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.