Ed Yong in The Atlantic:
On the surface, people are more or less symmetrical. Aside from small differences, our right sides mirror our left. The same isn’t true for our innards. The heart, stomach, and spleen typically sit slightly to the left, while the liver and gall bladder sit to the right. That’s the usual set-up, but it’s mirrored in one in every 10,000 people, who have a condition called situs inversus. Donny Osmond has it. So did James Bond’s adversary Dr. No, who once survived a murder attempt because his would-be assassin stabbed the left side of his chest and missed his heart.
Whether standard or inverted, there is asymmetry, which raises the obvious question: What creates it? We begin life as a single fertilised cell, which divides again and again into the trillions of the adult form. At what point in that process does left begin to differ from right?
The standard answer, as least for the embryos of back-boned animals, is that tiny beating hairs called cilia push fluid in a typically leftward direction. This current concentrates molecules on one side of the embryo, including those that steer its subsequent development, including one called Nodal. Hence: asymmetry. That’s why people with genetic disorders that disable their cilia have 50:50 odds of owning a right-sided heart.
But that can’t be the whole story because by the time the cilia start to beat, the embryo is already asymmetrical. There must be some earlier symmetry-breaking event.