[adastra22]

You can’t. But if population A and population B share a ancestor X years ago, and they also speak languages that appear to have drifted apart by X many years, the inference that their ancestor spoke a common proto-language is the simplest explanation.

[eddiewithzato]

Well you can and in fact they have narrowed down the language to a haplogroup even. R1b in the case of greek for example