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.

I need to choose my headers and subheaders more carefully, it seems. If they are not controversial enough, there doesn't seem to be too much interest...

To prevent any misunderstanding: I certainly don't doubt (some form) of common descent. I don't think it's an anti-evolutionary thought in any form, either. I just wonder how certain it is, and which arguments support, that there was exactly ONE. Either ONE for absolutely all lifeforms that ever existed, or ONE for currently existing lifeforms. Where the former would of course be much harder to establish definitively.For example, is this question an active field of research, or is it instead more or less assumed in the "background" and considered that research that focuses on other issues would also immediately reveal different 'first ancestors'?Are there any 'lifeforms' that maybe (even just during a short period after their initial discovery?) fueled speculation that they could be an example of such 'alternative lineage'? I'm thinking about virusses, prions, the 'Mars bacteria'? It would be interesting to hear more about their position in this issue.And to get back to another topic of my post: IF we find that current lifeforms have exactly ONE common ancestor, doesn't that strongly indicate that life started exactly ONCE in the past (on earth)? And that this indicates that it is unlikely that life is really "common" in the Universe and somewhat flexible when it comes to the chemical components and physical circumstances?Now, there could be different interpretations of this... A first one, on one extreme, could be that life IS something we would call "extremely unlikely", and also (which maybe is another way of expressing it) very strict when it comes to the available building blocks and circumstances.But of course it could also be that there ARE different alternatives, which appear quite regularly, but that natural selection quickly establishes our particular form of "life" as the absolutely dominant one. This would mean that, after a sufficiently long period of time, only Carbon/DNA based life would remain on any planet, even if it hosted other lifeforms before, or if other lifeforms continued to appear regularly throughout history AFTER the first carbon/DNA life first originated.Hmmm.. I guess this is just my initial post somewhat reworded But it's the direction I would like this thread to take.

Of course very little can be said about it with any degree of certainty. Still, intuitively one would expect a very different situation from the current one if abiogenesis were reasonably flexible in terms of the possible chemical components and circumstances.If we look at carbon/DNA life, we can see that it has conquered many, extremely different, environments. And within those environments, it has also split up into numerous ecological niches. Or in other words: there is plenty of opportunity for lifeforms to find a suitable place.If abiogenesis was really flexible, I would expect this situation to also exist on a sort of 'meta' level. In other words: we would have a mixture of lifeforms of different chemical basis and possibly with different means of self-replication with modification, each positioning themselves in the environments and ecological niches that suite them best. Surely, some alternative forms would have certain advantages against carbon/DNA, if only in the most extreme circumstances?I wonder whether this hypothesis has been investigated thoroughly already. I looked up 'extremophiles' in Wikipedia, but it didn't specifically address the genetics (I guess that means all known extremophiles are known to be included in our 'tree of life'...)All this thinking has me really looking forward to the discovery of possible primitive life on Mars or Titan

Thanks for posting in my thread. (I expected you when I started this, lol)We'll have lots more responses now. That sounds ok... Fair enough... It could be, IF you conveniently disregard the multitude of indications that at least some of the similarities are highly unlikely to be of the 'functional' kind.We all know the examples of the gene for vitamin C production and others.And the fact that similar adaptation problems are tackled via many different routes (flight, eyes.. to name two). Which clearly indicates that there is a huge random component in the whole process. Which subsequently indicates that lifeforms descending from different 'first ancestors' would be extremely unlikely to share such similar DNA. IF you disregard the many counter-indications... I'm not sure we'll be able to stay on-topic here

Hi,well I'm sorry for not answering sooner and the slight misunderstanding it causes. I was actually thinking about what worthwhile I could add in a reply, and it turned out it wasn't much .I did not post the message to express opposition against the idea of common descent. I just wondered anyone had considered some of the remarks I had, so your message answered that to some degree.I still retain a slight feeling that, given the many cases of symbiosis etc that we observe among carbon/DNA life, other types of life could be expected to not be pushed aside completely and occupy some niche, however small. I guess some of your literature covers that issue. I just don't have a solid enough background in chemistry to really go for that, I fear. (and motivation in combination with that, lol)But thanks for the reply!