Sapiens

This allowed us to cooperate in large numbers (more than the ~150 natural limit for a tribe). The Agricultural Revolution (12,000 years ago):

Keep an eye on your tribe's "Happiness" and "Health" bars. If they are too cold or hungry, they will stop working. Sapiens

This period gave us unprecedented power, culminating in the potential to replace natural selection with "Intelligent Design" (AI, genetic engineering). This allowed us to cooperate in large numbers

Assign multiple sapiens to new skills simultaneously to speed up progress. Sapiens