An artist’s rendering of a prehistoric jawed fish from the Late Devonian called Dunkleosteus. These sorts of large, active vertebrates evolved shortly after the deep ocean became well-oxygenated.
© 2008 Nobu Tamura/CC-BY-SA.
Another wave of reality breaks over the impervious rocks of creationist dogma, in the form of news that an international team of researchers led by the University of Washington has shown a correlation between the rapid radiation of marine vertebrates and the evolution of plants on land. As trees and other vascular plants spread, more atmospheric carbon became locked into their woody stems, reducing carbon dioxide levels. At the same time, increased photosynthesis raised atmospheric oxygen levels, which in turn oxygenated the oceans, making oxygen available in depths that had previously been anoxic.
Creationists will, of course, need to ignore the fact that this finding flatly contradicts their claims that evolution only occurs within “kinds” and that all diversification happened in a brief burst of warp-speed evolution following a genocidal global flood some 4,000 years ago. The timeline alone is utterly inconsistent with their favourite creationist fairy tale.
Scientists once believed this major oxygenation event had occurred about 500 million years ago, but the new research shows that episode was short-lived. A more significant oxygenation occurred around 390 million years ago. Initially, oxygenation would have taken place in shallow coastal regions where vertebrates first evolved. As oxygen penetrated deeper into the oceans, vertebrates followed into the newly opened niches, leading to a rapid proliferation of jawed vertebrate species — the ancestors from which terrestrial tetrapods later evolved.
The team reached their conclusions after measuring selenium isotopes in 97 sedimentary rock samples from five continents, dated between 252 and 541 million years ago. These rocks had been deposited near the edges of continental shelves, where shallow seas transition into the deep ocean. Selenium occurs naturally in several isotopic forms, and the ratios in which they were deposited depend on the level of oxygen dissolved in seawater. These isotopic signatures thus provide an indirect measure of oxygenation levels at the time the rocks were laid down.
Why Selenium Isotopes Track Ancient Oxygen. Selenium (Se) is a trace element in seawater that occurs in four stable isotopes: Se-74, Se-76, Se-77, and Se-78 (plus two rarer ones, Se-80 and Se-82). These isotopes behave almost identically chemically, but tiny differences in their masses mean that biological and chemical reactions can separate them slightly — a process called isotopic fractionation.The research findings are published in Proceedings of the National Academy of Sciences of the USA (PNAS), and the study and its significance are explained further in a news release from the University of Washington by Gillian Dohrn.
The key link with oxygen comes from how selenium is cycled between seawater and sediments:Over time, these processes leave a distinctive isotopic “signature” in the sediments. By measuring which isotopes are more abundant in ancient rocks, scientists can tell whether the seawater above them was well-oxygenated or depleted of oxygen.
- In oxygen-rich water: selenium is mostly present as soluble selenate (SeO₄²⁻), which is stable and doesn’t easily get buried. When some is removed into sediments, the isotopic ratios remain close to those in seawater.
- In oxygen-poor or anoxic water: microbes use selenium compounds in their metabolism, reducing selenate to insoluble forms (like elemental Se or selenides) that settle into the sediments. This microbial reduction preferentially removes lighter isotopes, leaving the remaining seawater enriched in heavier isotopes.
This works because selenium is globally distributed, cycles between water and sediments relatively quickly, and its isotopic fractionation is sensitive to oxygen availability — making it a reliable tracer of past ocean redox conditions.
Note for creationists tempted to try their idiotic 'changing radioactive decay rates' fallacy:
Selenium isotope analysis is not a dating method at all, and it has nothing to do with radioactive decay. All the selenium isotopes used here are stable, meaning they do not decay over time.
What the researchers measure is the relative abundance of these stable isotopes in the rock record, which shifts depending on the oxygenation conditions of the seawater in which the sediments were deposited.
So:In short: selenium isotopes tell us what the environment was like, not how old the rocks are.
- Radiometric dating = relies on radioactive isotopes decaying at fixed, measurable rates → used to determine the age of rocks.
- Stable isotope geochemistry (like selenium, carbon, oxygen, sulphur) = relies on slight natural fractionation between isotopes under different environmental conditions → used to reconstruct past environments.
How oxygen made the deep ocean home to animals, spurring rapid evolution
Millions of years ago, the deep ocean was largely devoid of oxygen and thus inhospitable to many lifeforms. Now, the same dark zones host an array of marine mammals and fish.
Researchers once speculated that this habitat expansion followed a great oxygenation event more than 500 million years ago, but they didn’t have enough data to support the link.
New research shows that deep-ocean oxygenation did support animal evolution, but it didn’t occur until 390 million years ago, when plants began to take root above ground. The accumulation of woody biomass altered atmospheric conditions enough to influence aquatic oxygen levels. The research draws a clear connection between oxygenation of the ocean and the evolution of most modern vertebrates.
The findings were published in Proceedings of the National Academy of Sciences on Aug. 25.When oxygen levels rose, animals grew larger and moved to places that were previously uninhabitable. This meant more room, and more competition. Animals evolved different strategies to survive, which led to new species.
The Neoproterozoic Oxygenation Event temporarily oxygenated the deep ocean, but not long enough to allow for permanent colonization.
Kunmanee Bubphamanee, co-lead author.
Department of Earth and Space Sciences
University of Washington, Seattle, WA, USA.
The first animals appeared in the fossil record during an era called the Neoproterozoic, leading researchers to postulate that this was also when ocean oxygenation occurred. Recent studies indicated that permanent ocean oxygenation instead occurred later, but still left “a 60-million-year window of uncertainty,” Bubphamanee said.
According to research, ocean oxygenation was a gradual process. Shallow areas near the shore were oxygenated first, and inhabited by breathing species. As oxygen permeated deeper and deeper, ocean-dwellers followed, leading to a rapid expansion of jawed vertebrates, or gnathostomes.
This study gives a strong indication that oxygen dictated the timing of early animal evolution, at least for the appearance of jawed vertebrates in deep-ocean habitats.
Assistant Professor Michael Kipp, co-lead author
Division of Earth and Climate Sciences
Nicholas School of the Environment
Duke University, Durham, NC. USA.
In this study, the researchers started putting together a global timeline for ocean oxygenation, using 97 sedimentary rock samples from five continents collected between 252 and 541 million years ago.
They pulverized the rocks and extracted selenium, an element that indicates whether there was enough oxygen underwater to support breathing animals. Selenium is an element with several isotopes of distinct mass. Different isotope ratios develop depending on the oxygen level present when the sediment was deposited.
The ratio of selenium isotopes in the samples indicated whether there was enough oxygen present in the deep sea to sustain animal life. In the older samples, collected before 390 million years ago, there wasn’t, but in those collected later, there was.
Selenium is great for tracking deep-ocean oxygen levels, but extracting it from rocks is tricky, so few researchers have done it.
Kunmanee Bubphamanee.
Compiling the existing dataset took the team more than five years.
The rock samples were collected from areas near the edges of continental shelves, where shallow seas give way to deep, open ocean. Their data supported the hypothesis that permanent deep-ocean oxygenation didn’t occur until 382 to 393 million years ago, during the Middle Devonian period.
At the same time, woody plants were spreading above ground and trapping carbon-rich biomass, such as animal remains, in the sediment. This released oxygen back into the atmosphere and fed phosphorus — a sort of organic fertilizer — into the ocean. The water, now oxygen and nutrient rich, could support more energy-intensive life than before.
The findings also reveal just how critical oxygen levels are to marine life. Although there is abundant oxygen in the atmosphere now, certain human activities can impact how much oxygen is present in the ocean.Oxygen enables more metabolically active lifestyles. Predation consumes calories, and animals burn calories using oxygen. Until the deep ocean had ample dissolved oxygen, it would not have been viable to live there as a large predator.
Assistant Professor Michael Kipp.Runoff from agricultural and industrial activity contains chemicals that fuel plankton blooms that suck up oxygen when they decay, causing levels to plummet. 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.
Assistant Professor Michael Kipp.
Co-authors include:
- Roger Buick, a UW professor in Earth and space science and astrobiology;
- Jana Meixnerová a UW graduate student in Earth and space science;
- Eva E. Stüeken, a reader in Earth and environmental sciences at the University of St. Andrews;
- Linda C. Ivany, a professor in Earth and environmental science at Syracuse University;
- Alexander J. Bartholomew, an associate professor of geology at SUNY New Paltz;
- Thomas J. Algeo, a professor of geology at the University of Cincinnati;
- Jochen J. Brocks, a professor in the Research School of Earth Sciences at Australian National University;
- Tais W. Dahl, an associate professor of geobiology at the University of Copenhagen;
- Jordan Kinsley, a postdoctoral candidate at Australian National University;
- François L. H. Tissot, a professor of geochemistry at CalTech.
Publication:
SignificanceWhat this finding demonstrates is that Earth’s history, far from being compressed into just a few thousand years as creationists insist, is that of a dynamic, ever-changing planet over deep time. Changes in the terrestrial realm — such as the rise of land plants — had profound and lasting consequences for the oceans, and life evolved in response to these changes. This is a very different picture from the static, perfectly tuned, and unchanging world demanded by creationist mythology.
The timing of permanent deep-ocean oxygenation is controversial; early work placed it in the Ediacaran, whereas more recent studies point to the mid-Paleozoic. Establishing the timing of this transition has profound implications for the ecological radiation and evolutionary diversification of metazoan life. Here, we better constrain the Paleozoic history of deep-ocean oxygenation using selenium geochemistry. We do not observe permanent deep-ocean oxygenation until the Middle Devonian (393 to 382 Ma), which overlaps with the “mid-Paleozoic marine revolution” in animal life. Ocean oxygenation thus provides a plausible explanation for the evolutionary changes that are documented in this interval. This rise of oxygen was likely driven by organic carbon burial during the spread of woody vascular plants across landmasses.
Abstract
The oxygenation history of Earth’s surface environments has had a profound influence on the ecology and evolution of metazoan life. It was traditionally thought that the Neoproterozoic Oxygenation Event enabled the origin of animals in marine environments, followed by their persistence in aerobic marine habitats ever since. However, recent studies of redox proxies (e.g., Fe, Mo, Ce, I) have suggested that low dissolved oxygen levels persisted in the deep ocean until the Late Devonian, when the first heavily wooded ligniophyte forests raised atmospheric O2 to modern levels. Here, we present a Paleozoic redox proxy record based on selenium enrichments and isotope ratios in fine-grained siliciclastic sediments. Our data reveal transient oxygenation of bottom waters around the Ediacaran–Cambrian boundary, followed by predominantly anoxic deep-water conditions through the Early Devonian (419 to 393 Ma). In the Middle Devonian (393 to 382 Ma), our data document the onset of permanent deep-ocean oxygenation, coincident with the spread of woody biomass across terrestrial landscapes. This episode is concurrent with the ecological occupation and evolutionary radiation of large active invertebrate and vertebrate organisms in deeper oceanic infaunal and epifaunal habitats, suggesting that the burial of recalcitrant wood from the first forests sequestered organic carbon, increased deep marine oxygen levels, and was ultimately responsible for the “mid-Paleozoic marine revolution.”
K. Bubphamanee, M.A. Kipp, J. Meixnerová, E.E. Stüeken, L.C. Ivany, A.J. Bartholomew, T.J. Algeo, J.J. Brocks, T.W. Dahl, J. Kinsley, F.L.H. Tissot, & R. Buick
d-Devonian ocean oxygenation enabled the expansion of animals into deeper-water habitats Proc. Natl. Acad. Sci. U.S.A. 122(35) e2501342122, https://doi.org/10.1073/pnas.2501342122 (2025).
© 2025 National Academy of Sciences of the USA.
Reprinted under the terms of s60 of the Copyright, Designs and Patents Act 1988.
As science advances, the picture that emerges is of a story of life on Earth that is far richer, more complex, and more awe-inspiring than the simplistic tale the Bronze Age authors of Genesis could ever have imagined.
If for no other reason, this stands as a comprehensive rebuttal of the claim that the Bible is the inerrant word of an omniscient creator god. The deep oceans came alive when plants transformed the air; our understanding comes alive when science transforms the myths.
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