Similarities in languages or religion (in the broadest sense) are not coincidental. We do indeed all origin from the same "tribe". Also, some shared myth are likely based on true occurences, so traumatic for the people then, that it stuck in the memory of the people and was passed on in tales for thousands of years. For example the great flood, which is part of the mythology all over the world. There is strong evidence that such floods happened at the end of the last ice age.
With other common myth its less clear what sparked them. One interesting example is that of the "dragons". It also appears in several unrelated cultures, in Europe, Asia and Australia. What actually caused it is not clear. Some dinosaurs would fit the image of a dragon quiet well, but for all we know, no human - and not even a distant relative of a human - lived so long ago that they would have seen living dinosaurs.
But they possibly found dinosaur fossils and made conclusions on what kind of animal this must have been.
Its all hard to say, since we have only half way decent records of the human history since the ancient Egypt, and even that is limited to the mediteranean region. But the real interesting period, the early hours of mankind, will remain mostly in the dark, I guess. Unless we discover time travel... :)