Personally, against all odds, I loved it.
If I wanted to be a jerk, I'd call it entirely predictable and not a very daring end, not to mention taking WAY too long..

But I'm not, and I'll congratulate the writers on an awesome finale, in which all the ties were bound together into the single most important event in actual human history.
I am a HUGE fan of paleohistory, and this is one of those instances where you actually wonder whether all of it might happen just this way.

Then again, I had the same with Tolkien's universe, and the links to regular mythology and geography. Very amusing, and entirely out of focus, but such fun to exercise one's brain with the possibilities.
History of the pre-tribal humans is so extremely vague that one can litterally fill in anything they want, from spacefaring ancestors of our race to simple walking monkies (forgive the inaccurate term).
Personally, I often wonder what would have happened in the time between the First-to-Last Great Ice Age and the Last Great Ice Age.. 200-odd k years of development where actual 'humans' (or something similar) existed.
Take into account that modern culture and civilization is a stunning 13k years young at best, and I find myself wondering what humanity could achieve in 200-odd k years.

150 thousand years from now? Really.. that much time, that much development.. one cannot stop (intended) to fathom the possibilities.

Drifted a little off-topic.. but I LOVED the finale just because of the appeal it has to me. Not because it was particularly good, or particularly moving, or otherwise awesome.. I think it was good, moving and pretty awesome, but not everyone will like it. I'm sure of that.
(drat, lots of I's there..)