Rising deep-ocean oxygen levels likely opened up new marine habitats and spurred speciation among early vertebrates, according to a new study in the Proceedings of the National Academy of Sciences.
The work links a permanent oxygen boost in the Middle Devonian, roughly 393–382 million years ago, to the colonization of deeper waters by jawed fishes and other animals, a shift that lines up with a major burst of biodiversity in the fossil record.
Co-lead author Michael Kipp of Duke University said that while oxygen has long been recognized as necessary for animal evolution, whether it could also be “the sufficient condition” behind diversification has been hard to pin down.
This study, he added, “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.”
What changed in the deep ocean, and when
For years, scientists debated whether the deep ocean was oxygenated once at the start of the Paleozoic, around 540 million years ago, or in multiple steps. The new analysis supports a phased story.
The team found evidence for two distinct oxygenation events in deeper waters along the outer continental shelves, the underwater fringes of ancient continents just before the seafloor drops away.
One was a short-lived pulse during the Cambrian, about 540 million years ago. After that, oxygen fell back to levels inhospitable to most animals. The second event began in the Middle Devonian, between 393 and 382 million years ago, and persisted thereafter.
That Middle Devonian step change coincides with what some researchers call the mid-Paleozoic marine revolution. Ecosystems reorganized, animals moved into new niches, and body sizes increased.
As oxygen became a persistent feature in deeper settings, jawed fishes (gnathostomes) and other groups appeared in the fossil record, invading and diversifying in those habitats.
The study frames oxygen not just as a catalyst but as a key driver of where and when ancient animals could thrive.
As Kipp put it, the initial Cambrian pulse may have opened doors briefly, but “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.”
The authors also draw a link between land and sea. Permanent oxygenation overlaps with the spread of woody plants, which are early precursors to forests, with hard, supportive stems.
Kipp notes that as these plants multiplied, “they released more oxygen into the air, which led to more oxygen in deeper ocean environments.” In other words, terrestrial innovation may have primed the deep ocean for a lasting biological expansion.
How the team inferred ancient oxygen, and why it matters now
To time these changes, the researchers turned to selenium. A trace element in marine sediments that records past oxygen conditions through its isotopes (atoms of the same element with different weights).
The ratio of heavy to light selenium isotopes varies widely where oxygen levels are high enough to sustain animal life.
Where oxygen is too low for most animals, that ratio stays relatively uniform. By measuring these isotope patterns, scientists can infer whether ancient waters crossed the threshold for animal habitability.
The team assembled 97 rock samples deposited under deep seawater between 252 and 541 million years ago.
Sourced from research repositories across five continents, the rocks all formed along outer continental shelves, exactly where a shift in deep-water oxygen would have reshaped living space.
The rocks were pulverized and dissolved in the lab, and their selenium was purified for analysis.
The isotope data revealed the two-step history. A transient Cambrian oxygenation and a Middle Devonian transition that endured.
“The selenium data tells us that the second oxygenation event was permanent,” said co-lead author Kunmanee “Mac” Bubphamanee, a Ph.D. candidate at the University of Washington.
“It began in the Middle Devonian and persisted in our younger rock samples.”
The findings carry a modern caution. Today, the ocean’s oxygen is generally in equilibrium with the atmosphere, but local and regional dead zones do form.
Some are natural, yet many arise when nutrient runoff from fertilizers and industrial activity fuels plankton blooms; as that organic matter decays, it consumes oxygen and can drive levels to undetectable values.
Kipp warned that the study highlights a simple connection: “This work shows very clearly the link between oxygen and animal life in the ocean.”
The balance that took hold roughly 400 million years ago enabled complex ecosystems to spread into the deep; “it would be a shame to disrupt it today in a matter of decades.”