Ancient Australian Fossils Show Early Eukaryotes Thrived Only in Oxygenated Waters

Researchers analyzed drill-core mudstones stored in Darwin that date back up to 1.75 billion years, extracting and microscopically inspecting organic residues to identify more than 12,000 fossilized single-celled organisms. The fossils represent the earliest known eukaryotes and were located in sediments deposited in oxygenated marine environments, from shallow mudflats to deeper open water. In contrast, oxygen-free layers contained only primitive prokaryotic forms.

Chemical tests of the rocks confirmed the presence of oxygen in the fossil-bearing strata. These findings bolster the long-standing view that rising oxygen levels drove the emergence of complex cellular life. The research, published in Nature, adds a crucial data point to debates about how and why eukaryotes first evolved.