Very little is known about the Scottish people in prehistoric times as they did not write books and kept no historical records. What we do know is that the emergence of the Picts coincides with the arrival of the Romans in history. What the Picts were doing before the arrival of the Romans, or where they originally came from, no one knows. For centuries they raised families and standing stones, kept themselves quiet, and enjoyed the rivers, lochs and glens of Scotland.
That all changed with the arrival of the Romans. It is clear that the formation of a Pictish nation that united the Highlanders was due to the arrival of the Romans. That the emergence of brochs in such vast numbers is associated with this period of Roman influence in Scotland is also clearly no coincidence.
In 367 AD, the Picts with the help of the Irish invaded England and together they pushed the Romans back from their last defensive positions at Hadrian's wall. Not long after that, the Romans left Britain. As an aside, I believe it's possible that the Picts were European refugees, possibly Israelites who moved here to escape the Romans.
The Picts so resoundingly defeated the Romans that they survive to this very day. Their descendants still flourish in the Highlands of Scotland and around the world. Their blood is strong. If you're looking for your Scottish Braveheart, the man we should be building monuments to and making movies about, he is here, Calach of Clan Caledon, the Swordsman, the Father of modern Scotland.