Here’s my conjecture. .Brain size would be limited so processing capability would be limited too. Calorie intake needs would be reduced so a vegetarian diet would be easy to support. They would live in caves and naturally occurring burrows. For fun lets assume they have the brain power of today’s human. They would need to protect themselves from predators like birds, coyote, snakes, etc otherwise they would be wiped out. They could either reproduce prolifically like mice and other rodents, or develop some form of weaponry fight off or kill the predators. The latter is the most likely scenario. They might have spears with improved points. Maybe they would discover toxic plants and make their own poison darts. With active, capable brains they’d have all day to figure out how to protect and feed themselves. And opposable thumbs would give them the dexterity to put some of their ideas to use.