How birds got their wings: Part III
The origin of flight is still hotly debated amongst biologists and paleontologists alike. There are 4 major hypotheses for how it came about. The first one is the cursorial hypothesis also known (quite explicitly may I add) as the “from the ground up” hypothesis. Many theropods were known to be cursorial, having limbs adapted to running, and so could run very fast. So, the cursorial hypothesis is exactly what the name suggests: birds evolved flight by running and jumping. Add in some flapping movements from the wings and Archaeopteryx may have been able to acquire enough lift to get airborne. Makes sense right?
This turns out to be quite an ambitious theory. First of all, the energy it would require is immense. It is unclear whether or not Archaeopteryx was able to generate enough power to actually leave the ground. Secondly, getting lift would necessitate its forelimbs (wings) to be extended symmetrically. This happens to create drag as well which reduces thrust, effectively slowing Archaeopteryx down, preventing it from gaining enough speed to generate lift....
A related hypothesis was put forth by Kenneth Dial in 2001. The principle is similar but there is a bit more context to this idea. It involves a small cursorial dinosaur running up a slope, such as a tree trunk or a hill, to escape a predator. Flapping motions with feathered forelimbs might have helped them to gain ground while doing this. However, the lack of shoulder mechanism in early birds for upward limb movements means that the necessary forelimb upstrokes would likely have been impossible.
The biggest opposing theory is the arboreal hypothesis (which as it so happens also has an unambiguous other name: the “from the trees down” hypothesis). Supporters of the theory believe birds’ reptilian ancestors glided from tree to tree, until they developed full flight. The evidence for this idea lies once again in Archaeopteryx’s anatomy. For instance, its dorsoventrally flattened tail may have promoted gliding. Archaeopteryx also had a hallux: a modified big toe which had the advantage of being on one side (and not opposed to the other toes as thought previously). This may have made it easier to climb. One reason this theory is so appealing is the fact that it’s more energy efficient than the first two, making it more believable to many scientists. No theory is perfect though as Archaeopteryx also has an elongated tibia, which makes its habit of climbing trees a bit hard to comprehend.
Finally, there is a theory which combines elements of all the other propositions: the Pouncing Proavis model. This playfully-named model addresses the origin of flight from a very different angle: that of small proavian predators! It suggests that small predatory dinosaurs might have gained an advantage from glide-assisted hunting and pouncing.
From the evidence that we have, it is impossible to tell which one is most likely. Most scientists favour an explanation which encompasses all these hypotheses. Firstly, because early birds lived in a range of habitats, it is possible that all of these theories contributed to their survival. Secondly, new research shows that flight evolved several times across the Dinosauria clade, demonstrating that birds do not have one flying common ancestor.
And that just about wraps it up for my mini-series on “How birds got their wings”. But don’t worry, this is not the end of our journey in time… Keep a look out for the next post to come ;)
