I think it goes back into prehistory- and was already old and perhaps not fully understood when it began cropping up in the written records.

The prehistoric example that comes to my mind is Lindow Man- a sacrificial victim who turned up well-preserved in a bog in Cheshire. Examination of the body suggested he had been a member of the elite- smooth hands, manicured nails- and that he'd been rather well treated up to the moment of his death. These things don't prove consent but rather suggest it.