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A creative interpretation of a primeval jawed fish from the Late Devonian known as Dunkleosteus. These significant, dynamic vertebrates developed shortly after the deep ocean became sufficiently oxygenated.© 2008 Nobu Tamura/CC-BY-SA
Countless years ago, the profound ocean was mostly lacking in oxygen, rendering it unsuitable for numerous organisms. Currently, those same shadowy regions are home to a variety of marine mammals and fish. Scientists previously theorized that this habitat growth was a result of a major oxygenation event occurring over 500 million years ago, although they lacked sufficient evidence to confirm the connection.
Recent studies reveal that deep-ocean oxygenation did foster animal evolution, but this process did not take place until 390 million years ago, coinciding with the establishment of plants above soil. The buildup of woody biomass significantly changed atmospheric conditions, consequently affecting aquatic oxygen concentrations. The findings provide a definitive link between ocean oxygenation and the evolution of most contemporary vertebrates.
These results were published in the Proceedings of the National Academy of Sciences on Aug. 25.
“As oxygen levels increased, animals became larger and ventured into areas that had previously been uninhabitable,” stated lead author Kunmanee “Mac” Bubphamanee, a doctoral student in Earth and space sciences at the University of Washington. “This created more space and heightened competition. Animals adapted with diverse survival strategies, leading to the emergence of new species.”
The initial animals emerged in the fossil record during a period referred to as the Neoproterozoic, prompting researchers to hypothesize that ocean oxygenation also took place then. Recent investigations suggested that permanent ocean oxygenation occurred later, but “a 60-million-year gap of ambiguity” remained, according to Bubphamanee. “The Neoproterozoic Oxygenation Event temporarily introduced oxygen to the deep ocean, yet not for a long enough period to permit lasting colonization.”
Research indicates that ocean oxygenation was a gradual occurrence. Shallow coastal zones were oxygenated first, inhabited by aerobic organisms. As oxygen infiltrated deeper layers, marine life followed, leading to a swift proliferation of jawed vertebrates, or gnathostomes.
“This research offers strong evidence that oxygen regulated the timeline of early animal evolution, at least regarding the emergence of jawed vertebrates in deep-ocean environments,” remarked co-lead author Michael Kipp, an assistant professor of Earth and climate sciences at Duke University, who initiated this study as a doctoral researcher at UW.
In this investigation, the researchers compiled a global timeline for ocean oxygenation, utilizing 97 sedimentary rock samples from five continents collected between 252 and 541 million years ago.
They ground the rocks and extracted selenium, an element that signifies whether sufficient oxygen was present underwater to support aerobic life. Selenium has various isotopes of unique mass. Distinct isotope ratios develop based on the oxygen levels present when the sediments were deposited.
The selenium isotope ratio within the samples denoted whether adequate oxygen existed in the deep ocean to sustain animal life. Older samples, gathered before 390 million years ago, revealed a lack of sufficient oxygen, while those obtained later indicated its presence.
“Selenium is excellent for monitoring deep-ocean oxygen levels, but extracting it from rocks is challenging, which is why few researchers have undertaken this,” Bubphamanee noted. Assembling the existing dataset required the team over five years.
The rock samples were gathered from regions close to the edges of continental shelves, where shallow seas transition into the deep, open ocean. Their data corroborated the hypothesis that permanent deep-ocean oxygenation did not occur until approximately 382 to 393 million years ago, during the Middle Devonian period.
Simultaneously, woody plants were proliferating above ground, sequestering carbon-rich biomass, including animal remains, within the sediment. This process released oxygen back into the atmosphere and supplied phosphorus—basically organic fertilizer—into the ocean. The now oxygen- and nutrient-rich waters could sustain more energy-demanding life than prior.
“Oxygen facilitates more metabolically active lifestyles,” Kipp stated. “Predation requires calories, and animals expend calories using oxygen. Until the deep ocean possessed sufficient dissolved oxygen, it would not have been feasible for large predators to thrive there.”
The findings further illustrate the crucial role oxygen levels play in marine ecosystems. While the atmosphere currently contains abundant oxygen, certain human actions can influence the levels found in the ocean.
“Runoff from agricultural and industrial activities carries chemicals that ignite plankton blooms, which deplete oxygen as they decay, causing levels to drop sharply,” Kipp explained. “This study clearly demonstrates the connection between oxygen and marine animal life. This was a balance achieved around 400 million years ago, and it would be regrettable to interrupt it today within just a few decades.”
Co-authors comprise 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 the Australian National University; Tais W. Dahl, an associate professor of geobiology at the University of Copenhagen; Jordan Kinsley, a postdoctoral candidate at the Australian National University; and François L. H. Tissot, a professor of geochemistry at CalTech.
This investigation received funding from the National Science Foundation, the Agouron Institute, and the NASA Astrobiology Institute Virtual Planetary Laboratory.
For further details, contact Bubphamanee at [email protected] or Kipp at [email protected].
Adapted from a news release from Duke University.
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