Phenotypic reconstruction of the last universal common ancestor and evolution of complexity
A fundamental concept in evolutionary theory is the last universal common ancestor (LUCA) from which all living organisms originated. The pervasive perception of LUCA as a minimal, primitive cell is anchored in the psyche of biologists, a very simple cell that has subsequently increased in complexity through time. LUCA must have lived in the sea under constant stress to find and exploit nutrients in a dilute, viscous environment. In this context, motility, size and shape are some of the traits that will have a highest impact on its ecological and evolutionary success. Yet, our understanding of how these traits have evolved in LUCA is entirely lacking. However, despite few recent attempts to link genes and phenotypic traits in prokaryotes, it is still inherently difficult to predict phenotype based on presence or absence of genes alone. Here, we take a different and novel approach based on phylogenetically informed ancestral phenotypic character reconstruction. We apply our ancestral state reconstruction to 22 of LUCA's phenotypic traits, which are fundamental descriptors of prokaryotic cells, across more than 3,000 bacterial and archaeal species. Our results depict LUCA as a far more complex cell than has previously been proposed, challenging the notion of increased complexity through time in prokaryotes.