Ancient archaeal toolkit predates Earth's oxygen rise
A Nature study proposes that the archaeal host lineage behind eukaryotes was already assembling complex cellular machinery in largely anoxic oceans about 2.9 billion years ago, well before substantial atmospheric oxygen. This suggests parts of eukaryotic cellular complexity predate oxygen, and that later oxygen helped expansion rather than origin. It does not prove early multicellular life; instead it highlights a CALM model and relies on molecular dating and genome comparisons, with further data needed to confirm the timeline.