Peter Berresford Ellis in his book "The Celts: a history" states "The Celtic idea of immortality was that death was but a changing of place and that life went on with all its forms and goods in another world."
This seems to me to be a logical and scientific way of looking at life and death, in that, given the complexity of the food chain (worms, fungi, rot, and other ghoulish things we try not to think about), we must all have in us atoms that were once in other human beings. (Also in animals, but I try not to think about that either!)
It seems highly likely, therefore, that we all carry within us, not just the programming (the genes) of our prehistoric ancestors, but the actual substance as well - at least in part.