Watching it now... Liking it a lot.
They explained away the "why go back in time to when you know the earth is going to face an ice age and humanity will get wiped out again?"
Two things about this comment--even if it wasn't a quantom reality;
1) There ice ages that we know about happened much, much earlier... like 400 million years earlier (Snow-ball Earth)... and much much later...like 30-50 million years later in cycles of 100,000 years or so. For reference, modern humans have only been around for about 60,000 to 100,000 years DURING the last major ice age. In fact some scientist believe that it was the ice age that caused the climate to change enough to cause hominids to evolve in the first place during an ice age 5 million years ago.
2) Two choices: Face certain extinction staying where you are, or try to carve out a place in a jungle with predatory animals. Simple choice really.