For instance, if all the dinosaurs above a branch possessed a unique adaptation (such as shoulder blades fused into a bird-like wishbone), then we can infer that the wishbone evolved on that branch.
We then identified the series of successive branches leading from the very base of the dinosaur tree all the way to living birds: this is the "bird stem lineage".
A close analogy would be taking a real tree and tracing the single path that leads from the trunk all the way to a "special" little bunch of leaves somewhere high in the crown.
It turns out that the bird stem lineage - the dinosaurs on the road to becoming birds - were evolving in a noticeably different manner to other theropod lineages around at the time.
This lineage kept shrinking in size, with each successive descendant smaller than its predecessor. By contrast, in other dinosaur lineages, body size was alternately increasing and decreasing, with no sustained trend in one direction.
Another recent study further shows that body size along the bird stem lineage often changed unusually rapidly (in addition to in a coherent direction).
Evolutionary novelties were appearing on the bird stem lineage at a faster rate than across the rest of the tree. Many were major innovations such as complex feathers, bigger brains, wings and wishbones. Stem-birds were out-evolving their contemporaries by changing around four times as fast. This continual and often rapid shrinking was probably directly related to the accelerated evolution of anatomical novelties.
Reduced body size, for instance, allowed bird-stem dinosaurs to explore new postures (bird-like walking where the thigh bone is held horizontal) and habitats (such as arboreal and, later, aerial habitats). This in turn would have created pressure to evolve radical new adaptations such as reshaping fluffy feathers into wings.
Perhaps the movement of small dinosaurs into trees was one reason for the appearance of gliding flight, using aerodynamic feathers. Small theropods with feathered arms, legs and tails - such as Microraptor and the recently described Changyuraptor - were very likely excellent climbers, as fossils have been found with small birds in their stomachs.
Ultimately, the dinosaurian lineage that was the most evolvable during the Mesozoic also proved to be the most long-lived (there are 10,000 species of birds alive today). It is not surprising the ability to adapt rapidly is a key to long-term survival.
Taking flight
• Evidence shows that dinosaurs evolved into birds
• They wouldn't have perished when the meteorite struck at the end of the Cretaceous period.
• Research shows that the ancestors of birds were the only lineage of dinosaurs to continually shrink in size for an extended period of time.
• They were also the fastest-evolving lineage of dinosaurs.
