The evolutionary transition from prokaryotes to eukaryotes—characterized by eukaryotic cells obtaining a nucleus and two membrane compartments—is still subject to considerable debate. It has been suggested that a nucleus developed first, followed by the acquisition of mitochondria, a theory that has partly relied on the single-celled eukaryote Giardia intestinalis because it has a nucleus and no mitochondria, suggestive of an intermediate step. The recent identification of mitochondrial remnants in other amitochondrial protists suggests this may not be true. In the November 13 Nature, Jorge Tovar and colleagues at Royal Holloway, University of London report that Giardia intestinalis possesses highly reduced mitochondria remnants called mitosomes (Nature, 426:172-176, November 13, 2003).