Decades-old rock cores drilled from beneath the ground by mining companies now sit on trays in a warehouse in Darwin, Australia. Inside these mudstones, researchers have discovered more than 12,000 fossils of tiny living things that date back 1.7 billion years. The fossils reveal details about the history of oxygen, complex cells, and how life as we know it came to be.
A new study published in Nature has identified what researchers say are the oldest known eukaryote fossils on Earth. These organisms belong to the group that contains animals, plants, algae, and fungi — any life form with a cell nucleus and internal structures. Prokaryotes, which lack nuclei and include bacteria and archaea, make up the other main branch of life.
A fresh study bolsters the case for oxygen’s central part in the rise of complex life. It marks a modest yet telling advance toward solving one of biology’s largest questions: how did eukaryotes come into being?
Mudstone Cores From Ancient Seas
Northern Australia’s drill holes reach hundreds of metres beneath the surface, and the companies that made them sought minerals rather than fossils. Instead, they uncovered hardened seafloor mud from an ancient inland sea that once covered much of the region more than 1.5 billion years ago.
Tiny creatures that dwelt upon the ocean’s floor were saved within the mudstone. The stone was broken and its substance dissolved, and what remained was organic residue from the fossils themselves. Examined beneath a microscope, more than 12,000 specimens were counted among the remains.
Fossils have been recovered from a variety of marine habitats, including coastal mudflats and open waters. However, their presence has been restricted to deposits formed under conditions where oxygen was present in the water. Samples collected from environments lacking oxygen yielded only simple, prokaryotic life forms.
The evidence backs a longstanding idea that oxygen was central to the development of early eukaryotes. It also contributes to a mounting case that during the initial stages of eukaryote evolution, oxygen levels were probably far lower than they are now.
The Cellular Divide
Life on Earth comes in two basic kinds when it comes to cell structure. One kind, the prokaryotes, has cells that are simple and for the most part exist as single cells. The other kind, the eukaryotes, is quite distinct. Their cells carry a nucleus and contain structures known as organelles, which handle particular functions within the cell.
The rise of eukaryotes changed the planet dramatically, setting the stage for the emergence of animals and ultimately humans. Modern genetic evidence supports the conclusion that the most recent shared ancestor of all living eukaryotes came about through a merger between two distinct prokaryotic organisms: an archaeon and a bacterium.
These fossils of single-celled organisms mark the earliest known evidence of eukaryotic life. Their cells display a degree of complexity that sets them apart from prokaryotes, while matching what is typical of eukaryotes.
The Oxygen Question
Most kinds of bacteria can survive and reproduce in conditions without oxygen. By contrast, nearly all living eukaryotes depend on oxygen for their existence. The reason is that aerobic respiration — which breaks down food with the help of oxygen — delivers the large quantities of energy that complicated life requires.
In recent years, the notion that oxygen has always benefited every eukaryote has faced challenge. The cause is the unexpected finding of strange eukaryotes that can flourish without it.
The geological record now holds growing proof that during the early days of eukaryotic life, oxygen was likely far less plentiful than it is today. That would have made low-oxygen waters the typical setting for marine existence.
Taken together, these observations now cast doubt on the long-held idea that eukaryotes have always relied on oxygen for survival.
The Darwin Collection
A collection kept in Darwin, Australia, holds the fossils. Drilling firms carried out the work long ago for mineral exploration. Among them is mudstone, which forms from hardened seafloor mud.
Eukaryote fossils are some of the oldest known, and the earliest of these date back to around 1.75 billion years ago.
To work out what the environment was like when the sediments were laid down, the study’s authors, Maxwell Lechte and Leigh Anne Riedman, examined the mudstones that held the fossils. They looked at the chemistry of those mudstones, which told them whether ancient seawater contained oxygen.
The findings reveal that eukaryote fossils turned up across a wide variety of habitats, including coastal mudflats and the open sea. However, these fossils appeared exclusively in sediments laid down in oxygenated conditions. Samples from oxygen-starved settings held only simple, prokaryotic life forms.
What the Findings Mean
These results back the idea that oxygen had a major part in pushing the development of early eukaryotes forward. They also show just how little we still know about the distant past where these organisms first came into being.
This research contributes further proof to the discussion, even though it fails to resolve the question of whether complex life required oxygen to appear.
The Road Ahead
One of the key unresolved questions in the life sciences concerns the forces behind the great leap that gave rise to early eukaryotes. Further research into these puzzling, ancient microfossils will certainly shed more light on where we came from — and where we stand in the grand scheme of things.
Scientists will keep returning to the fossils in Darwin, and each visit promises more data.
Key Facts Box
- Fossils: More than 12,000
- Oldest known eukaryote fossils: 1.75 billion years old
- Study published in: Nature
- DOI: 10.1038/s41586-026-10533-4
- Samples: Mudstone cores from northern Australia
There is evidence that ancient life forms inhabited environments with plenty of oxygen, which backs up the notion that oxygen held importance. Yet these remains fail to establish that oxygen was what drove the rise of eukaryotes.
This research marks an advance rather than a final answer. It focuses the investigation, yet it does not resolve it. The ancient past still holds many secrets, and much about the earliest eukaryotes is still a mystery.
This study was carried out jointly with Susannah M. Porter, Galen P. Halverson, and Margaret Whelan.
Source material: “1.7-billion-year-old fossils reveal a crucial clue to the rise of complex life,” ScienceDaily.
Get the Notebook.
The day's best stories and every fresh verdict, in plain English, in your inbox by seven. One email a day, no more.