Ancient horizontal gene transfer and the last common ancestors

Background: The genomic history of prokaryotic organismal lineages is marked by extensive non-vertical inheritance of genes, with horizontal gene transfer (HGT) between many groups of organisms at all taxonomic levels. These HGT events have played an essential role in the origin and distribution of biological innovations. Analyses of ancient gene families show that HGT existed in the distant past, even at the time of the organismal last universal common ancestor (LUCA). Most gene transfers originated in lineages that have since gone extinct. Therefore, one cannot assume that the last common ancestors of each gene were all present in the same cell representing the cellular ancestor of all extant life. Results: An ancient population of organisms existing as part of a diverse ecosystem at the time of LUCA would likely be sharing genetic material between lineages. If these other lineages persisted for some time, HGT with the descendants of LUCA could have continued into the bacterial an...