Paleontologists have studied 1.75 billion-year-old microfossils from the ancient ocean floor of Australia, revealing that early eukaryotes—the ancestors of all plants, animals, and fungi—thrived in patches of oxygen-rich ocean floor for over a billion years before they ventured into the open ocean.
Eukaryotic fossils from Northern Territory, Australia. Image credit: Lechte et al., doi: 10.1038/s41586-026-10533-4.
Eukaryotes encompass a wide range of life forms, including humans, plants, animals, fungi, and various microorganisms.
Understanding their origins is crucial for grasping the evolution of life’s diversity and complexity on Earth.
“Our goal was to uncover the environments in which early eukaryotes existed, particularly to determine if these early fossils had acquired mitochondria, enabling them to thrive in aerobic conditions,” stated Professor Galen Halverson from McGill University.
“Interestingly, the earliest eukaryotes we studied already demonstrated some level of dependence on oxygen,” remarked Dr. Leigh-Anne Readman, a paleontologist at the University of California, Santa Barbara.
“The distribution of these fossils indicated they lived on or within the ocean floor,” she added.
In this groundbreaking study, paleontologists examined microscopic fossils preserved within fine-grained rocks found in the Macarthur and Billindudu basins of Australia’s Northern Territory.
Today, this area features diverse landscapes, from outback terrains and savannas to the lush environments of Kakadu National Park.
However, between 1.75 billion and 1.4 billion years ago, it was a shallow inland sea with lagoons, tidal flats, and calm coastal waters.
To decode the habitat of these ancient eukaryotes, researchers analyzed the rocks’ chemistry.
By examining oxygen-sensitive elements like iron, they confirmed that the seawater inhabited by these early eukaryotes was oxygen-rich, despite most oceans lacking oxygen during that time.
“We now understand that the earliest known eukaryotes lived predominantly in oxygen-abundant benthic (seafloor) environments near the coast,” Professor Halverson explained.
“This compelling evidence suggests that oxygen availability was a significant factor in the early evolution of eukaryotes,” Dr. Readman noted.
Historically, many scientists believed early eukaryotes existed without oxygen or floated within water columns.
The revelation that oxygen was integral to early life on Earth overturns longstanding assumptions.
The location of these fossils provided additional insights into how these primitive organisms existed.
Dr. Maxwell Lechte, a paleontologist from the University of Sydney, stated: “The fossil distribution implies that eukaryotes likely inhabited the ocean floor and didn’t venture into the open ocean until about a billion years later, when significant environmental changes occurred.”
This discovery aligns with recent studies on microorganisms closely related to eukaryotic ancestors, indicating their ability to utilize oxygen.
“Eukaryotes constitute most of the visible life around us,” Professor Halverson remarked.
“Understanding their origin remains a pivotal scientific challenge, key to comprehending the biodiversity we see on Earth and the potential for life on other habitable planets.”
A recent research paper detailing this study was published in Nature this month.
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MA Lechte et al. Early fossil eukaryotes were benthic aerobic organisms. Nature published online on May 20, 2026. doi: 10.1038/s41586-026-10533-4
Source: www.sci.news
