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.

But how does nature distinguish between the top and the bottom sides of a limb, and which adjustments to the genetic machinery were necessary during evolution to make this possible? An international research team led by Konstanz-based biologist Joost Woltering has the answers. In their recent article published in Molecular Biology and Evolution, they describe how ancient genes from the midline fin of fish had to be "redeployed" to establish the dorsal-ventral axis in our limbs.

An anatomical puzzle
The evolutionary journey from ancient fins to the human hand began roughly 500 million years ago. Around that time, the genetic program for fins typically found on a fish's back—the midline fins—was copied and activated on the flank of one of our aquatic ancestors. This gave rise to the first fish with paired fins. About 350 million years ago, these paired fins evolved into the paired limbs of vertebrates, including our arms and legs.