Ancient Australian fossils, dating back more than 1.7 billion years, have given scientists new clues about how complex life on Earth began. These fossils, found in mudstone cores from northern Australia, are among the oldest known eukaryote fossils in the world. Eukaryotes are complex cells that form the foundation of all animals, plants, algae, and fungi. A study published in Nature shows that these early eukaryotes lived in environments where oxygen was present, unlike simpler prokaryotic microbes, which can survive without it. The fossils were discovered in sedimentary rock samples that had been drilled by mineral exploration companies decades ago and stored in an open-air warehouse in Darwin, Australia. Scientists analyzed these samples and found more than 12,000 fossils, offering a detailed look at the cellular structures of these ancient organisms. The study suggests that early eukaryotes lived in a range of environments, such as coastal mudflats and the open sea, but only in places where oxygen was available. The findings support the idea that oxygen played a key role in the development of early eukaryotes. While some recent studies have shown that certain eukaryotes can live without oxygen, this research suggests that the earliest eukaryotes relied on oxygen to survive. This challenges earlier assumptions that oxygen levels were too low to support such life during this time, as geological evidence had previously suggested. The research underscores the value of examining both the genetic makeup of modern organisms and the fossil record to understand the evolutionary history of eukaryotes. The study's authors believe that further research into these ancient microfossils will continue to shed light on the origins of complex life and its impact on shaping the Earth's ecosystems.