One day in 1788, students at the Hunterian School of Medicine in London were opening a cadaver when they discovered something startling. The dead man’s anatomy was a mirror image of normal. His liver was on his left side instead of the right. His heart had beaten on his right side, not his left.

The students had never seen anything like it, and they rushed to find their teacher, the Scottish physician Matthew Baillie, who was just as stunned as they were. “It is so extraordinary as scarcely to have been seen by any of the most celebrated anatomists,” he later wrote.

His report was the first detailed description of the condition, which came to be known as situs inversus and is thought to occur in about 1 in 20,000 people. Baillie argued that if doctors could figure out how this strange condition came to be, they might come to understand how our bodies normally tell the right side from the left.

Over two centuries later, the mystery of left and right still captivates scientists.

“I know what it is, you know what it is, but how does the embryo learn what it is?” asked Dominic P. Norris, a developmental biologist at the Medical Research Council in Harwell, England.