I've just been holidaying in Angus, Scotland- a heartland of Pictish culture (c. 3rd-10th C BCE). Have had great fun tracking down as many sculptured stones as permitted by museum's winter opening hours and speculating on the meaning of the symbols and functions of these pre-literate monuments.
I would love to understand better the connection (or lack of) between these Iron Age Picts and preceding Neolithic/Bronze Age cultures of Scotland (~5000-500 BCE).
Were the Picts descendants of the Neolithic/Bronze Age peoples of Scotland who built so many megalithic monuments, or germanic migrants after a Bronze Age collapse?
Were the Picts Celts? In favour would be if Pictish was, as I've read, a P-Celtic language. How is this known, given that Picts (famously) left no writing?
What is the reason for the strong congruity in art motifs between Pictish, Anglo-Saxon and Viking artefacts, e.g. knotwork, stylised animal representations, ‘beasts’. Do they have recent common antecedents?
Are the Picts the broch-makers of northern Scotland, e.g. Carloway, Gurness? Are they defenses against raiding by fellow Picts, or Nordic seafarers?
So many questions, so much to learn! Any light on these issues much appreciated.
Edit- Originally wrote Picts as 3-10th C BCE when I meant CE. Pretty massive error, really.
Briefly, there's a bit of a problem calling them 'Picts' because that's a much later appellation and possibly one of convenience (in the same way 'Celt' covers a multitude of people who were not Roman or Germanic, but in essence is useless beyond that). Pretty much any book covering the Picts means the ones from Julius Caesar's invasion onwards, not the late Iron Age ones. Barry Cunliffe's Iron Age Communities in Britain doesn't even have the term in it. That means that any answers to your questions above will probably fall in the AD rather than the BCE section, so at some points I have to ignore your dates. I suspect you might be actually talking about Iron Age Scotland rather than the Picts but I'll let you clarify.
There's such a large gap between the Neolithic/BA that I'd be hesitant to draw any conclusions - we're talking about 4000 years from the Neolithic to the first mention of the Picts! Even an early IA date is suggesting that there's 1000 years of continuity.
See the intro bit above about Celts. If we stick to AD Picts, then we have to deal with several assumptions - that there was a people called 'Picts' and that they had a language, and that language survived in some form that we can discern. The academic discussion then splits into two schools. The first is that Pictish is a P-Celtic language (related to Gaulish and Brittonic), the second is that it's pre-Celtic (and non-Indo-European, and hence a Bronze Age descendant). I'd say the first school has more adherents, and it's based upon the fact that we think we can discern some Pictish origins in place-names and possibly personal names. As the only written accounts are well into the first millennium AD section, that's what you're limited to with any sense of certainty, in which case it's P-Celtic and not non-Indo European (as argued eloquently by Kathryn Forsyth). How far that goes back is speculation.
As all Pictish remnants I've seen are post-AD, they're all linked together with the Vikings and Anglo-Saxons because there would have been links between them at some point. Ritchie says that it's the way in which they were used which was unique, more than the actual symbols, which in the AD centuries has some close Christian motifs. But I'm assuming you want Picts and not LBA or IA inhabitants.