Not sure if it is fair to call something "pure speculation", just because the exact mechanisms are not yet known. Are there not different degrees of likelihoods? Strictly speaking we can not prove it yet, but given everything we know (complex systems, chemistry, history of earth,...), self-synthesis of life seems still more plausible than a god-like creator. Of course we haven't proved it yet, but at least it seems to warrant further research. And I claim that isn't arbitrary - one could research all sorts of things, maybe we could try to build a big telescope to talk to god or whatever - but some approaches, paths of research seem more promising than others.