The Origin of Left and Right

We often think of ourselves -and other animals- as symmetrical because we have two two eyes, two hands, two legs, etc that are pretty much alike. But we are not symmetrical. Our hearts are on the left, and our liver is on the right, except for a tiny minority of people who have those reversed. And almost all of us have one dominant hand that we use for skilled tasks. Scientists have been looking back into the evolutionary line to see when and how animals developed asymmetry, and why one direction is so dominant in a species. Pond snails were a good species to study, since their shells spiral either right or left from the time they begin to grow them.

In 2010, Reiko Kuroda showed that these asymmetries begin in the snail’s earliest moments. Like all of us, they begin life as a single cell, which divides into two, and then into four. At this stage, all the cells are the same size and shape. But the next division is unequal, pinching off a small cell on top of a big one—picture four ping pong balls sitting on top of four tennis balls. The ping pong-sized cells then rotate to nestle between the furrows of the tennis-sized ones. If they rotate clockwise, the snails end up with a right-handed shell. When Kuroda nudged them anticlockwise, using glass rods and exceptional dexterity, she produced left-handed shells.

Scientists Angus Davison Mark Blaxter looked further at snail development and found differences in the very first cell division -and traced the difference in the two cells to one protein. Manipulating this protein also caused frogs to grow their hearts on the “wrong” side of their bodies. Read more about the research into animal asymmetry at the Atlantic.