It comes from studying the origins of languages. It explains how Persians are related to Europeans. They all came from one people in one place who spoke one language. It's not rocket science.
DNA, pure science says that it is literally impossible for all races to have come from ONE people. That is evolution whether it calculated on a very long time term or just a day's worth of time. Languages might have 'dna' to 'study', but it has NOTHING to do with races of people.