The advancing ghost cult made ancestor worship inevitable since it became the connecting link between common ghosts and the higher spirits, the evolving gods. The early gods were simply glorified departed humans.
Ancestor worship was originally more of a fear than a worship, but such beliefs did definitely contribute to the further spread of ghost fear and worship. Devotees of the early ancestor-ghost cults even feared to yawn lest a malignant ghost enter their bodies at such a time.
The custom of adopting children was to make sure that someone would provide offerings after death for the peace and progress of the soul. The savage lived in fear of the ghosts of his fellows and spent his spare time planning for the safe conduct of his own ghost after death.
Most tribes instituted an all-souls’ feast at least once a year. The Romans had twelve ghost feasts and accompanying ceremonies each year. Half the days of the year were dedicated to some sort of ceremony associated with these ancient cults. One Roman emperor tried to reform these practices by reducing the number of feast days to 135 a year.
The ghost cult was in continuous evolution. As ghosts were envisioned as passing from the incomplete to the higher phase of existence, so did the cult eventually progress to the worship of spirits, and even gods. But regardless of varying beliefs in more advanced spirits, all tribes and races once believed in ghosts.