environments," said Kipp, who began this research as a Ph.D. student at the University of Washington.
The cause of the first, temporary oxygenation event during the Cambrian is more enigmatic.
"What seems clear is that the drop in oxygen after that initial pulse hindered the spread and diversification of marine animals into those deeper environments of the outer continental shelves," Kipp said.
Though the team's focus was on ancient ocean conditions, their findings are relevant now.
"Today, there's abundant ocean oxygen in equilibrium with the atmosphere. But in some locations, ocean oxygen can drop to undetectable levels. Some of these zones occur through natural processes. But in many cases, they're driven by nutrients draining off continents from fertilizers and industrial activity that fuel plankton blooms that suck up oxygen when they decay," Kipp said.
"This work shows very clearly the link between oxygen and animal life in the ocean. This was a balance struck about 400 million years ago, and it would be a shame to disrupt it today in a matter of decades."
Funding: MAK was supported by an NSF Graduate Research Fellowship and Agouron Institute Postdoctoral Fellowship. Additional support was provided by the NASA Astrobiology Institute's Virtual Planetary Laboratory.
Citation: "Mid-Devonian ocean oxygenation enabled the expansion of animals into deeper-water habitats," Bubphamanee K., Kipp M., Meixnerová J., Stüeken E., Ivany L, Bartholomew A., Algeo T., Brocks J., Dahl T., Kinsley J., Tissot F., Buick R. Proceedings of the National Academy of Sciences, August 2025, DOI: 10.1073/pnas.2501342122
