Well since you ended up by dismissing it as insufficient, what else is that but to say it doesn’t count? The question we were considering is the potential evidence for a simpler pre-LUCA stage of life, and I have a hard time seeing how, if such a stage has not left any descendants with a similar level of complexity, we could ever hope to obtain evidence of such a stage if you are going to use the lack of homology between cellular structures or systems(or the actual lack of that structure or system) as evidence of separate ancestry.
What would evidence for simpler life look like? Well, fewer ancestral genes, missing ancestral genes, simpler ancestral genes? But if these entail the lack of a defining attribute of cellular life as we know it, you take it to imply separate ancestry rather than simpler life. But then what would evidence of simpler life have to look like? In the absence of surviving descendant with a similar level of complexity, it seems you have made it impossible for such evidence to exist.
I think this is a good illustration of the distinction between an observation and one’s interpretation of it.:
Observation: The membranes of archaea and bacteria have distinct chemistries and the respective enzymes of their lipid biosynthesis show no sign of homology.
Design interpretation: Archae and bacteria were independently designed.
Non-design interpretation: The common ancestor of archaea and bacteria did not have a membrane.
Right, but there’s no observation for which one could not come up with an almost infinite number of different interpretations. There is no pattern we could observe you could not account for with some ad-hoc hypothesis. You could say every imaginable collection of data, or any imaginable pattern is just a remarkable coincidence, or that it was designed to be the way it is because the designer desired that specific outcome.
Things fall down to the ground not because there’s a mutual forces of attraction between objects with mass, but because invisible pixies are pushing them around. Or they’re actually all moving around in random directions, it’s just a remarkable coincidence that some of them seem to systematically fall down. Or even worse, the data is an illusion, we’re in a sort of simulation and it only appears to us af if they fall down. In fact they remain stationary, or they don’t even exist.
One can have specific reasons for favoring one interpretation over the other. Personally, my own holistic view of the cell as a complex, integrated system leads me to favor the design interpretation.
Me, I like consistency, parsimony, explanatory power and scope. That’s why I don’t suddenly stop thinking ancestral convergence and consilience of independent phylogenies is evidence for common ancestry right before we hit the final node in the tree of all known life.
And one can use one’s preference for a particular interpretation as an impetus for further investigations in order to pry the two hypotheses apart. For example, the hypothesis of an independent design of archaea and bacteria would predict that other similar discontinuities between the two cell types would exist.
Why? It’s not clear to me why that would be the case. Why would the designer not just re-use the same basic designs in both the bacterial and archaeal clade? It doesn’t look like a prediction to me, it looks like a sort of ad-hoc rationalization that’s supposed to make it compatible with the already collected evidence, which you’re now trying to sell as a prediction.
I’m not buying it. You don’t have any model that actually predicts this. Where is your model? What are it’s mechanisms, what are it’s starting conditions? I want to verify that it predicts what you say. Can I implement it in a computer and see that yes, in fact, on your design model the designer will come to produce organisms with “similar discontinuities”?
I think evidence that would convince everyone is an unrealistic standard. I get that there has to be some sort of criterion by which to judge whether the evidence is good or bad, persuasive or not, but there will be some inherent subjectivity to that and all we can do is argue about it.
The sense-antisense complementarity between the conserved cores of Class I and Class II aminoacyl-tRNA synthetases is certainly something we would expect to see if the two superfamilies ultimately derive from the opposite strands of the same ancestral gene. It would be entirely rational for a researcher to take this result as an impetus to further investigate the hypothesis.
But I don’t see this as evidence that must compel accept of the hypothesis in everyone, in the sense that one would have to drop one’s own investigation of a competing hypothesis. One of the reviewers of the paper makes an interesting point: (… )A design hypothesis would predict that the sense-antisense complementarity is the result of some functional/structural constraint. This suggests an avenue for further research and shows that design thinking can lead to expectations that differ from those of the non-design approach.
This is where I think your alternative theory runs into inconsistency and arbitrariness. Why suddenly invoke the potential for a structure/function constraint criterion at the final node in the tree just because it potentially implies a lack of some later evolved structure?
Presumably the two next nodes in the bacterial tree (say) also imply a lack of later evolved structures, yet you don’t posit separate ancestry there. At that taxonomic level you would be fine(?) with accepting ancestral convergence and consilience of independent phylogenies as evidence for common ancestry, and you might even allow yourself to rationalize that some putative lack of a homologous key gene can be explained by either a lack of conservation allowing you to detect a homologous relationship, or gene loss, or replacement in one or both clades. But not at the final node.
Yet you could have invoked the same criterion as an argument for separate ancestry at any node in the tree. Reconstructed ancestral nodes in the tree of primates exhibits ancestral convergence to the reconstructed ancestral node in the tree of rodents because of some sort of structural or functional constraint, or by mere remarkable coincidence(what could not be taken to be a mere remarkable coincidence in this fashion? Homology itself could be claimed to just be a remarkable coincidence). Yet presumably you don’t think so and are fine with taking that as evidence for common descent. At every other node in the tree, except the final one.
