In an open-air warehouse in tropical Darwin, Australia, sit dozens of trays containing cylindrical cores of rock, drilled decades ago by mineral exploration companies chasing mineral deposits hundreds of meters below the ground. Few people knew what those cores actually held: more than 12,000 microscopic fossils that represent the world's oldest known eukaryote fossils, preserved in mudstone from an ancient inland sea that covered northern Australia over 1.5 billion years ago.
These tiny windows into Earth's deep past have just solved a longstanding puzzle about one of the most pivotal moments in evolutionary history: how complex life emerged from simple single-celled bacteria. Published in Nature, the discovery offers the first clear evidence that oxygen played a crucial role in the origin of eukaryotes—the fundamental cellular leap that eventually made possible all animals, plants, fungi, and the rest of us.
The gulf between prokaryotes (bacteria and archaea) and eukaryotes is vast. Prokaryotes are simple, mostly single-celled organisms that lack a nucleus. Eukaryotes, by contrast, are cellular powerhouses: they feature a nucleus and specialized structures called organelles, each performing specific biological jobs. This eukaryotic revolution transformed the planet and made complex life possible. Scientists now agree that the first eukaryotes arose from a symbiotic merger of at least two prokaryotic microbes—an archaeon and a bacterium.
But the world in which early eukaryotes evolved has remained mysterious. A central question has lingered: did oxygen drive their emergence, or did early eukaryotes thrive without it? Recent discoveries of enigmatic eukaryotes that survive in oxygen-free environments, combined with geological evidence that oxygen was scarce when eukaryotes first appeared, have challenged the long-held assumption that oxygen was always essential to complex life.
To answer this question, researchers dissolved samples of the mudstone cores stored in Darwin and examined the organic residue under a microscope, identifying more than 12,000 fossils in the process. They then analyzed the chemistry of the mudstones themselves to determine whether oxygen was present in the ancient seawater when the sediments were laid down. The fossils—which date from 1.75 billion years ago, making them the oldest eukaryote fossils known anywhere on Earth—came from diverse habitats: coastal mudflats, open ocean, and everywhere in between.
The results were striking. Eukaryote fossils appeared only in samples from oxygenated settings. In oxygen-free environments, only simple prokaryotic forms were found. This suggests that even the oldest known eukaryotes on Earth, living between 1.7 and 1.4 billion years ago, were dependent on oxygen. The data support a hypothesis long held by scientists: that oxygen played a key role in driving the evolution of early eukaryotes, paving the way for the emergence of all complex life.
What makes this discovery remarkable is how it arrived: through the patient examination of rock cores that sat unexamined in a warehouse for decades, waiting for the right questions to be asked.