"What do you believe is true even though you cannot prove it?"

That's the question edge.org asked of about 150 of the world's most interesting thinkers. I'll probably go through them slowly. Here's an extract from Jarad Diamond's answer.

The big animals of [Africa, Europe, and Asia] survive, because they had millions of years to learn fear of human hunters with very slowly evolving skills; most big animals of [North and South America and Australia] didn't survive, because they had the misfortune that their first encounter with humans was a sudden one, with fully modern skilled hunters.