Early eukaryotes, the lineage that later gave rise to animals, plants and fungi, may have depended on oxygen from the start. However, they mostly stayed on the seafloor. That narrow habitat could help explain why complex life took so long to spread widely. The oldest widely accepted eukaryote fossils come from seas that were anything but inviting. Oxygen was scarce, the chemistry of the water shifted from place to place, and much of the ocean floor remained hostile to complex organisms. Yet in that patchwork world, a crucial branch of life appears to have found its footing. A study in Nature argues that some of the earliest known eukaryotes, organisms in the domain that later gave rise to animals, plants and fungi, were already tied to oxygen between about 1.75 billion and 1.4 billion years ago. Moreover, the fossils also point to a more grounded lifestyle than many scientists had assumed. These organisms seem to have lived on or within the seafloor, not drifting freely as plankton in the water above. That picture matters because …