Your body knows how to build an arm. It did it twice – one for each side of your body.
Before birth, without a surgeon, a scaffold or a computer model, a small cluster of cells constructed bones of the correct length, joints that moved in the correct directions, muscles, tendons, blood vessels, skin and nerves that connected to the spine.
Nearly every cell in your body still contains the genetic instructions that made this possible. Yet if an adult loses an arm, those instructions remain silent. The wound closes, scar tissue forms. End of story.
But many animals regrow…
