Phenotypic reconstruction of the last universal common ancestor reveals a complex cell

A fundamental concept in evolutionary theory is the last universal common ancestor (LUCA) from which all living organisms originated. While hypotheses regarding the phenotype of LUCA do exist, most are at best based on gene presence or absence. However, despite recent attempts to link genes and phenotypic traits in prokaryotes, it is still inherently difficult to predict phenotype based on the presence or absence of genes alone. Here, we apply a novel approach based on phylogenetically informed ancestral character reconstruction with 22 fundamental descriptors of prokaryotic cells to reconstruct LUCAs phenotype. We infer that the last universal common ancestor of all living organisms was likely an ovoid cell with a large genome, which probably had a cell wall and was actively motile. Potentially obtaining energy from the oxidation of inorganic compounds, tolerant of saltwater and thriving at temperatures above 70{degrees}C, LUCA was probably found living freely in anaerobic conditions in water of neutral pH. Our results depict LUCA as likely to be a far more complex cell than has previously been proposed, challenging the evolutionary model of increased complexity through time in prokaryotes. Given current estimates for the emergence of LUCA we suggest that early life very rapidly evolved considerable cellular complexity.