Researchers studying drill cores stored in Darwin, Australia, have uncovered more than 12,000 microscopic fossils from an ancient inland sea that covered northern Australia over 1.5 billion years ago. The cores, originally drilled by mineral exploration companies, contain mudstone formed from seafloor mud. By dissolving the rock and examining the organic residue, the team identified the oldest known eukaryote fossils on Earth, dating back as far as 1.75 billion years.
The fossils were found in environments ranging from coastal mudflats to the open sea, but only in layers deposited in oxygenated water. In oxygen-free layers, the team found only simpler prokaryotic microbes. This pattern suggests that even the earliest eukaryotes depended on oxygen, contradicting recent suggestions that some early complex life may have thrived without it.
The study, published in Nature, strengthens the long-held hypothesis that oxygen played a crucial role in the evolution of eukaryotes—the group that includes all animals, plants, algae, and fungi. Understanding the environmental conditions that drove this major evolutionary leap remains a key question in the life sciences, and these ancient fossils offer a rare window into that pivotal moment.