Question

### Passage

For over a century, evolutionary biologists and paleontologists have been divided by a classic debate regarding the mechanical origins of avian flight: did the ancestors of modern birds take to the air from the trees down, or from the ground up? This debate is not merely about habitat preference; it concerns the fundamental aerodynamic and energetic pathways that led to the evolution of the avian wing. The traditional arboreal hypothesis, first proposed in the late nineteenth century, posits that early avian ancestors were tree-dwelling animals that moved through the forest canopy. According to this model, flight began with gravity-assisted gliding. Proto-birds would leap from elevated perches, using primitive feathers to slow and extend their descent. Over evolutionary time, natural selection favored individuals who could actively flap their forelimbs to control their trajectory and extend their range, eventually transitioning from passive gliding to active, powered flight. Proponents of this view argue that gliding is aerodynamically simpler and energetically less demanding than launching directly from flat ground, making it a logical precursor to flight.

In stark contrast, the cursorial hypothesis proposes that flight evolved from the ground up, entirely bypassing an arboreal phase. This theory suggests that bipedal, running theropod dinosaurs used their feathered forelimbs to assist in terrestrial locomotion. Initially, proto-wings might have been utilized as nets to capture prey or as stabilizing appendages to maintain balance while running at high speeds. Proponents argue that as these animals ran, flapping their forelimbs created aerodynamic lift, which gradually allowed them to take longer leaps, clear obstacles, and eventually achieve sustained flight. The cursorial model aligns closely with paleontological evidence showing that the closest dinosaurian relatives of birds, such as dromaeosaurs, were cursorial predators with anatomy built for running rather than climbing. However, critics have long pointed out a major physical flaw: the physics of launching from a flat surface present an enormous energetic barrier that primitive, proto-wings could not have easily overcome without pre-existing flight muscles.

This long-standing binary deadlock was challenged in the early 2000s by a groundbreaking discovery made by biologist Kenneth Dial: Wing-Assisted Incline Running (WAIR). While studying chukar partridges, Dial observed that young, flightless chicks and adults with clipped wings flapped their wings not to lift themselves into the air, but to press their bodies against steep slopes. By flapping, the birds generated aerodynamic downforce—much like the spoilers on a sports car—which increased traction and allowed them to run up near-vertical obstacles, such as tree trunks and cliffs, to escape predators.

WAIR suggests a novel ecological path for the evolution of flight. Rather than choosing between the trees and the ground, it proposes that proto-birds used their developing wings to navigate vertical environments. The primary claim of the WAIR hypothesis is that the initial evolutionary pressure selecting for feathered forelimbs was not flight itself, but the utility of aerodynamic downforce in helping ground-dwelling animals ascend steep inclines to evade danger. This model elegantly bridges the gap between terrestrial running and climbing, showing how transitional, half-wing structures could provide immediate survival benefits before they were capable of generating the lift required for true, self-sustained flight.

### Question

Based on the passage, match each evolutionary theory of flight (on the left) with its primary claim regarding the initial function or mechanism of proto-wings (on the right).