I learned this from watching a MOOC presented by the U of Chicago, about neurobiology – that has made me a wiser person.
We have three autonomic nervous systems, and these are full of nerve cells, with their own sensing organs – that can also operate on various muscle groups. These operate independently, with only minimal input from the brain.
The two main autonomic systems are the sympathetic and the parasympathetic. When we detect a threat – our sympathetic system takes over (in about a millisecond) – and says “Run!”, and we run, and keep on running, because our sweat glands kept us from overheating. Predators had to stop chasing us. This system saved our lives many times in our distant past – when we were seen as a tasty meal. It also notes something else – what kind of threat we are reacting to. A big predator, or a smaller one – such as a snake.
After we had run for a while (and this interval is determined by trial and error, from our past) the parasympathetic system takes over – and we stop to think things over. Thinking takes much longer – perhaps several seconds. But it is also critical for our survival. We cannot run forever – we also have to eat and rest.
We have always existed as members of a group – and the entire group was always on the alert, watching and listening for danger. If anyone detected danger – everyone started running, away from it. Just like any prey species.
But we had the ability to stop and think – and possibly, turn around and go on the attack. Predators learned to be especially careful of going after us. A hundred people armed with clubs can take on a much smaller group of predators. And it was not long before we had none.
And we took over the world – everywhere, except in the Antarctic.