Frequently we read through historical and Bilbe accounts about peoples who are no longer here in modern times. What decimated them? Some were absorbed by other cultures, relocated due to changing geography like natural catastrophes, victims of war, or other circumstances
One unique disappearance of people were the Rapa Nui of the remote pacific Island known as Easter Island. We know from general knowledge that the inhabitants used up most of natural resources making those huge statues (probably religiously influenced) and pretty much died out. How did that happen? Did people just watch each other starve to death?
Regardless of the various theories of where the Rapa Nui came from, they undoubtedly arrived by boat. The boat or boats evidently had rats as stowaways. It is suggested that at some point, there were maybe 15,000 RapaNui. They deforested the larger trees, perhaps to roll the pieces of their statues, during assembly. The smaller trees that thrived in their shade were threatened. Fishing for meals may have dwindled the amount of edible ocean life. Rats competed for the tree nuts. Some may have (successfully?) attempted exiting by boat.
AS times became more crucial, there is evidence that the population members attacked each other, maybe separating into tribes in hopes of finding safety in groups. At the archaeological time that statues stopped being built, parts of the population became cave dwellers. Although many had genealogical ties, it seems they turned on each other for survival.
In 1877 there were about 100 inhabitants on the Island. You may think of Bible accounts where people turned against one another. This seems to suggest that this does indeed happen.
Just my ruminations about. "Where have all the people gone."