About 900 years ago there was a sea battle between two opposing Japanese armies.
The emperor was only seven years old, and legend has it that when his armies were defeated, the surviving members threw themselves into the water, and were turned into crabs. The defeat was so dramatic that a festival commemorating this event takes place in Japan every year to this very day.
Now here's the amazing part, there really are crabs living in the waters where this event took place 900 years ago, crabs with scowling Samurai faces on their shells. It's not just a matter of hearsay, plenty of people have caught the Samurai crabs and thrown them back because of the human faces imprinted on their backs. Are these the ghosts of the drowned emperor and his men?
Well, no, but the Samurai crabs do show us how quickly evolution works. What we have here is an instance of artificial selection, or a change in the physical characteristics of another species owing to the influence of humans.
Think of it this way, first comes the memorable sea battle, spawning legends. Next a fisherman finds a crab with markings on its back that look something like a human face. Does he eat it? No, that crab is thrown back. Now it has a survival advantage over crabs whose markings don't happen to look like anything to humans.
A cycle begins. Every generation that has a more human looking face stands a better chance of survival than the last. In time, perhaps over a thousand years, the originally random markings on Samurai crabs really do look like Samurais.