Gene discovery may reveal how scaly dinosaurs became feathery birds
It is finally becoming common knowledge to the general public that birds really are dinosaurs. What’s more, an ever-increasing number of discoveries gives us incredible insight into the form and diversity of feathers in various non-avian dinosaurs and early birds. We have a growing understanding of how feathers spread and changed in various lineages, their functions, and why they might have evolved in the first place but a fundamental gap remains in our understanding – how did they evolve?
Feathers are composed of keratin, which also makes up scales (and for that matter claws and parts of skin – and, indeed, our own hair) and they are both growth of the skin, so presumably they have some kind of shared evolutionary history – but that is about as far as researchers have got. There are a number of suggested pathways to get from scale to feather, but while some are well thought of, none are especially well thought of. Complicating the process are the odd patterns that evolution has thrown up from time to time.
Although modern birds have scaly feet, these are, in fact, very heavily modified feathers and not some hold-over from their dinosaurian ancestry. Various dinosaurs and early birds show that they were completely covered in feathers – even down to their toes – and had presumably exchanged scales for feathers at this point. Only later did “scales” reappear on the feet of birds, so sadly their own feathers cannot immediately tell us about how scales might have transformed.
In 2014, a new and very unusual dinosaur called Kulindadromeus was unearthed in Siberia, which appeared to show not just feather-like filaments, but that some of these were growing out of scales. Although this little animal is from a very different lineage that that which gave rise to birds, it suggests at least that some feather-like structures might have evolved directly from highly modified scales.
Enter a new study published last week (Wu et al., 2017), which looks at how scales and feathers form. The researchers took genes thought to be important in feather development and had them expressed in embryos of alligators and chickens during the development of scales and feathers respectively. They also identified some new genes that regulate the genes for development and changed their degrees of activity to further alter these changes.
In modifying these various genes, they were able to produce new kinds of modified scales. True, these were not feathers from alligators, but they do show that relatively simple changes to a few genes can cause the early development of scales even in modern alligators to produce things with much in common with the ancestral feathers that we see in early non-avian dinosaurs. In short, it’s not a massive step from these to something like a true early feather, not least when you consider that the alligator lineage split from that of birds over 250m years ago. Couple that with the fact than any early “proto-feathers” that conferred a benefit on their owners would be under natural selection to retain and refine them, and it’s not a big jump to suggest that feathers may have formed relatively easily.
Modification of the genes in the chickens produced a variety of feather forms, including those seen in various dinosaurs, so the gap from scale to feather is at least partly bridged from the other side by this study. Some of these too match the theoretical steps between feathers and scales that have been proposed, and this suggests that we have been on the right track for a while, if lacking in some supporting evidence which is now being discovered.
Clearly there is some way to go, but the identification of only a few genes, still present in modern crocodilians such as alligators, that can be changed quite easily into rather simple feather-like filaments is most intriguing. There is a huge diversity in the feathers seen in the dinosaurs on the run-up to birds, as well as those in other dinosaur lineages and their relatives the flying pterosaurs, and this shows the possibilities when it comes to filaments. A few more tweaks to these genes might show us the complete pathways from scale to feather – and then the real question of how birds became birds and first grew their feathers can, perhaps, be answered.
References:
Wu, P., Yan, J., Lai, Y.C., Ng, C.S., Li, A., Jiang, X., Elsey, R., Widelitz, R., Bajpai, R., Li, W.H. and Chuong, C.M., 2017. Multiple regulatory modules are required for scale-to-feather conversion. Molecular Biology and Evolution.