I haven't found this written out somewhere, but it's generally implied in discussions on evolution, but the uninformed might not entirely understand evolution without first considering that the "single ancestor hypothesis" isn't necessarily the central hypothesis of the debates. That argument is simply the popular interpretation, I suppose. An alternative is that DNA, RNA, the ribosome, the organelles of the cell, etc., evolved in synergistic/cooperative manners in which there were many, many different emergent structures or functions or processes that then were integrated together into the modern cellular self-replicating systems that we see today. There was not necessarily ever a "first cell" from which all modern descendents were born. And by descendents I mean the replicational systems more than anything, not so much the eukaryotes and animals and so on, which to a very large extent seem to show shared traits -- this could be due to shared ancestors -- but at the same time it could also be due to shared/common historical contexts that led to the selective pressures and scenarios in which those ancestors (of all of the entities, not 'the only ancestors that everything shares') happened to become those organisms that had the similar traits as the ones that we're now commenting on. Yes, we can find evidence that different branches of different trees had different/similar traits. But I'm not talking about evolution or natural selection at that level. Rather, consider the systems and processes that emerge all the time 'out of the blue' (I mean out of thin air, not out of the sky), like murky oil spills and so on. This is where the systems and processes, before they could leave significant bone-based fossilizations, could be hypothesized. This is sometimes called the RNA world, but it could easily be from long before RNA, etc. Yes, some ancient archaebacteria have been found fossilized, and even some microbubbles can be found in rocks on some parts of the world, from about the time that the first cell membrane is hypothesized to come from -- but again, as can be seen in any simple chemistry lab, chemical reactions usually don't leave behind a footprint besides some ash or some byproduct of that nature, easily destroyed by winds or hundreds of millions of years of natural degradation and whatnot. But. There is some good news. Those original emergent chemical conditions were dependent on the environment -- which we can determine from the rock records to some extent -- as well as by the fact that we figure they would have to be fairly simple, not some monstrous process more complicated than current organisms [although this alternative hasn't been extensively explored to my knowledge]. So if the chemical systems were simple, then we may be able to happen upon a 'possibility space' where we can test out what happens if we throw them together and see if we get any interesting results that look close to modern molecular biology.
As for the "complex primordial soup hypothesis" -- the long, complicated molecular processes that might have resulted in the evolution or collaborative development of the molecular biology on which we are based. I find this unlikely largely because of the maintenance required to maintain complex molecular structures. There is a limit to the ability to repair information in a molecular system, a limit to preserved functionality, and at some point the functionality is going to drop off quickly and nothing is going to get done. So I'm not going for the "dragons making ants" scenario. But what about lightning-quick molecular structures that emerge in a split second and then vanish again? If such rapidly emergent structures (RESes) do in fact exist, then it should be interesting to explore them in a lab and see what can be done with them.