They're not saying that there's only one universal ancestor only that of all the universal ancestors, one must have been the most recent. Which is almost certai

They're not saying that there's only one universal ancestor only that of all the universal ancestors, one must have been the most recent. Which is almost certainly true, if in fact there exists a universal ancestor. Perhaps you meant to ask if there could have been no universal ancestor? In that case I think most evidence points the other way. There could possibly have been multiple 'origins' of life but those seem to have died out or have been driven to extinction by our ancestors.