To survive winter, you have to plan ahead. Gathering enough food when it grows and being able to store it so that it doesn't rot or get eaten by animals requires an ability to think and plan. Those who didn't work hard in the summer and gather enough food to meet the planned amount for winter, starved and died in the winter. Those who lived taught their kids who inherited that knowledge and the ability to think further and expand on it. When food grows year round and you never have to worry about freezing to death, you can get by being lazy and dumb.