Ancient oxygen boom transformed the deep ocean, sparking a fish evolution explosion
By isabelle // 2025-08-27
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  • A permanent oxygen surge 390 million years ago transformed lifeless ocean depths into thriving ecosystems and fueled the rise of jawed fish.
  • Woody plants on land released extra oxygen into the atmosphere, which then enriched the oceans and enabled deep-sea colonization.
  • Fossil and isotope evidence reveals two major oxygenation events, one temporary in the Cambrian period and one permanent in the Middle Devonian.
  • This oxygen boost triggered an evolutionary explosion, leading to diverse marine life, including massive predatory fish like Dunkleosteus.
  • Modern human activity now threatens ocean oxygen levels, risking the collapse of ecosystems that took millions of years to develop.
What could transform the ocean's lifeless depths into thriving ecosystems? A sudden surge of oxygen did exactly that roughly 390 million years ago, paving the way for the rise of jawed fish—the ancestors of nearly all vertebrates today—according to groundbreaking research. A team of scientists, led by Michael Kipp (Duke University) and Kunmanee "Mac" Bubphamanee (University of Washington), uncovered evidence that a permanent rise in deep-ocean oxygen during the Middle Devonian period (393–382 million years ago) allowed marine life to colonize previously uninhabitable zones. This oxygen boost didn’t just open new habitats; it triggered an evolutionary explosion among jawed fish, setting the stage for the diverse marine life we see today.

The missing link: Woody plants and oxygen

So where did all that oxygen come from? The answer lies not in the ocean, but on land. As woody plants—the building blocks of Earth's earliest forests—expanded across the land, they released additional oxygen into the atmosphere. This oxygen eventually made its way into the oceans, penetrating deeper waters and creating livable conditions for the first time. This wasn’t the first time deep-ocean oxygen levels spiked. The researchers found evidence of an earlier, temporary oxygenation event around 540 million years ago during the Cambrian period. But unlike that fleeting surge, the Middle Devonian oxygen rise was permanent, coinciding with a marine revolution at a time when jawed fish (gnathostomes) and other complex organisms began dominating deeper waters. To piece together this ancient puzzle, the team analyzed selenium isotopes in 97 sedimentary rock samples from five continents, dating back 252 to 541 million years. Selenium, a trace element in seawater, acts like a chemical fingerprint—its isotope ratios shift depending on oxygen levels. Where oxygen is abundant, selenium isotopes vary widely. But in low-oxygen zones, those ratios stay flat. By measuring these ratios, the researchers confirmed that two major oxygenation events occurred: one temporary (Cambrian) and one permanent (Middle Devonian). This oxygen influx didn’t just allow fish to survive in deeper waters; it supercharged their evolution. Fossil records from this period show a dramatic increase in fish diversity, including the emergence of large, predatory species like Dunkleosteus, a armored fish that could grow up to 33 feet long.

Why this matters today

While the study focuses on ancient oceans, its implications ripple into the present. Today, human activity such as fertilizer runoff and industrial pollution is draining oxygen from the seas, creating dead zones where marine life cannot survive. The research demonstrates how essential oxygen is for marine animal survival. The delicate balance that took hundreds of millions of years to develop could be undone by human activity in a matter of decades. The findings also challenge long-held assumptions about how life thrives—and where. For centuries, scientists believed that photosynthesis (which requires sunlight) was the sole source of Earth’s oxygen. But recent discoveries, like "dark oxygen" produced by deep-sea polymetallic nodules, suggest that life may have originated in ways we’re only beginning to understand. As debates rage over deep-sea mining and its potential to destroy fragile ecosystems, this research serves as a reminder: The ocean’s depths are still a frontier, full of mysteries we’ve barely begun to unravel. Sources for this article include: Phys.org PopularMechanics.com Phys.org
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