I know this will be an unpopular comment, but I've had a long-held theory that past lives / reincarnation is a real phenomenon with a biological mechanism that is as-yet unknown.

The theory being that trauma at time of death is somehow encrypted for the next generation and passed on shortly after death, particularly if a corpse enters the water or food supply.

The basis for this is a little embarrassing.. I had memories of being a bridge engineer, frequent dreams of falling off a very specific truss bridge in my head in childhood. To this day I'm terrified of certain types of truss bridges, which are coincidentally quite common upriver of where my parents lived when I was born, and where our water came from.

This theory would also explain why river burial is commonly associated with reincarnation. Committing the tissue of a deceased person into a community water supply would increase transfer of memory.