Here is the story of how simple video-game creatures evolved a memory.

These simple creatures were devised by a group of scientists to study life’s complexity. There are lots of ways to define complexity, but the one that they were interested in exploring has to do with how organisms behave.

Every creature from a microbe to a mountain lion can respond to its surroundings. E. coli has sensors on its surface to detect certain molecules, and it processes those signals to make very simple decisions about how it will move. It travels in a straight line by spinning long twisted tails counterclockwise. If it switches to clockwise, the tails unravel and the microbe tumbles.

A worm with a few hundred neurons can take