Here’s what we know about the evolution of flight: By about 150 million years ago, the forests were filled with flying — or perhaps just gliding — dinosaurs like Archaeopteryx, possibly similar to the ancestor of modern birds. What we don’t know is what primitive wings were used for before bird ancestors could fly.
There are two main theories for how avian flight evolved. According to the “trees-down” theory, primitive wings were used to glide down from heights. The “ground-up” theory holds that bird ancestors used their wings to “run flap” along the ground, making them faster and better able to scamper up steep inclines that got in their way.
The problem with the ground-up theory is the huge speed required to achieve liftoff. By comparison, incremental improvements in gliding could have led to flight. The fossil evidence has been too scant to settle the matter.