Life is still nothing but microbes

Steranes — molecular fossils of eukaryotes — extracted from 2.7-billion-year-old Australian shales were long taken as evidence that complex cells, and oxygen-producing cyanobacteria, were already present. Drilling with ultraclean methods later showed the molecules were not indigenous: concentrations inside pristine cores match laboratory blanks, while conventionally collected core exteriors are contaminated by orders of magnitude. The biosphere of this era was microbial and nothing else, and the episode stands as a caution about how easily ancient rocks pick up modern carbon.