Norman Davies says this in his book, God's Playground, Volume I regarding Miezsko I:
He started the recorded history of the Poles which has continued without a break from that day to this.
Even I have noticed that when I read about Miezsko's father, Siemomysl, on Wikipedia, exact dates of birth and death are not known. However, after Miezsko, most of the dates of events are exactly recorded. Same is the fate of youtube videos regarding Polish history
Why is it that the recorded history of Poland began after Miezsko I's reign?
How does his baptism or unification of land led to suddenly pens jotting down events?
What was amiss before his reign?