From some of local Pashtuns people, I have heard that they belong to Greek Origin. There explanation was, When Alexander conquered Persia (Achaemenid Empire). he left some of the soldiers here.
But I have seen a second angle to this, When Alexander came to current day Afghanistan he found lot of Greek people already living there. They were uprooted from Greek and forced to settle there by Persians and these people later made empire of Bactria. To Support
In my personal view the Pashto is very different languages spoken in this area. I can understand some to almost every words of Sindhi, Balochi, Persian, Pahari-Pothwari and Punjabi (I am Saraki) but I couldn't understand Pashto. When I searched I came to found that Pashto contains Greek words. That explains a lot.
And there is another theory that Pashtuns are Indo-European who migrated from central Asia like other Indo-Aryan and Kalashi people are from Greek descendants.
Which scenario is more plausible?