Physiologically, Homo Sapiens have existed for between 300,000 and 500,000 years but archaeologists believe that we had the same type of basic, almost instinct-level, culture for most of that time: simple basic tools that never showed any innovation, simple fur clothing, no evidence of art, jewelry, or other indication of higher cultural structure.
Then, somewhere between 50,000 and 40,000 years ago, that all changed with what some archaeologists call The Great Leap Forward.
Suddenly we had cave paintings, more complex and sometimes multi-stage tools, jewelry, evidence of complex housing, boats, etc and that is where modern human cultural structures and behavior started to form. That is the period that archaeological discoveries have pinpointed that seems to be the start of humanity's drive to improve.
But what started it? Why did modern humans exist physiologically for 250,000-450,000 years before all of a sudden getting this cultural drive for improvement seemingly out of nowhere?
To me this seems to suggest a solid basis for a story but not sure under which primary tag it would go... not time travel, not do-over, not alternate history...
The premise: Future scientists/engineers are working on an experiment in quantum physics/ quantum tunneling. They eventually succeed in creating a quantum tunneling effect and don't realize that their experiment had additional effects than what they observed... the effect of "snapshotting" their memories and injecting them into a migrating family-group of Cro-Magnons 40,000+ years in the past. These "snapshots" have JUST their memories, not their personalities, and they do NOT override the original memories of the Cro-Magnons... they just add to them.
So now this group has all these memories of seemingly fantastic things from that group of scientists and engineers...but also the background knowledge that the individuals learned. So they know it is not magic but simply the culmination of millennia of initially slow but eventually rapid technological advances. After a period of shock and mental adjustments, they start to use some of that knowledge to make small improvements in their lives and, as they travel, in the lives of those they encounter. They do not have the technological base available to make drastic advances, but the small changes they are able to make rapidly catch on and start the cultural shift towards modern humanity.
I invite anyone interested to use this as a story plot.