A common type of ancestor, right? I mean, maybe all life evolved from the same type of single-celled organisms that, it so happens, originated in several different places in the world--not necessarily from the exact same ancestor. Or does it really mean from the exact same ancestor?
Good to see you hear, John! --LS
As I have seen the term used, it does indeed mean that all observed life descended from the very same ancestor, which is actually quite likely given the nature of speciation and the brutal pruning of the tree of descent. I include the weasel-word there because we know so little about what might have started the process in the first place that we can barely even speculate about forms of life earlier than those we have seen. Since we have not seen any replicator sufficiently simple to have conceivably arisen spontaneously (we only assume there was one), all those we have seen are products of reproduction as we see it now, so they are almost with mathematical certainty descended from a single individual. Even if similar mutations or chance events had arisen independently early on, it is mathematically likely that only one survived (it is even more likely that none would, but obviously at least one did). -- Lee Daniel Crocker