How the Pufferfish Got Its Puff

The puzzle--and joy--of evolutionary biology is to find the lost paths that life took to arrive at the strange forms it has today. Take, for example, the pufferfish. At first sight, it seems miserably adapted to the tropical waters where it makes its home: it is an unassuming, small fish, so slow you can easily catch it by hand. But when a predatory fish or bird attacks, a pufferfish goes through a unique transformation: it rapidly gulps water and swells into a huge, spiky, hard-shelled ball three times its normal size. How could anything like this evolve from an ordinary fish?