The intense debate over the origin of eukaryotes is being fuelled by the lipids in the cell membrane of an engineered bacterium (see Nature 569, 322–324; 2019). These could be evidence for a ‘two-domain’ (2D) model, in which eukaryotes diverged from a subgroup of the archaea. But in our view, the lipids offer better support for the ‘three-domain’ (3D) model, in which the two groups of organisms share a common ancestor.

Bacteria and eukaryotes have a similar set of lipids in their membranes. Archaeal membranes contain a different set. In both the 2D and 3D scenarios, there could have been an intermediate organism with mixed lipids. This might have arisen either during the transition from archaea to eukaryotes (2D) or at the start of the archaeal lineage (3D).

John van der Oost and his colleagues present an argument that could favour the 3D scenario — namely that the engineered bacterium, which also contains archaeal membrane lipids, is more resistant to heat shock than normal bacteria are (A. Caforio et al. Proc. Natl Acad. Sci. USA 115, 3704–3709; 2018). This might explain why archaeal lipids were selected when archaeal ancestors started colonizing hot springs. The 2D model requires archaeal lipids to have been replaced in eukaryotes by weaker bacterial lipids — which seems to us implausible.