Hundreds of millions of years ago, Earth’s first forests helped pump oxygen into the deep seas, transforming once-barren waters into thriving habitats.
This permanent oxygen boost allowed fish with jaws and other marine animals to expand, diversify, and grow larger, sparking a revolution in ocean life.
Colonizing the Deep Seas
Around 390 million years ago, marine animals began moving into deep ocean zones that had previously been uninhabited. New research suggests this expansion was made possible by a lasting rise in deep-sea oxygen levels, fueled by the spread of woody plants on land (the early ancestors of Earth’s first forests).
This long-term oxygen increase took place during a remarkable burst of diversity among jawed fish, the group that would eventually give rise to nearly all vertebrates living today. The evidence points to oxygenation as a potential driver of evolutionary shifts in these ancient species.
Tracking Ancient Oxygenation
“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.”
Scientists once believed that deep-ocean oxygenation occurred only once, at the start of the Paleozoic Era, roughly 540 million years ago. However, newer studies point to a stepwise pattern, beginning with oxygen making shallow coastal waters habitable and later extending into the deeper ocean.
Rock Clues From the Seafloor
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.
Global Rock Samples and Analysis
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.
Two Great Oxygenation Events
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.
Forests Rise, Oceans Change
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.
Lessons for Today’s Oceans
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.
A Warning Across Time
“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.”
Reference: “Mid-Devonian ocean oxygenation enabled the expansion of animals into deeper-water habitats” by Kunmanee Bubphamanee, Michael A. Kipp, Jana Meixnerová, Eva E. Stüeken, Linda C. Ivany, Alexander J. Bartholomew, Thomas J. Algeo, Jochen J. Brocks, Tais W. Dahl, Jordan Kinsley, François L. H. Tissot and Roger Buick, 25 August 2025, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2501342122
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.
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1 Comment
They seem to have forgotten that what downwells into the deep oceans has to upwell to the surface somewhere else. Any “temporary” loss of downwelled oxygen requires downwelled organic biomass to remove oxygen by aerobic respiration. That recycles CO2 back into the atmosphere. This is all part of the natural carbon cycle that has been taking place since the Archean. Nothing new. The origins of higher complex life just before the Cambrian “explosion” is a separate issue.