Legs to Flippers

A whale's flipper hides an old secret. X-ray it and you find the bones of a four-legged limb — an upper arm, forearm, wrist, and fingers — the same plan as your own arm, just reshaped for water. Whales even keep broken leg genes switched off in their DNA, and a rare whale is born with small leg buds, a throwback to its walking ancestors.

The front limbs became paddles slowly: finger bones lengthened, wrist joints fused stiff, and webbing filled the gaps. Each tiny change made swimming a little better.