This leaves ___ for theories - and the schientists still argue ____ them
How was that possible ___ an animal without feathers become a bird? Birds often feed ___ the rubbish left by people.What should i put here?

Question

Answer

This leaves US/ME/THEM/WHOMEVER for theories. It means « we just have theories ramaining. »

The scientists still argue ABOUT them. (About the theories.)

How was that possible THAT an animal without feathers becomes a bird?
 “That” introduces the noun clause “an animal without feathers becomes a bird?”.

Birds often feed ON the rubbish left by people. 
It means that birds eat the rubbish that people left.

I hope that helps.