An article in the Washington Post,”Scientists solve the mystery of where feathers, fur and scales come from,” describes recent work reported in Science Advances that solves a longstanding puzzle regarding the first stages in the development of reptile scales.
It has been known for many years that mammals’ hairs and birds’ feathers both develop from small accumulations of cells in the skin, scattered all over the body, known as placodes. One of the key genes involved in initiating their development is called EDA. Of course, although their development commences in what are homologous structures, the subsequent developmental pathways leading to hair and feathers are very different, resulting in two skin appendages of very different design.
As mammals and birds share a common ancestor with reptiles, the puzzle has been why no placodes had been found in reptiles. The absence of placodes in reptiles was an anomaly, implying that reptile scales were non-homologous to hair and feathers. However, as the Science Advances paper reports, it turns out that reptiles do have placodes after all and that scales do develop from the same homologous structures and are indeed homologous to hairs and feathers.
The researchers studied a lizard, a bearded dragon, in which scales failed to develop. They found that the lizard had a mutant form of the EDA gene — again, the same gene that initiates the development of placodes in birds and mammals. This prompted them to look again, more carefully than before, for placodes in reptile embryos. As expected, they were able to show that placodes were indeed present and the development of scales does start from the same collections of cells as do hair and feathers. They rightly infer that the three appendages are derived from homologous structures.
Of course just because the starting cell condensations are homologous, it does not follow that the causal factors involved in the subsequent development of the three very different appendages is in any way similar or homologous. The developmental trajectories of these three appendages diverge markedly after the “placode stage” and the factors responsible for their divergent evolution remain to be identified.
What role did selection play? Was their origin slow or saltational? What was the role of internal constraints? To date, no one knows the answers to these key questions.
Moreover the origin of the homologs, the “starting points” or ground plans themselves, poses intriguing questions. How important was selection in the origin of the placodes and their scattering over the body surface? How much was due to emergent internal factors, and how much to external imposition by natural selection?
It is difficult to imagine that the placodes were gained one by one until they covered the body surface. What would be the selective advantage of one placode, and indeed what would be the selective advantage of cell condensations of this sort before the developmental trajectory leading to fur, feathers, and scales was in place?
Divergent developmental pathways from homologous starting points is a universal phenomenon in evolution. One well-known case is the development of the various classes of vertebrates: fishes, amphibians, mammals, and birds. These all pass through a homologous stage during early embryogenesis (the phylotypic stage) when their embryos are very similar. Judging by the evidence reported in the paper, the same is also true of the three skin appendages.
One thing is clear. The discovery that the three appendages develop from a similar starting structure — a placode — may solve the problem of where fur, feathers, and scales “come from.” However, the question of how they subsequently diversified is as mysterious as ever.