When did archaebacteria first appear?

Prokaryotic fossils have been dated to 3.5 billion years ago, but it is not possible to determine by their shape if they were Archaea. Some lipids, consistent with eukaryotes or archaea, have been detected in shale dating to 2.7 billion years ago. The split between bacteria and archaea probably occurred earlier than that; the lipid remnants could be from either one or both. In light of genetic research since the 1960s, it is no longer held as a certainty that eukaryotes developed from archaea; they may have developed as separate, parallel lines of descent from primordial organisms. That is why the term "archaebacteria" has fallen out of use; Archaea are now classified as a separate domain of living organisms from Bacteria.