How did the complexity of many organisms living today evolve from the simpler body plans of their ancestors? This is a central question in biology. Take our hands, for example: Every time we type a message on our mobile phone, we are using an evolutionary "masterpiece" that evolved over millions of years. Notably, we typically grasp and manipulate objects with the palm of our hand—its ventral side. The back of our hand, or dorsal side, plays almost no role. This differentiation of our limbs, with a ventral side adapted for contact and a dorsal side protected by nails or toenails, is essential for life on land.