Stored in an open-air warehouse in tropical Darwin, Australia, are trays of cylindrical rock cores drilled decades ago by mineral exploration companies. Some of those cores are mudstone, hardened seafloor mud from an ancient inland sea that once covered much of northern Australia.
Inside them, researchers found tiny fossils that may help explain where some of Earth’s earliest complex cells lived.
The fossils, described in a Nature study, are between about 1.75 billion and 1.4 billion years old. The researchers reported that these early eukaryotes, the broad group that includes animals, plants, algae and fungi, were found almost entirely in rocks deposited under oxygenated bottom waters.
That finding points to a more specific picture of early complex life. Rather than drifting mainly through the open water like plankton, the study suggests these organisms lived on or within the seafloor, in places where oxygen was available.
Ancient Mudstone Preserved More Than 12,000 Fossils
The fossils came from mudstone cores stored in Darwin and linked to ancient marine environments in northern Australia. According to the University of Sydney, the team crushed samples of the mudstone cores, dissolved them, and studied the organic residue left behind under a microscope.
The researchers identified more than 12,000 microfossils. They also examined the rocks that held the fossils to reconstruct the environments where the sediments had formed.
That combination mattered. The team was not only asking whether eukaryote fossils were present. They were asking what kind of seafloor they were preserved in, whether the ancient seawater contained oxygen, and whether the fossil distribution matched a life spent floating in the water column or living at the bottom.
The study reports that eukaryote fossils appeared in settings ranging from coastal mudflats to the open sea. But they were present only in samples deposited under oxygenated conditions. Rocks formed in oxygen-free settings contained only simpler prokaryotic forms, according to the University of Sydney account.
Early Eukaryotes Appear Tied to Oxygen-Rich Habitats
Eukaryotes are cells with more complex internal organization than bacteria and archaea. Modern eukaryotes include organisms with nuclei and organelles, including mitochondria, the energy-producing structures used by nearly all eukaryotes alive today.
The Nature paper reports that the size and morphological complexity of these ancient fossils suggest the organisms probably possessed mitochondria. The authors also argue that their fossil distribution indicates they were aerobes, meaning they used oxygen in some capacity. The study leaves room for different kinds of oxygen use, including organisms that may have required oxygen, tolerated variable oxygen levels, or lived with very low oxygen.
That distinction is important because early Earth did not look like the modern planet. Oxygen was far less widely available when these organisms lived, and oxygen-free marine habitats would have been common. Because of that, scientists have debated whether the earliest eukaryotes depended on oxygen from the beginning or whether oxygen use became more important later.
The new study does not claim to solve every question about eukaryote origins. It does, however, add fossil and geological evidence to the argument that oxygen was already important for some of the oldest known fossil eukaryotes.
The Seafloor Evidence Challenges an Older Assumption
The researchers’ most distinctive claim is not only that these organisms used oxygen. It is that they were likely benthic, meaning they lived on or within the seafloor.
That conclusion comes from where the fossils were missing. If the organisms had lived mainly as plankton in surface waters, their remains would be expected to settle into both oxygenated and oxygen-free seafloor sediments. Instead, the Nature paper reports that the eukaryote fossils were nearly absent from otherwise fossil-bearing anoxic samples.
A UC Santa Barbara article on the research says the team concluded that oxygenated seafloors were likely an early habitat for eukaryotic life. Leigh Anne Riedman, a co-lead author and paleontologist at UCSB, said the team found that the oldest eukaryotes they had seen “already needed oxygen in some capacity” and were living “on or within the seafloor” based on how they were distributed across samples.
The finding adjusts a view that had treated many early eukaryotes as likely plankton-like organisms. Some of the fossils resemble modern plankton in form, but the researchers argue that their geological pattern tells a different story.
Oxygenated Seafloors May Explain Slow Early Evolution
The broader question is why complex cells appeared so early in the fossil record but did not immediately become diverse and widespread.
The Nature paper proposes that eukaryotes may have been largely restricted to oxygenated seafloors for much of the Proterozoic eon. In that model, suitable places for early eukaryotes would have existed, but they may have been limited to specific oxygen-rich zones on or near the seafloor.
That could help explain why eukaryotic fossils appear long before the later rise in eukaryote diversity. The authors suggest these organisms may not have expanded into planktonic habitats until the Neoproterozoic era, roughly 1 billion to 540 million years ago.