How Ancient Oxygen Surges Transformed Oceans and Sparked Evolution
The Oxygen Flood That Changed the Seas Forever
Nearly 390 million years ago, life beneath the waves experienced a dramatic shift. New research reveals that marine animals began venturing into deeper waters thanks to a permanent increase in oxygen—an event that coincided with the spread of woody plants on land, precursors to Earth’s first forests.This oxygen boom not only expanded the boundaries of marine habitats but also sparked a wave of diversification among jawed fish, the evolutionary ancestors of most vertebrates alive today.
Oxygen as the Hidden Driver of Evolution
“It’s known that oxygen is a necessary condition for animal evolution, but how much it actually drives diversification has been harder to prove,” explained Michael Kipp, assistant professor at Duke University and co-lead author of the study. “Our findings strongly suggest oxygen dictated the timing of jawed vertebrates moving into deep-ocean habitats.”For years, scientists believed the deep ocean became oxygenated only once, around 540 million years ago during the Cambrian explosion. But this new research, published in Proceedings of the National Academy of Sciences, shows the process occurred in two phases—one temporary, and one permanent.
Tracing Ancient Oxygen Through Rock
To uncover the timeline, researchers analyzed 97 marine rock samples dating between 252 and 541 million years ago. These samples, collected from ancient continental shelf environments across five continents, were tested for selenium isotopes—a chemical fingerprint that reveals whether oxygen levels were high enough to sustain marine animals.The data showed two major events: an initial, short-lived spike around 540 million years ago and a second, lasting surge between 393 and 382 million years ago during the Middle Devonian. Unlike the first, this second wave of oxygenation never reversed and continues to shape ocean chemistry today.
The Middle Devonian Revolution
This permanent oxygenation coincided with what scientists call the “mid-Paleozoic marine revolution.” As oxygen levels stabilized in deeper waters, jawed fish—known as gnathostomes—spread and diversified rapidly. Fossil evidence suggests that marine animals also grew larger, likely because oxygen supported greater body sizes.Researchers also noted the timing overlapped with the rise of woody plants on land. These early trees released more oxygen into the atmosphere, which in turn increased oxygen flow into the oceans.
Lessons for Today’s Oceans
While the study focuses on ancient seas, its implications are strikingly modern. “This was a balance struck nearly 400 million years ago, and it would be a shame to disrupt it in just decades,” Kipp said.Today, some coastal regions experience oxygen-depleted “dead zones,” often caused by agricultural runoff and industrial pollution fueling plankton blooms that strip oxygen as they decay.
The ancient record is clear: oxygen has always been the foundation of marine biodiversity. Disrupting that balance now could threaten the very ecosystems humans depend on.
