Some abiogenesis considerations

Perhaps this has been discussed before, but anyways:There is obviously a lot of speculation about abiogenesis. Particularly in the area of SETI, there is a lot of discussion about the likeliness of the development of life, what kind of life it would (have to) be etc. I'm sure opinions vary from life being extremely rare (or even just one, earth-based instance), and being exclusively carbon and DNA-based, to views that favour life as being almost an essential/inevitable product of the Universe under reasonable circumstances, and available in many chemical makeups and many hereditary/reproductive mechanisms.In general, I have always held the view that both extremes could still well turn out to be true, because we simply lack substantial evidence or strong indications for either. (life on earth as one single datapoint, so to speak)However, lately I'm starting to lean towards the "extremely rare" hypothesis, or alternatively the assumption that, even if life is abundant, it would still very likely be based on very similar chemistry (carbon and/or DNA or something very very similar). With a preference for the "extremely rare" hypothesis.This is based on my very basic observation (which may well be wrong/uninformed) that ONE common ancestor for all currently existing lifeforms seems to be the prevailing idea in science at this point (?)The (admittedly quick) conclusion that I draw from this, is that it is unlikely that development of life is BOTH almost inevitable, and at the same time shows a lot of tolerance when it comes to the exact chemical makeup. Because if this were true, many different types of life would be expected to originate in different places at roughly the same time, and/or different types of life would keep appearing throughout time. It looks like this is not the case, OR alternatively this may have happened (and IS happening at this moment?), but these "alternatives" turn out to have absolutely no chance in competition against "our" type of carbon/DNA based life. In which case they would appear in very limited areas, and disappear quickly again. But if we leave out the "many chemical makeups possible" idea, it still seems unlikely that one particular type (carbon/DNA) is highly likely to appear spontaneously. Because in that case, we would expect this event to have taken place in many different locations and timeframes, ending up with life that is chemically related/similar, but with many different first ancestors.So, what I would like to see addressed here is, first of all, how strong the indications are that current life has indeed exactly ONE common ancestor. What reasoning/evidence is this idea based on (or ISN'T it the strong prevailing idea after all?) And are there escape routes for the "life is inevitable and can take on many forms" idea, given the possible counter-arguments I gave above? Any other related ideas are also welcome.

Thread copied to the Some abiogenesis considerations thread in the Origin of Life forum, this copy of the thread has been closed.