The concept of the three domains of life (the bacteria, archaea and eukaryotes) goes back to Carl Woese in 1990. Most scientists now see the eukaryotes (cells with a true nucleus) as a secondary domain, derived from bacteria and archaea via an endosymbiosis. That makes the last universal common ancestor of life (LUCA) the ancestor of bacteria and archaea. While these domains are strikingly different in their genetics and biochemistry, they are nearly indistinguishable in their cellular morphology — historically, both groups have been classed as prokaryotes. In terms of their metabolic versatility and molecular machinery, prokaryotes are if anything more sophisticated than eukaryotes. Yet despite an exhaustive search of genetic sequence space in virtually infinite populations over four billion years, neither domain evolved morphological complexity to compare with eukaryotes. The evolutionary path to morphological complexity does not seem to depend on genetic information alone. The most plausible explanation is that physical constraints stemming from the topological structure of prokaryotes blocked the evolution of morphological complexity in prokaryotes, and that the endosymbiosis at the origin of eukaryotes relieved these constraints. In this lecture, I shall argue that the dependence of all life on electrical charges across membranes to generate energy explains the structural constraints on prokaryotes, and the escape from these constraints in eukaryotes.
Archaea, Bacteria, Eukaryotes, Tree of Life, Origin of Life, Bioenergetics, Membranes.