It was thought for example the celts brought the language to Ireland but there's references to places and descriptions of places that go back in our language that predates well before the "arrival" of the celts.
Common understanding up to now was 10,000 yrs ago we were all hunter gatherers living in nomadic tribes etc....but recent discoveraries in Turkey show that up to 12,000 yrs ago there were very established, settled cultures buildings sophisticated monuments....
So DNA science and modern archeology is debunking alot of what we thought was true.