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I had my concerns that this might end up focusing on something other than AGW, but here goes.
We observe life and seek an explanation as to why we observe life. The "hypothesis space" is essentially infinite - limited only by reality, and within reality limited only by human imagination and ingenuity. But we need only concern ourselves with those hypotheses which have so far been stated, and we can conclude (A) one of the hypotheses is sufficiently close to the actual truth to be useful, (B) none of the hypotheses are of any use and someone needs to do some more work. Assuming (A), we can then state reasons as to why we find certain hypotheses to have a close to 0% probability of being correct and why we find one of them to have a >>0% probability of being correct, or at least not wrong.
No one would claim that the current understanding of evolution is absolutely correct and will remain unchanged for all time, but the weight of expectation is that it is far more likely that the theory will move incrementally towards a perfect description of reality in ever smaller steps without ever getting to its destination, than that God will come down from his heaven on judgment day and tell us we have got it all wrong.
AGW seems to me to pull a number of stunts which bypass this process altogether. Firstly, what observation are we trying to explain? It seems to me the observation that AGWers are trying to explain is largely something that they predict we will observe over the next 100 years rather than something we have already observed (and that many of the expected observations such as hurricanes inundating cities built below sea level in hurricane-prone areas, are pretty much dead certs over a 100-year period.) Secondly, if there is a observation that needs explaining (e.g. warming between 1980 and 1998 which almost no one disputes actually happened), what are the other ideas in town? Proponents of evolution at least acknowledge the existence of crackpot religious ideas and the more patient evolutionists expend considerable effort on explaining why the crackpot religious ideas have a close to 0% probability of being correct. AGWers simply dismiss alternative hypotheses without a second thought (in much the same way that religious crackpots dismiss evolution without a second thought). Even if the suggestion that that warming was not anthropogenic in origin is totally crackpot, someone should be able to provide a thoughtful explanation as to why.
Your point about humans potentially having evolved many times is taken. It's a possibility, but a rather remote one. Evolution works by applying selection to (effectively) random genetic changes and it is vanishingly unlikely that near enough the same thing will happen twice, especially at a distance of time with very different circumstances.
I agree. With everything you wrote including the vanishingly small potential that there is likely to have been almost identical instances of human evolution separated by periods of huge change that resulted in different basic circumstances existing at possible starting points.
Very unlikely indeed. But still difficult to disprove.
I could extend it and say there may have been 'similarities'.
Perhaps humanity has not so much evolved from a previous distinctly different biological state more than once but may have evolved once and become highly developed as a social and knowledgeable civilisation (in a modern way we could understand) and then failed for some traumatic reasons - too much carbon sequestration perhaps or maybe galloping temperature tipping points, any reasons, does not matter what - and resulted in ALMOST total destruction but some survived and returned to regenerate another version of 'civilisation' developed over time from a new blank mind. (Or not ...)
Alternatively humanity has not existed previously but creatures with similar abilities to invent and adapt to a greater extent than other creatures and perhaps use tools and some equivalent of science for their own betterment .... and so on.
As with AGW there is always the potential that some event might prove to be a decisive proof, one way or the other, that such events did in fact occur, but the chances of that are very very small based on out current knowledge, skills and potential for radical discoveries. AGW is the same, in my opinion, other than the effective battle ground is the future rather than the past. Whilst that might suggest a greater potential for improving and refining the knowledge available to come closer to refining the hypothesis in much shorter geological time scales it may equally prove to pose more scientific questions (based on new knowledge) than are answered.
Perhaps the problem lies not with science and scientific principles but with the concepts of working with them reliably detached from any other influences so that over time the target question at least remains the same (as it likely does most of the time with fundamental historically oriented questions.)
But that won't happen so it will come down to a moving target of what people are trying to prove. Or rather many targets I would suspect. None of them are likely to be absolutely provable without large error bars and nor are the moving targets, billed as 'future', likely to be disprovable in a reasonable (if any) period of elapsed time. The entire theory, from a social context, seems to rely on that being the case 0 in which event it can hardly be considered scientific.