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Scientists studying ancient Australian rocks uncovered more than 12,000 fossils from among the oldest known eukaryotes on Earth, dating back 1.7 billion years. These early complex cells were found in oxygen-available settings ranging from coastal mudflats to open sea, while oxygen-free environments contained only simpler microbes. The findings strengthen the hypothesis that oxygen availability was a crucial driver in the emergence of complex life, offering a clearer picture of the environmental conditions that shaped early evolution.