Oldest known eukaryote fossils turn up only in oxygenated Australian mud

The result rests on a negative sample, because the oxygen-free settings in the study still preserved simpler microbes ^[3]. Without that, an obvious objection would stand: anoxic mud might simply destroy delicate cells, and the eukaryotes' absence there would say nothing about where they lived. Rock that kept prokaryotes but no eukaryotes is much closer to a habitat signal.

The finding also lands in an open argument. Nearly all living eukaryotes use oxygen, because aerobic respiration supplies the energy complex cells need ^[9]. In recent years, though, researchers have found eukaryotes that thrive without it ^[10]. The geological record also increasingly suggests oxygen was much scarcer when eukaryotes were first evolving, with oxygen-free marine habitats the norm ^[11]. If anoxic water was the default, it is a pointed result that early eukaryotes stayed in oxygenated water across settings as different as coastal mudflats and the open sea ^[2].

The authors explain why they went to rocks and not genomes. Genetic studies of living microbes can say a lot about eukaryote ancestry, but they wrote that "only the fossil record can tell us about long-extinct lineages" ^[15]. The account is their own, published in The Conversation about their Nature paper ^[14][4]. The authors did not report how oxygen levels were read from the mudstones, how many samples came from anoxic settings, or how the more than 12,000 fossils ^[1] split across environments.

The fossils cannot show whether oxygen was needed for eukaryotes to arise at all. Gene-based work traces the last common ancestor of living eukaryotes to a union of at least an archaeon and a bacterium ^[12]. Fossils as old as 1.75 billion years ^[7] record where early eukaryotes lived once they existed, and say nothing directly about the conditions under which the lineage formed. An association in one ancient inland sea ^[6] fits oxygen being a requirement. It fits equally well with eukaryotes that had already become oxygen users living where the oxygen was.

The summary's wording is that the findings "strengthen the idea" that oxygen mattered to the emergence of complex life ^[13], and that is about as far as the evidence reaches. I think the pattern is good evidence that these early eukaryotes depended on oxygen, provided the oxygen reconstructions in the paper hold up. On the question of origins, it is evidence from one sea that covered much of northern Australia more than 1.5 billion years ago ^[6].