For thousands of years people have spread out all over the globe then we have a worldwide catastrophic event (see tree rings), people from everywhere are exiled all over the earth and all connections are lost and forgotten.
Then a few hundred or a thousand years later when humanity recovers from the disaster, a new wave of explorers begins again and rediscovers these exiled people. (Who by then have no idea where they came from)
The one thing that all explorers 'find' during their explorations, every where they go, every time, are, more people. (Many of them with similar customs, legends and myths.)