Cursorial Hypothesis

Also known as the "from the ground up" hypothesis. The cursorial theory of the origin of flight proposes that some fast-running animals with long tails used their arms to keep their balance while running. Modern versions of this theory differ in many details from the Williston-Nopcsa version, mainly as a result of discoveries since Nopcsa's time.

Baron Nopcsa theorized that increasing the surface area of the outstretched arms could have helped small cursorial predators to keep their balance, and that the scales of the forearms became elongated, evolving into feathers. The feathers could also have been used as a trap to catch insects or other prey. Progressively, the animals would have leapt for longer distances, helped by their evolving wings. Nopcsa also proposed that there were three main stages in the evolution of flight. First, passive flight was realized, in which the developed wing structures served as a sort of parachute. Second, active flight was possible, in which the animal achieved flight by flapping its wings. He used Archaeopteryx as an example of this second stage. Finally, birds gained the ability to soar. It is now thought that feathers did not evolve from scales, as feathers are made of different proteins. More seriously, Nopcsa's theory assumes that feathers evolved as part of the evolution of flight, and recent discoveries prove that assumption is false.

The modern version of the "from the ground up" hypothesis argues that birds' ancestors were small, feathered, ground-running predatory dinosaurs (rather like roadrunners in their hunting style) that used their forelimbs for balance while pursuing prey, and that the forelimbs and feathers later evolved in ways that provided gliding and then powered flight. The most widely-suggested original functions of feathers include thermal insulation and competitive displays, as in modern birds. The debate over the cursorial hypothesis is complex and not yet resolved.

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