One hundred million years ago, there was a school of fish
swimming along with the ocean current.
These fish were very basic, being so long ago,
and they had no senses or central nervous system.
They only had a single nerve cluster on their heads
that connected to the muscles down their backs.
On this day, one hundred million years ago, the current that carried this school of fish
went right over a thermal vent that was releasing super heated water.
The water was so hot it killed everything that went through it.
As the fish got closer to the vent,
the water in the current heated up,
and the nerves on their heads fired off a signal.
Most of the fish felt the signal, but their muscles did nothing
and they drifted to their deaths.
Most of the rest felt the signal, successfully making them swim faster,
but they did not turn and went straight into their deaths.
Some were able to turn, but were not strong enough to fight their way out of the school
and were carried along to their deaths.
A few were strong enough to fight their way out of the school.
They turned, bouncing and fighting their way out of the school, and traveled in a different direction.
Some of the other fish that were a part of that fight ended up following along.