This story is not a certainty, just a theory that anthropologists and archaeologists have made. I try the idea that man arose in southern Africa and China (or from Africa migrated to China) the fact is that they did not exist everywhere, and only occupied all the land because they migrated very often.
Once in the valleys of the great rivers, where they had water, fruits and game in large quantities, they ended up settling more and developing agriculture, the great science of being able to produce what you are going to eat, and not having to go out into the world looking for it. With this the population increased so much that in 4000 BC they began to enter Europe. In general, along the Danube River, along the banks of the river, they founded small villages and entered Europe.
It may be a theory, it may not be very interesting, but this migration is where the Celts came from.
Trivia