To go back to the question why East? Did they sit down and ask those who had travelled furthest what the going was like? It is not necessarily the easiest route. We also seem to rather fix the Earth as it is today. Were they driven by environment, a desire to travel, curiosity or war? Why do we have a need for trekking humans anyway? If humans just bred and spread, that would be a lot more likely but very slightly boring. This is why I struggle with the romantic, brave humans narrative, it just seems too grasping, too self-important and if you look at the way nature treats us, how it swots us aside when it feels like it, you realise that in the grand scheme of things, we really aren't that important.
Then there are the dinosaurs. Successful on a pretty heroic scale for millions of years, they seem to have, relatively quickly, died out. As most people now know I think, it was a meteorite what done it. Massive explosion, loads of dust blanking out the sun, cold, lack of plant food etc etc. But how did some survive, like mammals and some fish and some dinosaur like creatures like crocodiles. We don't know, it must have been something special. So, to me the theory isn't proven, but there it is. It's the best we have they say. Well, if it doesn't answer the questions, its probably not right, don't ya think? Why do we have this arrogant need to impose partial ideas as fact?