A trove of fossils in China from around the time birds were evolving continues to yield insights into how feathers and flight evolved. Science still does not have a good story for the evolution of birds, but the cast of characters has gotten bigger. With so many new clues, Archaeopteryx has now been dethroned as 'oldest bird'. Birds came off the dinosaur family tree near Archaeopteryx, but not from its branch.
Recently National Geographic ran an article on feathers that summarizes much of the known fossil record, how it relates to the chain of ancestry and what it tells us about the evolution of flight.
At the time of Archaeopteryx, numerous dinosaur species had feathers or protofeathers, and evolutionary pressure was diversifying and selecting feather traits. Many animals clearly unable to fly possessed feathers, so they did not originate to enhance flight. Flight originated once feathers already existed.
The better story emerging from the fossils is this: flight evolved from swimming and diving. In their squabbles over whether birds flew from the ground up or down from the trees, scientists neglected this third option. Dinosaurs started entering water for prey, gradually evolving specialized scales with thermal properties and spending more time in the water. Their aquatic habits eventually included true swimming, which lead to the gradual evolution of larger and more powerful muscles and aerodynamic feathers. Swimming set the stage for aerial flight.
The origin of flight in birds is the story of the evolution of the breastplate to which a bird's wing muscles attach. This bone shows specializations seen only in birds, and flight muscles attach in a way that is essentially an inversion of forelimb musculature as it appears in every other four-limbed vertebrate including humans. We know that flying reptiles like Pterodactyls are unrelated to birds because they lacked an inverted musculature and large breastplate.
Birds have a muscle configuration all their own, and the wishbone is part of their unique skeleton. It is found only in birds and in dinosaurs like Archaeopteryx, Sapeornis and few others. While those fossils have wishbones, they lack large muscle-attachment sites on the breastbone and show no indication of significant strength in their forelimbs. So inverted musculature evolved prior to the origin of birds. It is not an adapatation to flight but a precursor.
Aerial flight requires strength, even just to glide. The power demand makes transitions from terrestrial to aerial locomotion unlikely, and the major retooling of the breastplate in birds is the sort of change that happens gradually, on geological time scales rather than generational scales. It is simply not reasonable to expect the evolution of flight to be as simple as the tales science has been exploring.
To understand the origin of flight, we need to think of a process taking many steps and millions of years, and the story should take place in water. Water is far more buoyant than air, so flying in water requires less power. We call that swimming. Gliding in water we call diving. An aquatic origin provides the time and opportunity for big changes in power and musculature. Diving lizards become swimming lizards, and shorelines become open waters. Eventually a master swimmer makes transition to aerial flight.
The oldest fossils that possess a bird's breastplate are the Hesperornithes, which resemble diving birds like loons. There are numerous marine varieties of birds, including albatrosses, ducks, gulls, pelicans and sandpipers, and they are deeply rooted in bird phylogeny. Terrestrial and forest birds arose later in avian history: fowl, hawks, songbirds, woodpeckers, these are younger varieties of bird.
Science has overlooked an aquatic phase in bird evolution, but it is staring us right in the face.