Some major transitions in evolution (such as the origin of multicellular organisms or that of social animals) occurred a number of times, whereas others (the origin of the genetic code, or language) seem to have been unique events. One must be cautious with the word ‘unique’, however. Due to a lack of the ‘true’ phylogeny of all extinct and extant organisms, one can give it only an operational definition. If all the extant and fossil species, which possess traits arising from a particular transition, share a last common ancestor after that transition, then the transition is said to be unique. Obviously, it is quite possible that there had been independent “trials”, as it were, but we do not have comparative or fossil evidence for them.