A comprehensive analysis of microfossils dating back 1.75 billion years, unearthed from ancient seafloor sediments in Australia, has led paleontologists to conclude that early eukaryotes—the primordial ancestors of all plant, animal, and fungal life—persisted in oxygen-rich benthic zones for over a millennium before venturing into more open aquatic environments.
Fossil eukaryotes from Northern Territory, Australia. Image credit: Lechte et al., doi: 10.1038/s41586-026-10533-4.
The eukaryotic domain encompasses complex organisms such as humans, alongside flora, fauna, fungi, and a multitude of microscopic entities.
Comprehending the genesis and environmental context of these early life forms is paramount to unraveling the evolutionary trajectory that led to the profound diversification and complexity of life on Earth.
“Our objective was to ascertain the environments inhabited by the earliest known eukaryotic life, specifically to investigate whether these nascent forms had already developed mitochondria, thereby equipping them with the capacity to thrive in aerobic conditions,” stated Professor Galen Halverson of McGill University.
“Our findings indicate that the most ancient eukaryotes identified to date required oxygen to some degree for their survival,” corroborated Dr. Leigh Anne Riedman, a paleontologist affiliated with the University of California, Santa Barbara.
“Furthermore, through their spatial distribution within the sampled materials, we were able to deduce their existence on or within the seafloor substrate.”
Within the scope of this investigation, the paleontological team meticulously examined fossilized microorganisms preserved in fine-grained sedimentary rocks originating from the McArthur and Birrindudu basins located in Australia’s Northern Territory.
Presently, this Australian geographical expanse encompasses diverse landscapes, ranging from arid outback regions and savannas to the intricate network of billabongs and lush forests characteristic of Kakadu National Park.
However, during the period spanning 1.75 billion to 1.4 billion years ago, this locale constituted a shallow, inland marine environment, characterized by numerous lagoons, offshore mudflats, and tranquil coastal waters.
To elucidate the ecological niches occupied by these ancient eukaryotes, the research team undertook an analysis of the geochemical composition of the rock strata themselves.
Employing elements sensitive to oxygen levels, such as iron, the researchers were able to confirm the presence of dissolved oxygen in the ancient seawater where these early eukaryotes resided—a notable finding, given that the majority of Earth’s oceans at that epoch were largely anoxic.
“We ascertained that the earliest eukaryotes, for which fossil evidence exists, dwelled predominantly in near-shore, oxygenated, benthic (seafloor) environments,” commented Professor Halverson.
“This discovery underscores the pivotal role that oxygen availability played in shaping eukaryote evolution from its nascent stages,” observed Dr. Riedman.
A significant number of scientific hypotheses had previously posited that early eukaryotes existed in anoxic conditions or were pelagic drifters.
The revelation that oxygen was integral to early terrestrial life challenges long-standing scientific assumptions.
The geographical provenance of the fossil discoveries provided additional insights into the lifestyle of these primordial organisms.
“The spatial arrangement of the fossils also suggests that these eukaryotes likely inhabited the seafloor, and their colonization of open oceanic realms may not have occurred until approximately a billion years later—a subsequent expansion that would have profoundly reshaped the biosphere once more,” stated Dr. Maxwell Lechte, a paleontologist from the University of Sydney.
These findings resonate with more recent investigations into microorganisms closely affiliated with the ancestral lineage of eukaryotes, which similarly indicate an aptitude for oxygen utilization.
“Eukaryotes represent the vast majority of macroscopic life forms observed on our planet,” Professor Halverson remarked.
“Understanding their evolutionary origins constitutes a pervasive and fundamental scientific inquiry, intimately linked to interpreting the extant biodiversity on Earth and potentially on other celestial bodies capable of supporting life.”
A scientific publication detailing this research was issued this month in the esteemed journal Nature.
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M.A. Lechte et al. Early fossil eukaryotes were benthic aerobes. Nature, published online May 20, 2026; doi: 10.1038/s41586-026-10533-4
