Thag and his tribesmen had taken down a mammoth that morning. The feast was still underway, but Thag was bored with the men grunt-bragging about their exploit and the women grunt-complaining about the tribulations of raising cave-kids. He retreated to his cave and sat outside it, absently rubbing two sticks together. His hand brushed against one of the sticks where he had rubbed it—hot. After rubbing some more he stuck a dried leaf on the hot spot, just to see what would happen. Smoke, a flame, fire! He dropped the burning leaf to the ground. How could this be? Fire came from the sky gods. The flame died out. He gathered leaves, put them in a pile, rubbed the sticks, ignited a leaf, and dropped it on the pile. Big fire! Warm—good on cold nights.
When Thag showed the tribe how he had tamed fire, they may have grunt-hailed him as a "genius," although he had only stumbled on to something because he was bored. While cave living may appeal to certain sensibilities—Nature! No technology! Extended families living together! A sense of community! etc.!—it had to have been excruciatingly boring for any mentally active cave-person.