I always got the impression they didn't evolve in a natural way. At one point in the future timeline, they found what appeared to be an ARC facility, plus there were hints that one of the bad guys (the one who sicced the bat creatures on Lester, allowing him to show off a tiny bit of badassery) somehow knew a lot more than was ever explained.
I figure at some point, some really intelligent person decides to do something brilliant, like create the perfect weapon out of the DNA of creatures from the past. The weapon got a little upset over dealing with the hairless apes, and either we killed the planet trying to take them out or they had munched all the humans for tasty noms. Or possibly we managed to bring a disease from the past through an anomaly that did the job and all the critters took over in the ruins. IIRC, there might have been a possibility of survivors elsewhere, if they were in a bunker somewhere, because we only saw a limited view of the future time.
I really need to go back and rewatch things in one go. They kept swapping timelines and motivations, then there was a production gap, so it makes it hard to keep it all straight in my mind what happened when and, well, when. I kind of gave up on getting a coherent storyline, but the f/x are decent and there are pretty boys (and a pretty girl too!) to look at and everyone seems to be having a grand time, so I'm finding myself not really caring.
Yep, but my experience was one long marathon all the way up to the most current episode (Season 4 episode 7). Everything is coherent except how the creatures from the future came to exist. We know that they destroyed everything in the future, but how did they get there and how were they created/evolved? Same with the wasp things.