About 390 million years ago, a profound transformation took place in Earth’s ancient oceans, altering the evolutionary trajectory of marine life in ways that continue to shape biodiversity today. Recent groundbreaking research reveals that a permanent increase in oxygen levels within deep-ocean environments was a pivotal factor enabling marine animals to colonize previously uninhabited depths. This oxygenation event, intricately linked to the proliferation of woody plants on land—Earth’s earliest forests—illuminates the complex interplay between terrestrial and marine ecosystems during the Devonian period.
Oxygen has long been recognized as essential to animal life, yet the specific influence of fluctuating oxygen levels on the timing and patterns of animal diversification has remained elusive. The new study, spearheaded by scientists from Duke University and the University of Washington, provides compelling evidence that elevated oxygen concentrations directly dictated the emergence and expansion of jawed vertebrates—known as gnathostomes—into deeper marine habitats. This influx represents one of the most significant evolutionary chapters in the history of complex life.
For decades, researchers believed that deep-ocean oxygenation was a singular event occurring at the dawn of the Paleozoic Era, roughly 540 million years ago. However, this narrative has been overturned by accumulating evidence suggesting that oxygenation unfolded in multiple phases. Initial increases in oxygen rendered nearshore waters habitable, followed by a subsequent, more enduring oxygenation pulse that permeated deeper oceanic zones much later in Earth’s history.
The team’s approach to unraveling this ancient oxygen history hinged on the innovative analysis of selenium isotopes embedded in sedimentary rock formations. Selenium, a trace element present in seawater, exists in different isotopic forms whose relative abundance shifts in response to the oxygenation state of marine environments. High oxygen levels stimulate greater variability in the ratios of heavy to light selenium isotopes, whereas uniformity in these ratios signals oxygen-poor conditions unsuitable for sustaining animal life.
By assembling and analyzing a globally distributed collection of 97 rock samples, spanning a temporal window of approximately 252 to 541 million years ago, the researchers traced selenium isotopic variability across the outer continental shelves—the submerged edges where continents descend into the deep ocean. These samples originated from five continents, representing geological archives of ancient marine environments once located along the margins of early land masses.
Sophisticated chemical procedures extracted and purified selenium from these pulverized sedimentary rocks, allowing precise isotope ratio measurements. The data unveiled two distinct oxygenation episodes in the deep marine realm: an ephemeral event during the Cambrian period around 540 million years ago, and a subsequent, irreversible oxygenation beginning in the Middle Devonian, approximately 393 to 382 million years ago, which continues to the present day.
This second oxygenation phase coincides with what paleobiologists term the “mid-Paleozoic marine revolution,” a sweeping transformation of marine ecosystems marked by the rise of diverse, active, and sizeable jawed vertebrates invading deeper oceanic niches. Fossil evidence corroborates that these shifts in animal morphology and ecological dynamics align temporally with the sustained oxygen enrichment of these habitats.
Crucially, this oxygenation is thought to have been fueled by the terrestrial expansion of woody plants—the ancestors of modern forests. As these plants colonized continents, their metabolic activities, including photosynthesis and organic matter deposition, increased atmospheric oxygen levels, thereby enhancing oxygen diffusion into the oceans. This terrestrial-marine oxygen linkage underscores the far-reaching effects of early land plant evolution on ocean chemistry and animal habitats.
The initial Cambrian oxygenation event, in contrast, remains enigmatic. It appears to have been a transient pulse insufficient for fostering lasting colonization of deep marine environments by complex animals. Following this brief episode, oxygen levels declined to inhospitable thresholds, impeding faunal diversification in these depths and shaping the evolutionary landscape for millions of years thereafter.
Beyond its paleontological significance, this research holds pressing implications for contemporary ocean health. Today, oxygen distribution in oceans remains uneven, with localized hypoxic zones—”dead zones”—arising from nutrient runoff and eutrophication. These zones threaten marine biodiversity and mirror, albeit on much shorter timescales, the dynamic oxygen fluctuations of Earth’s distant past.
The study, published in the Proceedings of the National Academy of Sciences, offers a nuanced perspective on the fundamental role oxygen has played in sculpting animal evolution. By disentangling the timing and permanence of ancient ocean oxygenation, the authors provide a powerful framework for understanding how Earth’s life-supporting conditions emerged and emphasize the delicate balance that sustains marine ecosystems.
As Michael Kipp, co-lead author and earth sciences professor at Duke University, notes, the interplay between oxygen availability and animal evolution is not merely historical but vital for guiding conservation efforts in the face of accelerating anthropogenic impacts. Preserving oceanic oxygen levels is paramount to maintaining biodiversity, echoing a balance struck nearly 400 million years ago during the Devonian.
This research eloquently illustrates the deep connections binding terrestrial plant evolution, atmospheric chemistry, and marine life diversification, offering new insight into the evolutionary innovations that paved the way for the complex ecosystems thriving in the present-day ocean depths.
Subject of Research: Animals
Article Title: Mid-Devonian ocean oxygenation enabled the expansion of animals into deeper-water habitats
News Publication Date: August 2025
Web References:
10.1073/pnas.2501342122
References:
Bubphamanee K., Kipp M., Meixnerová J., Stüeken E., Ivany L., Bartholomew A., Algeo T., Brocks J., Dahl T., Kinsley J., Tissot F., Buick R. “Mid-Devonian ocean oxygenation enabled the expansion of animals into deeper-water habitats.” Proceedings of the National Academy of Sciences, August 2025, DOI: 10.1073/pnas.2501342122.
Image Credits:
© 2008 N. Tamura/CC-BY-SA
Keywords:
Mid-Devonian, ocean oxygenation, marine evolution, jawed vertebrates, selenium isotopes, Paleozoic marine revolution, deep ocean habitats, woody plants, sedimentary geochemistry, ancient oceans, hypoxia, evolutionary biology
