I don't think there weren't ancient / primitive civilizations or tribes, it's just that they were fully erased from history as they didn't left any written history or well constructed monuments and/or big cities that we could find after 100-150.000 years (no need of "great events" to erase cities, just build the houses with mud bricks, wait 30-40k years).

At looking for clues about why those initial waves went exting, maybe we could take a look at how more recent tribes or even old civilizations like Incas, Mayas, etc. made it through a couple of centuries, and eventually got "extint".

Some of those groups were surrounded by other tribes, and tried to expand their territories, with different outcomes.

Some didn't tried to expand, they just got well adjusted to their original territory and didn't need more resources.

But everyone in those groups eventually disappeared from history.

Even when the american continent was discovered, the remaining "true" mayas didn't remembered or indicated that those big monuments, cities ever existed already taken / hidden by the jungle. Many of those also, were just found literally hundreds of years after having seen the last true mayas.