^^^
They have DNA from over a dozen Piasts, and they know who married which princess, so they could do this, I guess.
Etymology of Piast:
"Two theories explain the etymology of the word Piast.
The first gives the root as piasta ("hub" in Polish), a reference to his profession. The second relates Piast to piastun ("custodian" or "keeper"). This could hint at Piast's initial position as a majordomo, or a "steward of the house", in the court of another ruler, and the subsequent takeover of power by Piast. This would parallel the development of the early medieval Frankish dynasties, when the Mayors of the Palace of the Merovingian kings gradually usurped political control."
^^^
The previous legendary dynasty were the Popielids, so if Piast was at first a majordomo, he could be a majordomo in the court of the Popielids:
https://en.wikipedia.org/wiki/Popielids
"An Ancient Tale: When the Sun Was a God" describes the legendary struggle for power between Popielids and Piasts:
https://en.wikipedia.org/wiki/An_Anc..._Sun_Was_a_God
Rebellious peasants attacking Popielid stronghold
* (scene from "An Ancient Tale" movie):
Another scene - Popielids hired a band of Viking mercenaries against Piasts' supporters:
*According to legends, the main Popielid stronghold was Kruszwica at the Lake Goplo:
https://en.wikipedia.org/wiki/Kruszwica
Palynological studies have confirmed that the area around Lake Goplo was continuously farmed since the Late Iron Age until the 10th century:
https://www.britannica.com/science/palynology
Similar continuity has been confirmed by palynological and archaeological studies near Kalisz:
https://en.wikipedia.org/wiki/Kalisz
https://en.wikipedia.org/wiki/Calisia
"Calisia (Greek: Καλισία, Latin: Calisia) was a "station" on so-called "Amber Road", mentioned by Ptolemy, formerly universally identified with Kalisz in Poland. Besides the similarity of the names, the identification was supported by the closeness between the latitude given by Ptolemy (52�50') and the actual latitude of Kalisz (51�45'27").
The validity of these arguments is currently in doubt (...)"