The first person to write about the Druids was the Roman general Julius Caesar, who conquered Gaul between 58 and 51 BC, and wrote a book about his exploits. In this book he comments extensively on the Druids (Julius Caesar, Commentaries on the Gallic War, Book 6, Chapter 14)
They wish to inculcate this as one of their leading tenets, that souls do not become extinct, but pass after death from one body to another, and they think that men by this tenet are in a great degree excited to valor, the fear of death being disregarded. They likewise discuss and impart to the youth many things respecting the stars and their motion, respecting the extent of the world and of our earth, respecting the nature of things, respecting the power and the majesty of the immortal gods.
Caesar also says (Book 6, Chapter 13): “This institution is supposed to have been devised in Britain, and to have been brought over from it into Gaul; and now those who desire to gain a more accurate knowledge of that system generally proceed thither for the purpose of studying it.” It makes good sense that the Druids originated in Britain where a huge number of monuments aligned on the midwinter of midsummer sunrise or sunset, the lunar standtill (which occurs every 18.6 years), or stars like Orion’s Belt, were built during the Neolithic and Early Bronze Age.
Caesar says the Druid priesthood was devised in Britain, and that seems to run counter to traditional views that the Celts originated in central Europe and spread west to Britain and Ireland. However, in the last 10 or 20 years the archaeologist Barry Cunliffe and the Celtic scholar John Koch have been arguing that the Celts actually originated on the Atlantic coasts of Portugal, Spain, Ireland and Britain in the Late Bronze Age (that is, around 1000 BC) and spread east.
We don’t know where the Late Bronze Age/Early Iron Age Druids were in Britain, but I suspect we should look at the huge middens (rubbish dumps) at All Cannings Cross, Potterne and East Chisenbury, all in Wiltshire, southern England, where a large amound of feasting and ritual activity took place, presumably organized by a priesthood of some sort.
At the Wiltshire middens, and at the later hillforts of southern England, the most common form of funeral ritual was excarnation – or defleshing – and secondary burial (in case you’re wondering this ritual was carried out to release the soul). I’ll leave you with the words of the pioneering archaeologist Maud Cunnington, who excavated the hillfort of Yarnbury Castle (Wiltshire) in 1932. She found numerous “detached fragment of humanity”, including skulls and thigh bones, in various ditches. Cunnington remarks that some have attributed this to a head-hunting cult, but this does not account for the limb bones: