I think the attitude toward the modifications is a relevant factor as well - wanting to be "more than human" in some respect, even if only a trivial respect such as "more awesome-looking than a regular human" or "more able to open beer bottles than a regular human" - but given that, yeah I'd be totally on board with considering some pre-Columbian Maya or other stone-age person "the first transhumanist".
I see a smooth transition into tool-using, then. Picking up a stick certainly gives you more capabilities compared to a stick-less hominid, and probably makes you much more awesome as well.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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