I definitely had the same thought about them being the origin of life of Earth but I figured since it was such an old cliche (it's the end of a number of EC Comics sci-fi stories from the 1950's and I have no doubt that EC stole that twist from even older stories) that I never thought they'd use it on BSG. I guess they outsmarted me since I didn't think they'd do it.The funny thing about the ending for me - and this is not meant as a brag or anything - but I had it figured from the first episode that we were seeing a prequel to humanity on modern day earth, and not our future.
Good point(s). However:
In the third season finale we see a group of characters brought together because they all heard an extremely popular rock song from the 1960's (or rather, a cover of this song) playing, originating somewhere in space. Based on this scene, I don't think it's a stretch to draw the conclusion that they are hearing radio waves from Earth in the 20th or 21st century, and that they are eventually going to reach Earth in the present or not-too-distant past (presumably the song took years to reach them in space). I.e., I don't think I'm the only one that thought this.
Obviously, I was mistaken & misinterpreted the "All Along the Watchtower" scene. I did like the series finale as it stands, i.e. the BG crew reaching Earth in the prehistoric era. It's the last thing I would have expected, and as such I consider it a very clever & imaginative ending.