You are bound to come across some version of the phrase "bellydance as we know it today", which generally means raqs sharki as it has been performed leading up the golden era and recorded in early Egyptian cinema. This style was popular with the concert halls and it formed from the dances of the awalim and Ottoman court dancers.
As Dr. Deagon points out, we tend to want to simplify the past, conceptualizing it as straight roads, with no on or off ramps. Instead (and in light of the ancient tradition of trade, the legacies of empires expanding and falling, to say nothing of colonialism) I recommend thinking of the timeline of bellydance as a series of rivers and streams that merge and diverge, mixing together and forging their own paths. Liquids are made of lots of molecules that stick together, and are always moving. Think of it, you can never look at the same river twice.