Evolutionary relationships among the four major kinds of cell. The horizontal red arrow indicates the position of the universal root as inferred from the first protein paralogue trees, i.e. between neomura and eubacteria. To determine whether the root is really there or within eubacteria, as suggested instead by many paralogue trees for metabolic enzymes, we must correctly polarize the direction of the negibacteria/posibacteria transition that took place in bacteria that had already evolved flagella. As argued in detail in the text, flagellar evolution and wall/envelope evolution both strongly favour a transition from negibacteria to posibacteria (continuous black arrow), not from posibacteria to negibacteria (broken red arrow). This places the root within Negibacteria and shows that the ancestral cell had two bounding membranes, not just one as traditionally assumed. A negibacterial root also fits the fossil record, which shows that Negibacteria are more than twice as old as eukaryotes [1, 129]. As negibacteria are the only prokaryotes that use sunlight to fix carbon dioxide this is also the only position that would have allowed the first ecosystems to have been based on photosynthesis, without which extensive evolution might have been impossible. Posibacteria, archaebacteria and eukaryotes were probably all ancestrally heterotrophs, whereas negibacteria are likely to have been ancestrally photosynthetic and diversified by evolving all the known types of photosystem and major antenna pigments.