One morning in spring 2000, Michael Levin flopped in his chair and clicked on his desktop computer. A newly minted assistant professor at Harvard, Levin, then 30, was looking to solve a riddle that had baffled science for centuries: How do our dividing embryonic cells know on which side of our bodies to grow our hearts, our livers, our gall bladders? Countless people throughout history have been born with some, even all, of their organs transposed, and yet functioning. Levin suspected DNA alone was not to blame; there had to be some other trigger. Days earlier, he had ordered an imaging test on a half dozen chick embryos at the verge of organized development. As he pulled up the results, he stared, amazed. Electrical charges, rendered in yellows and reds, lay across the cells in patches, left to right, as clearly as a neon "This Way" arrow. Levin sat back and blinked. He was witnessing, for the first time in history, embryo cells telling each other left from right via electricity.