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Frank Harold

Frank responds to Mark:

Well, you can't please everyone, and I evidently failed to please you! As I understand your comment, you reject my suggestion (not assumption) that the host for the proteobacterium was an "urkaryote", an ill-defined organism related to the Archaea; and that notion may indeed be mistaken. But on what basis can one reject it? I quite agree that full-fledged phagocytosis had to await the acquisition of mitochondria, but does that really mean that no endosymbiosis was possible prior to that stage? After all, we do have a few cases of endosymbiosis among prokaryotes, and now there is evidence for endocytosis in planctomycetes. My suggestion relies on special pleading, but so do all the ideas being bandied about in the literature.

I think the insuperable problem with the entire field of early cell evolution is that no hypothesis can be falsified. There is no safety even in sequences, as the debates surrounding their interpretation shows quite clearly. It all happened long ago, under circumstances very different from those that prevail today, and the trail is dead cold. If you (or any other reader) think otherwise, I would welcome an articulated alternative thesis, and would be happy to engage with it. Conversation may not solve the problem, but may help us clarify why it is all so intractable.

With best wishes, Frank Harold

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