Islam has yet to undergo rigorous scholastic investigation as to its origins.
There are rumors that such studies are under way, but none have yet been published.
Spencer performs a great service by collecting existing scholarship in one book, but the definitive study has yet to be published.
I’ve read for example that the arab armies that invaded egypt were actually mostly iranian.
Go to Amazon and look at books by Ibn Waraq.
He if it is a he (the name is a pseudonym!) is doing that.