Google Docs keeps all your drafts in one file. You can easily view or restore earlier versions using Version History. This is particularly helpful when you are collaborating on a doc with others and want to see the changes that they have made.
Click File menu.
Version History
See Version History
At the right hand side of your document you will see all of the most recent versions of that document.
Clicking on each one allows you to see the doc as it was in that version. If you wish to restore that version of the doc click the three dots and then click Restore this version
Any edits made by each person are shown in the same colour as the dot beside their name
Note: you need to be the owner or have editing rights to the doc to use Version History.