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The Jataka Tales are moral tales, probably written down in this form in the 4th century, ie probably a 50 to 150 years after the life of Gautama Buddha.
Yes, many of these stories predate Buddha. Some of the stories are variants of stories believed to have been told by Aesop in the 7th century BC, for example.
I don't think Gautama Buddha added anything to these stories. The claim that they are about his earlier lives is probably something that arrives only well after his death.