The Origin of Avian Flight: The Great Debate
Avian flight is a remarkable evolutionary feat. But, where did it begin? Scientists are still stumped. Due to the lack of historical data that is available, it is very difficult for researchers to define when and how it began. There are two main competing hypotheses collectively known as top-down vs. bottom-up: 1) Top-down (aka: From Trees Down): scientifically known as the Arboreal hypothesis. The arboreal hypothesis begins with a tree-dwelling, feathered dinosaur/bird species. This species climbed tree trunks and leaped between them to catch prey and/or avoid predators (comparable to a flying squirrel). Flight in this species arose in a series of evolutionary adaptions that increased gliding ability to increase distance, then flapping and controlled tail feathers to increase manoeuvrability while gliding, and then finally, full flapping allowing for constant flight. These adaptions would have been characteristics such as decreased body mass, hollow bones, stiffer feathers, etc. This hypothesis can be supported by some evidence, such as the anatomy of Archaeopteryx (one of the earliest known bird/dinosaur species. See attached photo), which had feet similar to today's tree-perching birds.
2) Bottom-up (aka: From Ground Up): scientifically known as the Cursorial hypothesis. This hypothesis was later developed as a counter to the top-down theory. It suggests that the long neck, long legs, and stiff tail of Archaeopteryx suggests that it was a ground-dwelling sprinting species. To maintain good balance, individuals would spread their wing-like arms and use them in conjunction with the stiff tail to be more agile runners, while also creating lift to conserve energy. The lift would also allow Archaeopteryx to leap long distances to attack prey. Over time, feathers became stiffer and the body became lighter to create additional lift, which eventually resulting in wing flapping. Eventually Archaeopteryx could lift off the ground and fly.
Both hypotheses are riddled with assumptions and disagreements on the interpretation of avian ancestors' anatomy and morphology, which is why it continues to be debated.
Which hypothesis do you support? Debate below! (I'm curious)
~Rosie
Image source: http://abacus.bates.edu/acad/depts/biobook/Pal10pix.htm