Muslims ....... When did Islam start ?

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When did Islam start? Im serious. Bcoz the "great" Dr Zakir Naik on his website claims that Islam has been there since time immemorial ever since the first man set foot on earth (but gives no proof there). He also claims that Islam was not started by Muhammed. It is in the FAQ of islam, most common ques asked by hindus. Now if Muhammed made / discovered / invented ( whatever u want to call it ) the concept of allah then what was there before him? I was wondering that before allah whom did muslims worship and what were they called? Did they worship christ before allah? Were they jews? Pagans? Hindus? Whom did the muslims worship? What caused people of Islam to worship different people / gods every few hundred yrs? Why did they change gods so often? Is there any proof that Islam was there before Muhammed?