There is evidence of human settlement in parts of present-day Scotland that dates back to 6,000 B.C. The inhabitants were hunters and fishermen. About two thousand years later, a second group arrived -- the Neolithic people. Some of their stone houses remain in Orkney; the well-preserved stone-built village, Skara Brae, attests to the wealth and stability of its builders. On the mainland, chambered tombs also show the sophisticated engineering of a settled, cooperative community. Then came the Beaker folk, named after the shape of their pottery. It is to these people that we owe the mysterious groups of huge stone circles and standing stones dotted hither and yon across the landscape. The Bronze Age, or rather, the early and late Bronze Ages, from about 2,000 to 600 B.C., introduced swords, knives, chisels, buckles, cauldrons and buckets, all evidence of a high level of civilization and creature comfort that was enhanced by the metal craft learned in the so-called subsequent Iron Age. Such objects were used by the indigenous Picts, who lived in the region north of the Firth of Forth, and the Celts, who had come to live in regions of Britain and Ireland further south. Stone Circles are traditionally ascribed to the Beaker People, but little or no excavation evidence exists to support this, and they are more likely to have been from the Bronze Age, c.2,000 - 500 B.C. The only datable material to have come from stone circles in the Tayside area is pottery generally assigned to the late Bronze Age. This may mean that the circles were erected later than previously supposed, but they appear to have been venerated over quite a long period of time, certainly more than one thousand years.
Very little is known about the Picts, other than what has been written by invaders or neighbours, such as the Romans, and Bede of Jarrow. They left no history of their own other than archaeological remains and the enigmatic carved symbols on raised stones. The Romans patrolling Hadrian's wall spoke of the "Picti" or painted people. As Roman influence faded, the Picts, often in alliance with the Scotti or Scots, their near neighbours in the West, raided across Hadrian's Wall. Saint Columba visited the Picts in 565 AD, and made a few converts. He spent some time with Bridei, son of Mailchon, who ruled an area spreading from Skye to Orkney from his centre near Inverness. In the 600's, the Angles started spreading north, and came into conflict with the Scots, Britons and eventually the Picts. Anglian expansion was effectively ended at the battle of Nechtansmere in 685. Christianity began to spread, however, and this is when the great cross slabs started to appear. By the early 800's, the Picts and Scots were intermingling, and eventually under Kenneth MacAlpin were united. The Battle of Athelstaneford, c. 843, marked the start of the Scottish nation in actuality, and the adoption of the Saltire as our national flag, the oldest in the world.
Christianity began to impinge on the Picts as early as the early 400's, when Ninian is said to have begun conversions among the southern Picts. Simple crosses in low relief have been found as far north as Fife. Saint Columba visited the northern Picts round 565 AD, travelling up Loch Ness by boat, and having the first recorded encounter with the Loch Ness monster... Adomnan claimed that Columba made some conversions, but by the late seventh century greater inroads had been made. In 710 King Nechtan sent for advice about the date of Easter to the monks of Wearmouth and Jarrow. Abbot Ceolfrith responded, and the Anglian type of Christianity began to take over from the Columban. The expansion of Class II type stones dates heavily from this period, where the heavy Celtic crosses begin to dominate over Pictish symbology. No certain Pictish churches or monasteries are known in Pictland before 800 AD, by which time Pictland was becoming Scotland, served by the Scottish Church.
An interesting aside on all of this - take a long, hard look at the outline of the stones of Calanais, on Lewis. A long avenue leads to a circle, with short arms leading above and to each side. Is this not the outline of the Celtic Cross - perhaps the first in existence, and certainly prior to Christianity's invention? Just one of the bizarre little questions which start to ask themselves when you start to study the stones...