A tetrapod humerus from the Late Devonian of Pennsylvania has a novel mix of primitive and derived characters. A comparative analysis of this fossil and other relevant humeri from the Devonian shows that the role of the limb in propping the body arose first in fish fins, not tetrapod limbs. The functional diversity of the earliest known limbs includes several different kinds of appendage design. This functional diversity was achieved with a humeral architecture that was remarkably conserved during the Devonian.