Each time your fingers curl around an object, you are using a structure shaped over hundreds of millions of years. Human hands, like all vertebrate limbs, have a clear top and bottom: a palm, its ventral side, specialized for grasping and touch, and a back, or dorsal side, covered in nails and protective skin.
That distinction may feel obvious, but it had to evolve. Early fish first bore midline fins — single fins running along their backs or bellies — before paired fins, and eventually limbs emerged at their sides. Those early fins lacked anything like a palm and back. A study published in Molecular Biology and Evolution suggests the origins of limb asymmetry trace back to those aquatic ancestors, when evolution repurposed an ancient gene that once served a very different function.
“The biggest surprise of the study was that we found this very ancient trace of the dorsoventral patterning system in the midline fins. The experiment to look at Lmx1b was a bit of a “long shot,” but we tried anyway and ended up finding the most ancestral genetic signature for the upper side of our hand and feet in a place where we really didn`t expect it,” team lead, Joost Woltering, told Discover.
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Tracing Limb Evolution Back to Ancient Fins
To understand what that gene was doing, the team compared how it behaves in different types of fins. They focused on Lmx1b, a gene that helps shape the upper side of vertebrate limbs. In embryos, cells that activate Lmx1b form the upper surface of a limb; cells that do not form the lower side.
In paired fins — the evolutionary precursors to arms and legs — Lmx1b is active along the dorsal surface. In midline fins, however, the gene is switched on toward the rear of the fin rather than along the top surface.
That difference suggests Lmx1b originally had another role, likely helping organize tissues toward the back of the fin. They found that the gene likely helped connect nerves to muscles in early fins, ensuring that different muscle groups were wired correctly.
Gene Signaling Shifts in Early Vertebrates
The team also looked at what turns Lmx1b on in different fins. In paired fins, the gene is activated by one developmental signaling system. In midline fins, it is controlled by a different one.
That difference shows evolution did more than reuse an old gene. It changed the instructions that tell the gene when and where to act. By altering those upstream signals, Lmx1b began marking the upper surface in paired fins, laying the groundwork for limb asymmetry.
“The finding in the pectoral fins really tells us that for the fin to limb transition, the basic genetic pattern was already there,” Woltering told Discover. “Land animals only had to ‘fill it in’ with downstream morphology and new genes.”
By the time vertebrates began adapting to life on land, the basic pattern separating upper and lower surfaces was already in place.
Evolution’s Blueprint: Building New Limbs From Old Genes
The findings point to a broader evolutionary pattern.
“It keeps showing us how important ancestral patterns and gene networks are when evolution goes on to build something. New morphology doesn’t arise on its own but usually on top of ancestral genetic networks,” explained Woltering.
Rather than inventing an entirely new genetic system, evolution reused and modified one that was already there. The blueprint for our hands began taking shape in aquatic ancestors long before vertebrates ever set foot on land.
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