Of course, the fact that it is a fusion is strong evidence for the evolution of humans (and yet one more example of how evolutionary explanations could be falsified).
If the fusion occurred in a single individual then they must have still been fertile with the rest of the population. Otherwise he or she would have left no descendants. This in itself is reason for doubting that the fusion caused the split - or any of the later splits in our lineage.
As to the timing of the event, I will simply note firstly that there are non-homo species between us and our common ancestor with the chimps (likely among the australopithecines) and secondly that we have relatively little data to go on. So far as I know we don't even have a chromosome count for Homo erectus. Which leaves us no basis for saying that all Homo species had 46 chromosomes.
If we simply use chromosome count as a basis the fusion event could have occurred before the split between our ancestors and those of the chimpanzees or after our ancestors split from the Homo erectus population. There's no way to tell without additional data.
quote: I'm sorry, but I don't understand why you added this comment. Would you please expand on what you mean? What would falsify this explanation?
The evolution of humans from (other) apes would be seriously called into question if we had no plausible explanation of the difference in chromosomal arrangement. The fact that not only do we have an explanation, but that the explanation is strongly supported by the evidence shows evolution passing a significant test with flying colours.
quote: But if we found two individuals that we considered to be both say Homo erectus by morphological data, and we found that one had 46 chromosomes and one had 48 chromosomes, wouldn't we then reclassify them as separate species
Do we classify the '44 chromosome man' as a different species? And on what criterion would we do so? Not on the morphological criterion or the criterion of reproductive isolation, which are the two most likely to be applied.
quote: I guess that is what I would consider "cause". Since we could not expect to point to one individual in history and say "this was the first of this species", we say things like geographical isolation "caused" the split; genetic drift "caused" the split; chromosome translocation "caused" the split. Is this improper thinking?
The split would have to occur at the point where the populations diverge and interbreeding ends (there are nuances, but that's close enough). Did the chromosomal arrangement cause that? I don't think that there is a strong case that it did - and even if it did, it wouldn't happen immediately.
quote: Before or after ... well that narrows it down
which was my point :-)
quote: So, what data do we have? Which ancestral humans do we have genetic data for (at least karyotype for this topic)?
Neanderthal and Denisovan are the only two, I think.
quote: That's kinda what I thought you meant, which is why I asked for clarification. Because that's not really true is it? Isn't that what IDers and creationists do with irreducible complexity? Since there is no plausible explanation as to how this could have evolved then it must be intelligently designed. And the answer is usually "just because we don't have an explanation right now doesn't mean there isn't one." So lack of a plausible explanation may cause us to question the hypothesis, but is not a means of falsification.
You need to be careful there. That is the position some ID supporters retreat to, since the actual IC argument has failed but even there they demand a huge amount of detail, which I do not.
The key issue is the level of knowledge we have. Obviously we do not have the knowledge to reconstruct the complete history of a complex adaption including all the relevant mutations, in order and the selective pressures that caused them to become fixed in the population. By my understanding we do have a fairly good idea of the sorts of mutations that are possible, and if we could not explain the chromosomal difference in those terms we would have a problem. If we did NOT have that knowledge, it would be a different matter.
quote: Why would we not if we had limited other clues to go on. Differing chromosome numbers usually indicates infertility. In the case of the 44 chromosome man, we have many other clues that he is still of the human race. With the limited amount of ancestral human remains we have, any significant differences would give reason to separate them into different species. And we would also not expect to find the first or only person who had such a rearrangement, but a representative of the population.
Of course you are assuming infertility, but it would seem a huge leap to conclude a new species without morphological differences. While cryptic species do exist I think that we would want a little more. And don't forget that the article on the 44 chromosome man says he would have fewer infertility problems than his parents, even if his partner had 46 chromosomes.
quote: No, of course it wouldn't happen immediately. Is there a more plausible explanation for the break between humans and chimps? Chromosome 2 is presented as evidence of human - chimp evolution (and I agree it's quite a strong piece of evidence) but if it is not a plausible explanation for the evolution of chimp - human, doesn't that still leave us without a plausible explanation?
Well it isn't a plausible explanation for the whole evolution of humans and chimps, at most it is a component of how the populations that evolved into humans and chimps diverged. The very fact that the new chromosomal arrangement was able to spread into the population and persist argues that the infertility problems were not too severe, at least at first.
However, there's a problem here. The chromosomal difference can only significantly contribute to one population split, one speciation event. But there must have been several between us and our common ancestor with the chimpanzees. How do you narrow it down to the split between our ancestors and those of chimpanzees rather than one of the later speciation events ?
I think that with an effective population size of only 5 humanity would probably go extinct. And that's without assuming a population of only 2 a couple of thousand years earlier which would make things even worse.
Aside from the genetic issues, though, I don't think that there is necessarily a barrier to the actual population size increasing hugely since then (although I think we'd still be behind current population).
That said, good as the genetic evidence is, if you put the flood 4350 years ago I don't think that the archaeological evidence shows any culture being wiped out and replaced at that time. Not even the Sumerians. If you want to avoid archaeology, you'd have to go further back in time.
quote: Surely you agreed that the sumerian culture disappeared at a certain point in time?
Disappeared as in vanished ? Certainly not. Conquered and absorbed by other peoples, yes. Slowly lost to change and time, yes. But certainly not - just gone, in a single event.
quote: There is indeed geological evidence for a substantial flood in the mesopotamian area at least in the vicinity of the time that the sumerian culture existed.
I believe that there's evidence of some serious flooding during the Sumerian period. In fact there seem to be multiple flood events - at different sites (i.e. the flooding is local, even within Sumeria, there's no single flood affecting the entire region).
quote: What date does archaelogy give for the disappearance of the sumerian culture?
Sumeria was conquered by the Akkadians around the time of your Flood (dating is uncertain), with a revival following the downfall of the Akkadian Empire. Agricultural fertility was declining due to increasing salt in the soil (itself a consequence of poor irrigation and the dry climate), weakening the region. Sumeria was absorbed into the Babylonian Empire circa 1700BC and never reestablished itself after that.