How much do we know about the origins of Piast–the dynasty who founded Poland in 10th century?
I saw some TV documentaries making claims that they were of Viking origins. Is there any evidence supporting that? Or did Piasts emerge from local leaders of Slavic tribes?
I suppose that latter is much more likely, but:
The first comprehensive indigenous historical writing on the Piast Dynasty, a chronicle by Gallus Anonymous, only dates back to the beginning of the 12th century. It includes some interesting origin myths of the dynasty, but they would belong rather to legends than to historical facts.
The first appearance of the Poles (polani) as a group of people/ the political community in Poland, also only dates back to around 1000 (Mühle 2011: 14). Before that, in the reign of Mieszko, he was called either 'King Mieszko who ruled over the Slavs called Licicaviki (Widukind of Corvey, III-66; Bachrach & Bachrach trans. 2014: 140)', or just 'King of the North (Lunde & Stone trans. 2010: 164)'. The change of pattern of settlements in Greater Poland, the distribution of hillforts (that is to say, power center in the local landscape), disappearance of other small group of people in the area - All alludes to the possibility that a somewhat radical political structure occurred in Greater Poland during or just prior to Mieszko's reign.
Two recent researches by Boguchi & Jankoviak (Boguchi 2016; Jankoviak 2012), pay attention to new evidence(s) : The hoard of Islamic silvers found in Poland. It is well-known that Islamic silver coins originally struck in Central Asia flowed into Northern Europe mainly from late 9th to the first half of the 10th century, by way of the 'Eastward Road (Austrvegr) of the Scandinavians-Vikings, along waterways network in Russia. The concentration of Polish hoards is located in its south-western corner. These researchers associate these evidence of the inflow of silvers into Poland with the state formation process of the later Poles, represented by the ascendance of Mieszko I.
In short, both Boguchi & Jankoviak suggest that the Piasts in fact obtained their wealth and power in expense of the vanquished neighboring people, by selling war hostages as slaves in this slave trade network in north-western Eurasia beyond the religious border. In their understanding, found silver hoards testify only a tip of the massive iceberg of this large-scale slave trade, a kind of by-product of the Piasts' incessant conquest wars. The Rus' in contemporary Arabic texts, usually translated (a bit oversimplified) as 'the Vikings' were notorious slave traders, and Jankoviak supposes that such Polish war hostages are favorite commodities of the Viking traders that would be exchanged with Islamic silver coins, dirhams. They strengthen this hypothetical triad combination among the conquest- slave trade - state formation also with archaeological evidences: A few very large, but not so suitable for defense, hillfort-like archaeological site were built in the 10th century Greater Poland, and they interpret such a site as a kind of 'concentration camp' of the slaves prior to their journey to the slave market out of Poland...... From where/ to where the Piasts took their slaves and sold them to the Rus' (the Vikings), however, I'm at least personally not so sure.
[Added]: By the time this inflow of Islamic/ Central Asian silver stopped in ca. 960/ 965, Mieszko could fortunately at least achieve the initial phase of political consolidation of his group of people, as testified by foreign authors like Widukind and Ibrâhîm ibn Ya'qûb. If their hypothesis is to the point, he and his group of people (later the Poles) must have been one of those who got maximum benefit from the 'Viking' slave trade network in early 10th century north-western Eurasia, possibly more than the Vikings themselves.
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(Edited): fixes typo.