If you make a claim you need to have evidence to back it up, otherwise you shouldn't expect to be taken seriously. You can't just say 'well the opposite hasn't been proven yet so I'm going to believe it'. Just because it's more believable to say that a chimp doesn't live in my fridge, I can't claim that one doesn't in a debate scenario unless I am prepared to give evidence. If nobody explicitly claims the positive, then there is no way of shirking the responsibility of evidence onto someone else. If nobody claims that there is a chimp in my fridge, but I claim that there isn't, why should other people prove the claim they didn't make? It's my responsibility to prove the claim I made.