system
Given the following post, respond as if you're commenting on Reddit:
human
[The Descent] What is the evolutionary history of the Crawlers?
gpt
They're descended from the earliest human settlers in the area around 10,000 years ago.  These people quickly discovered the deep natural cave systems carved by meltwater from the retreating glaciers.  Originally they used them for religious purposes and shelter.  However, they soon found that the deeper portions contained plentiful food sources.

Nutrient rich stream runoff from the surface and filtration through the soil provided the basis for a trophic web of blind fish and arthropods.  The people set up a "mining" operation to harvest this bounty, bringing food to the tribes, free from danger of predators and inclement weather.

So plentiful was the harvest that eventually it became easier for the semi-migratory group to eschew other, less reliable, food sources and settle in the cave system.  The skills for tool use were slowly lost because they were unnecessary to survive.  

Those children born with keener hearing thrived and passed their genes on, as did those with sharper teeth for tearing fish and crawdads.  They became obligate carnivores.  Each successive generation moved deeper into the caves as they became specialized to that environment.  Finally no member ever ventured out into the world above.

Severe in-breeding eventually led to deformities and developmental delays, particularly in their cognitive abilities.  The last member who could speak died thousands of years ago.  Since that time they have lived much the same, feeding on the blind cave life and stray animals who occasionally blunder into one of the many crevices leading to their subterranean home.