In the last few decades we’ve realized that birds descended from theropod dinosaurs, and that the evolution of feathers preceded the evolution of flight. Indeed, many biologists still consider birds to be dinosaurs, since the group “dinosaurs” leaves out some of the descendents of dinosaurs—birds. Regardless of whether one adheres to this convention, we know from genetic and fossil evidence that birds are united with reptiles in one group: the Archosauria, which includes the common ancestor of crocodilians and birds, and all the descendants of that ancestor up to and including modern birds and crocodilians. That means that crocodilians are the living reptiles most closely related to birds (if, that is, you don’t consider birds to be reptiles).
Comparing birds with archosaurs, then, can tell us something about the evolutionary changes that produced our feathered comrades. And this is what was done in a new paper in Nature by Bhart-Anjan Bhullar et al. What they showed is that the evolution of certain features of birds that distinguish them from their reptilian relatives, namely their large heads, eyes, braincase, and beak—probably evolved through paedomorphosis. Paedomorphosis is the evolutionary process in which a descendant retains the juvenile state of an ancestor, retaining certain juvenile features of the ancestor into adulthood and reproductive age. Paedomorphosis is one example of heterochrony: the process (discussed by Steve Gould in his book Ontogeny and Phylogeny) whereby evolution operates by changing the timing of development.
The classic example of paedomorphosis is the axolotl (Ambystoma mexicanum), a Mexican salamander that becomes sexually mature while retaining the juvenile features that are lost when other salamanders mature, including the gills and the caudal fin along the back:
In the new Nature paper, the authors realized that the juvenile stages of ancient archosaurs resembled the adult stages of birds, their descendants, implying that perhaps the evolution of birds involved a process of heterochrony. Birds show smaller snouts and bigger eyes and braincases than those of their adult archosaurian relatives, but young archosaurs have the same big heads, big eyes, and short snouts as do adult (and juvenile) birds.
Here’s figure from the paper showing the difference between juveniles and adults of three living and fossil species: a crocodile (b), a primitive dinosaur (Coelophysis; c), and the famous Archaeopteryx (d; a stem group bird, known in Darwin’s time but now realized to be a transitional form between dinosaurs and birds). Note the similar change in morphology between adult and juvenile in the alligator and dinosaur, but the general retention of juvenile morphology in Archaeopteryx:
To show this pattern statistically, the authors did some fancy manipulations known as Principal-Component Analysis (PCA), parsing out those groups of morphological changes that unite related species. To do this they used both juveniles and adults of ancient archosaurs and theropods, modern birds, and alligators. I needn’t go into detail, but what they got was confirmation of the impressionistic pattern shown above: archosaurs (including alligators) all undergo elongation of the face, diminution of the eye orbit, and reduction of the neurocranium during development. Modern birds don’t change nearly so much, and early (fossil) birds change, as expected, in an intermediate way (for example, they undergo a bit of skull elongation). As the authors note:
Evidence for heterochrony is clear. Whereas adults of taxa distantly related to birds (non-eumaniraptorans) cluster together, basally branching bird relatives (eumaniraptorans) cluster with the embryos and youngest juveniles of other non-avian archosaurs (Figs 2 and 3), with the more crownward avialan Confuciusornis [an early bird] nearly identical to embryos and particularly close to the perinate enantiornithine (Figs 3d and 4).
The figure shows the configuration of the embryonic alligator skull, in green, superimposed on the adult alligator skull in red to the left and on the adult skull of the primitive bird Confuciusornis to the right. You can see how much more the the juvenile alligator resembles the adult bird than the juvenile alligator resembles the adult alligator. It’s hard to avoid the conclusion that, during the evolution of birds from their reptilian ancestors, evolution acted to retain the reptilian juvenile skull into adulthood.
Here, by the way, is a wonderful fossil of Confuciusornis, showing its paired tail feathers, and a reconstruction of what the adult looked like:
So maybe birds’ heads evolved simply by stopping the genetic changes that made the heads longer (and the eyes and braincases relatively smaller) during development of their ancestors. Why would evolution act this way? One answer seems obvious: birds need to rely heavily on vision to fly, and they need large brains to harbor the vision centers as well as the neuronal apparatus enabling them to fly.
Note that the postcranial skeleton (the rest of the bird) shows no signs of padeomorphosis. Indeed, some features of the bird body are peramorphic with respect to dinosaurs: that is, they seem to show an extension (rather than a truncation) of development past the adult stage. The authors say that’s okay because the brain is modular (i.e. genetically independent) of the rest of the skeleton, and can evolve independently, so the paedomorphosis could have involved genetic changes affecting only the skull. Something similar happened during human evolution: as we can see from our early australopithecine relatives, our postcranial skeletons evolved much faster than our cranium. Lucy, for example (an A. afarensis) has basically an apelike skull sitting atop a skeleton which is much more similar to that of modern humans.
There are other things in the paper that will be of more interest to specialists (e.g., when the various stages of paedomorphic change occurred and what happened with the beak), but I’ve given enough, I think, to show that once more an intriguing evolutionary hypothesis was sitting under our noses all along. Juvenile archosaurs have been around for years, but nobody thought to compare them to early and modern birds.
Bhullar, B.-A. S., J. Marugan-Lobon, F. Racimo, G. S. Bever, T. B. Rowe, M. A. Norell, and A. Abzhanov. 2012. Birds have paedomorphic dinosaur skulls. Nature, advance online publication, doi:10.1038/nature11146.