Could This Scientist Re-Wire Humans To Grow Extra Limbs?

Michael Levin wires frogs to regrow severed legs—or grow extra ones. Can he do the same for a human?

- Adam Piore

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.

For decades, genetics taught us a simple truth: Each cell in our body (and there are billions) contains the blueprint that tells us how to grow. That might not be the whole story. Levin and a few others now say that tiny bioelectric signals surging through and among our cells act as an instruction to kick-start gene expression. These signals point cells in the right direction as they start to grow into things like hearts, and influence the shape and function of the body. For two decades, Levin has set about proving it.

In doing so, he has created a startling Island of Dr. Moreau zoo of freaks. He forced tadpoles to grow an eye on their gut; induced frogs to sprout six legs; and caused worms to grow two heads, which, when severed, will grow back just like a salamander’s severed tail—all by manipulating the faintest of bioelectric signals.