Throughout our lives we have learned to associate feathers with birds and flight. Every single extant species of birds is covered by some variation of feathers—eagles have long and sturdy ones specialized for flight, penguins have short, densely packed feathers to keep them warm in the cold arctic waters, and peacocks have a brilliant plumage to help them attract mates.
However feathers didn’t used to be exclusively for birds. About 200 million years ago hundreds of species of dinosaur bore what could be called the first edition of the feather all over their body and not a single one of them was avian. The early versions of the feather were very different from the ones we are familiar with today, and, contrary to the beliefs of many scientists of the 20th century, did not evolve from the scale. Whereas a scale is simply a flat, keratin rich fold of the epidermis, feathers are tubular structures with a completely different type and dispersement of the structural protein. In fact, more and more evidence is suggesting that feathers were an evolutionary novelty, evolved and weeded through natural selection to provide for the ever-increasing demands of heat control.
One of the first bird-like dinosaurs, the Sinosauropteryx, lived in a relatively cold time and place that was unusual for the Early Cretaceous period. With an active lifestyle, a small size (which didn’t allow for much heat retention), and temperatures going as low as 10°C in the winter, this creature needed a way to maintain a high body temperature to avoid imminent death. As scales are poor thermal insulators, the answer was to find a new body covering. In the case of the birds this was feathers.
So what did this very first “protofeather” look like? Well imagine a 2 cm long hollow cylinder, similar to fur, but thicker and more rigid (or just look at the picture below). This filament did not come out of specific tracts or follicles; rather, it seems to have grown homogeneously across the body of the dinosaur, replacing scales in some regions and growing alongside in others. If you look at modern birds you will notice that most of them still have scales on their legs. Feathers did not grow in these regions not because they didn’t need to be kept warm, but because they would hinder fighting and would break easily.
The next stage in the evolution of the feather was actually fairly staightforward: for better thermoregulation these protofeathers needed to cover a greater area, which was easily achieved by making them thinner. However evolution didn’t stop there—over time these filament fused together into one at the base, allowing each combined “feather” to retain more heat while taking up the same amount of space.
As can be seen in the image, these feathers are far from what we are able to witness today; however the old protofeathers and the modern ones are almost exactly identical in their capacity to insulate. If thermoregulation was the only factor in the selection for these feathers, evolution would probably slow down around this point and we would be witnessing a completely different class of flightless and feathered creatures on our streets. So why would feathers continue developing into longer and more structured ones? This might seem like the completely opposite thing to do, since long feathers require more care and more energy to grow and could potentially have a deleterious impact on their heat retention property.
Well, as hinted in the previous paragraph, thermoregulation was not the only factor in the development of the feather. And, although for the first few stages it was the chief property to have, it slowly faded into the background for stages 3, 4, and 5. Judging by modern avians and the brilliant red and blue pigments found in the feathers of some of the protobirds (particularly the Anchiornis and the aforementioned Archaeopteryx), feathers continued to evolve through sexual selection. The bright and colorful plumage of male birds could have attracted female mates, resulting in longer and more colorful feathers.
Another factor that led to the continued evolution of feathers was male-on-male interaction. In most species of birds multiple males fight for the right to mate with the females. These confrontations typically begin with the males showing of their plumage and trying to make themselves appear bigger, and, if the other male does not run away in fear, end in an gruesome fight. This theory is supported by the placement of the feathers in ancient birds such as the Archaeopteryx, who had the longest feathers on their arms and their tail, the most visible parts.
Stage 4 of the evolution of feathers was the development of barbules and hooklets on each barb that locked with adjecent ones and connected the feather together making it harder to break apart. Rather than having the feather be weak and flimsy, this change gave it its traditional flat apperance. Such a change could also be explained by the male-on-male fighting—in a fight a higher position is often the difference between victory and defeat. It is a lot easier to hit down on your targer than to strike up, and the wing feathers of birds could have evolved to give the competitors the necessary lift to gain a higher ground. Over time, combined with other factors, they might have given the birds full flight.
The last stage of the evolution of feather was to make the feathers assymetrical, giving them even more lift and eventually allowing for full flight.
The fact that feathers evolved for display and threat also explains a previously unanswered mystery: the muscles and nerves connected to every single feather. Longer feathers are not as effective in scaring the opponent unless the are able to be lifted from the body of the bird, a bit like how a cat raises its fur. Thus, millions of years of evolution have given birds the ability to control individual feathers and “ruffle” them. This phenomenon is also inredibly important for effective flight.
I really hope you liked this post since I spent so much time doing the research, making the diagrams, and actually typing it up. The next update should be in two weeks and will be about mammal fur as well as some other characteristics, like their defining feature: the mammary gland. I look forward to reading your comments below and if you have any question feel free to ask me, I will do my best to answer.