How Feathers Grow

Feathers are one feature, highly pertinent to both avian and nonavian dinosaurs, for which this has been done in elegant and satisfying detail by Richard O. Prum of Yale and several colleagues.

Their work is all the more interesting because, in the absence of evidence from genetics and developmental biology, a theory of feather evolution had been developed that seemed to make sense but turned out to be impossible. "According to this scenario," Prum and Alan H. Brush wrote in Scientific American in March 2003, "scales became feathers by first elongating, then growing fringed edges, and finally producing hooked and grooved barbules."

To understand why this couldn't have happened, it's necessary first to understand the structure of that lovely feather floating in the wind, or contributing to the fluffiness of your pillow. Feathers are essentially long tubes with branches. The branches also have branches, and those branches again have something like branches, except that the last twiglike extensions are hooks or barbules that hold the feather together.

There are two different sorts of feathers. One is the blue jay or pigeon feather you may find on the ground, the turkey feather you can buy if you tie flies to catch trout. The other is found in great numbers as the down in your sleeping bag, comforter, or winter coat. The first is pennaceous and the second is—and this has to be one of the great words of biology— plumulaceous. The pennaceous feathers have the branching described above, while the plumulaceous feathers have very little main stem and instead a tangle of lesser branches, with barbules that link together, forming the air-trapping matrix that keeps birds warm, and people as well, in their sleeping bags and puffy mountaineering coats.

The first step in understanding what feathers are and how they evolved was achieved simply by tracking embryonic growth at a microscopic level. Feathers grow out of the skin or epidermis, the outer layer of cells in the developing embryo. Part of the skin starts to thicken, and then to grow out into a tube, while around the growing tube a cylinder of cells form the follicle. The follicle keeps generating a kind of cell that produces keratin, the substance in fingernails and hair. The new cells at the bottom push the old cells at the top, "eventually creating the entire feather in an elaborate choreography that is one of the wonders of nature," Prum writes.

One aspect of the growth is indeed wonderful, and complex. As the hollow tube grows, something happens with the part of the follicle called the collar, which is the source of the growth of keratin-producing cells that push the central, hollow shaft of the feather out from the skin. It begins producing ridges on the central shaft that grow in a helix on the tube, turning into the main branches as the feather grows. Then the barbules grow from these branches. All of this happens at once, which gives a hint of the mystery and wonder in the way organisms grow from one cell to a complex creature, with so many cells forming so many and such complex patterns, all timed to occur at the right moment and directed to the right place. The feather is just one small example of this sort of change in concert.

Prum and other colleagues proposed that in the course of this development they could see the way feathers had evolved. Primitive structures, like those they identified in the early stages of feather development, must have appeared first in evolution. Animals must have existed that had only these tubelike structures. Only later did the feathers that let birds fly emerge.

In other words, feather evolution, like feather growth in the embryo, proceeded by discrete steps. And one step had to be completed before the next one could occur. Each step depended on what had gone before. The final product, the feathers that enable the flight of falcons and swallows, came long after the feather first evolved. And since those first feathers had absolutely no connection to flying, feathers had to have evolved for some other purpose. The feather has been one of the features creationists have long pointed to as an impossibility for evolution. How could feathers, a truly novel development, not just a longer arm or a thicker skull, evolve on their own and just happen to be useful for flying? Prum and colleagues showed exactly how that could and did happen. Features emerged that served one purpose, and as other features were built on them they changed into the structures we see today.

First to evolve were simple tubes, hollow cylinders, then barbs that formed tufts on the tubes. In the next stage feathers became tubes with branches that had tufts, or barbules. There was one more step, which was for the barbules to change shape to have hooks at the end. These hooks are what allow a feather to close and feel as if it is one piece, repelling water, or pushing on air. After the stage of the hooking barbules, the change that produced true flight feathers could have taken place. This was an asymmetric feather, with more on one side of the central tube.

Prum and John F. Fallon and Matthew Harris at the University of Wisconsin-Madison went deeper into development, using techniques to observe which genes were active at which stages and in which locations in the growing feather. They found two well-known genes and the proteins they coded for. Sonic hedgehog and one of the bone morphogenetic proteins, BMP2, were present in different places and different concentrations promoting growth (sonic hedgehog) and the differentiation of new kinds of cells (BMP2). BMP2 was also limiting cell proliferation.

First they would appear where the feather germ was starting, later at the beginning of the ridges that turned into the first branches. The two proteins directed the growth of the feather, and did it in stages, just as Prum and colleagues were suggesting, with each stage possible only because of the one before it. Without the feather germ there could be no ridges or branches or tufts. The general picture they saw in development and proposed in evolution was that first came the central shaft, the hollow tube. Then came downy tufts. Finally came the helical ridges, organized branches, and barbules that made modern feathers, the sort that can be found on a starling or on Archae-opteryx.

On a cold night when you crawl into the warm cave under a down comforter, you are taking advantage of millions of years of evolution, mediated by two genes and the proteins they code for—sonic hedgehog and bone morphogenetic protein .

This made the steplike sequence of development clear, but more evidence was needed to link the developmental sequence to an evolutionary sequence. Some of this was readily available in the great variety of feathers in modern birds. Each evolutionary stage of feather development could be seen on some living bird. So, Prum was not inventing any structures that were unknown. All these feather types had appeared on birds at one stage or another.

Nothing they had learned had falsified their hypothesis. Nothing had proved it either. Of course, in historical sciences, like paleontology or evolutionary molecular biology, proof is not possible in the way that it can be obtained in a physics experiment. But predictions can be made and evidence produced that supports or refutes the validity of the predictions. Prum and his colleagues, in describing the sequence of evolution, were, in effect, predicting that extinct organisms existed that had primitive feathers, mere tubes, and downy feathers, and that these should have existed before Archaeopteryx.

Paleontology came to the rescue with the discoveries of feathered dinosaurs, which I described in the last chapter, in the 1990s in China. These were just what had been predicted.

As Prum writes, "The first feathered dinosaur found there, in 1997, was a chicken-size coelurosaur (Sinosauropteryx); it had small tubular and perhaps branched structure emerging from its skin." Later, other dinosaurs were found with pennaceous feathers. The variety of feathers, including the simple tufted sort that would correspond to the second stage of feather evolution in the Prum plan, all of them on dinosaurs, gave further support to this idea of feather evolution.

Prum's exhilaration in the Scientific American article produced one of the great scientific sentences: "These fossils open a new chapter in the history of vertebrate skin." Indeed.

Birds became a subset of theropod dinosaurs. Dinosaurs acquired feathers. T. rex may even have had them. The idea of feathers evolving from scales was undermined. Scales don't grow as cylinders, but with a distinct top and bottom. And it became clear that feathers did not evolve for the purposes of flight. Why they evolved we don't know. Nor can we say when they evolved. And we have found that we will probably never be able to say when birds evolved. All evolution in reality is a continuum, with no sharp distinctions. And nowhere is this clearer than in the transition from theropod dinosaur to avian dinosaur. Arguments now exist over whether some of the Chinese dinosaurs are birds.

In the work on feathers Prum demonstrated and articulated the direction that paleontology and evolutionary biology must take: the same direction that others have favored. As he concluded, "Feathers offer a sterling example of how we can best study the origin of an evolutionary novelty: Focus on understanding those features that are truly new and examine how they form during development in modern organisms." In fact, he refers to it as a "new paradigm in evolutionary biology" and one that is likely to be very productive. In a forgivable pun he ends by saying, "Let our minds take wing."

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