As for the commonality in words: Pashto, Greek, Serbian and pretty much all languages that are spoken in Europe are related. Better yet, most languages spoken in a belt from the Caucasus to the north of India are related too. All these languages are descendants from Proto-Indo-European (the name stems from the two main directions of spread: Europe and India). This language was spoken either north of the Black Sea or in a region located in modern day Turkey. From there on out, groups of people spread out. This spread happened around 4000-8000 years ago, with most experts preferring the later dates. During the spread, dialects formed which eventually 'became' languages. Because of their shared ancestry, these languages share a lot of linguistic elements, like words.

As for the matter whether the Pashto are indigenous: I'm not familiar enough with the concept and definition of 'indigenous' to give that an answer.