That’s interesting. How did they lose the use of fire?
I have often thought about the whole matter of humanity: what is it that makes us discernible as human beings? We are made in the Image of God, but in what particular aspect? Is it speech (the Word)? Is it love (that is, charity, disinterested love)? Is it memory? What are the signs?
Population bottleneck. Technology requires certain population levels to maintain. If the human race were choked down to say, half a billion, I bet we'd lose the ability to sustain modern semiconductor manufacturing. 50 million and kiss any new aluminum goodbye, less than a million, and blacksmiths start having trouble teaching mining and smelting iron ore from generation to generation. Less than a few thousand? the finer points of starting fires get lost. A few less and keeping a fire banked gets a little iffy...
Dunno. For starters, I'd vote for empathy.
Sadly, this would rule out a lot of the creatures we think of as human on this planet..
I would vote for sapience.