skin or thin plastic, metal and wood. A bass drum's head can be made of materials like: Plastic Polymer,etc. A bass drum shell can be made out of: Wood Metals (steel,Iron,copper,aluminum) Other materials
Yes, they are removable. And yes, they also effect the sound of the bass drum. That is the reason that Truth and some other drum manufacturers offer custom drum kits without the tom mount on the bass drum. You can still get a perfectly beautiful sound from a bass drum with a tom mount though.
No one really knows who when or where the first form of the bass drum was invented. it could be possible that a hollow long with dried up animal skin could have had some object accidentally dropped on it and made a 'boom' sound of the bass drum.