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How did paired fins contribute to the evolution of vertebrate limbs?

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A translucent X-ray fish (Pristella maxillaris) swims in an aquarium, showing its visible backbone and internal structures through its body. — Chordate X-ray fish (*Pristella maxillaris*), is an example of a chordate with a visible backbone.

Most authorities agree that the limbs of land vertebrates evolved from the paired fins of fishes, specifically those of the Sarcopterygii (lobe-finned fishes).12

In ancestral fishes, paired fins functioned primarily as horizontal stabilizing keels to prevent rolling or pitching movements. While these primitive fins were attached by broad bases and did not support body weight, certain extinct fishes began using their fins for "stepping" on the bottom, a behavior still observed in modern lungfishes.13

The transition to land vertebrates involved several key evolutionary changes:
* Structural Homology: The tetrapod limb pattern—featuring a single proximal bone, two distal bones, and joints such as the elbow, knee, wrist, and ankle—is derived from the skeletal arrangement found in the paired fins of lobe-finned fishes.23
* Weight Bearing: Unlike fish fins, which are used for steering and balancing, land vertebrate limbs evolved to support body weight. This required the development of limb girdles that became increasingly linked to the vertebral column.13
* Propulsion: As vertebrates moved onto land, the primary source of locomotion shifted from axial muscle undulations to the limbs, which evolved to press against the ground and create thrust.23