By Sarah Kaplan
The current diversity of feathers, fur and scales is part of what made their origins so mystifying to scientists. There are almost no known intermediate forms to illustrate how they might be related to one another. That’s largely because the features are so fragile — while bone and teeth can be preserved as fossils, delicate skin appendages are usually lost to time. In the absence of physical evidence from the past, scientists try to interpret the present, for instance by studying developing embryos, for clues to how traits evolved.
Early on in embryonic development, feathers and fur look startlingly similar — both begin as tiny, thick accumulations of cells on the skin known as anatomical placodes. This shared morphology indicates that the features have the same evolutionary roots, which would seem to make sense, since birds and mammals evolved from a common ancestor some 320 million years ago.
But that ancestor was also the predecessor of modern reptiles; in fact, reptiles and birds are far more closely related than birds and mammals. Yet reptile scales develop very differently than feathers and fur — or they seemed to, at any rate. Not a lot of scientists study reptile embryos, Milinkovitch noted (“model species” like fruit flies and mice tend to get most of the attention), but those who did generally couldn’t find evidence of anatomical placodes.
The placodes — dark blue spots corresponding to groups of cells expressing a specific early developmental gene — are visible on the embryonic skin of (from left to right) a mouse, a snake, a chicken and a Nile crocodile. Each of these placodes will develop into a hair, scale, or a feather. (UNIGE 2016 Tzika, Di-Poï, Milinkovitch)
That left with biologists with two possible explanations, Milinkovitch noted, neither of which was particularly satisfying.
“Either the placode was ancestral for everyone and then it was lost multiple times in independent lineages of reptile … or birds and mammals invented placodes independently,” he said. The second possibility seemed particularly unlikely because research had revealed that the same exact gene, called EDA, controlled placode development in both groups.
That’s where things stood when Di-Poï began parsing the genome of the naked bearded dragon his adviser had brought back to the lab. He pinpointed the mutation that prevented scales from developing, only to discover that it was EDA — the same gene responsible for feathers and fur.
That prompted the duo to take a closer look at the embryos of normal bearded dragons during development. They realized that the tiny creatures did have anatomical placodes, they just appeared and dispersed differently than the versions biologists are accustomed to seeing in mammals and birds.
“You have to look at the right places at the right time otherwise you don’t see them,” Milinkovitch said. “Now of course, once you know this it’s much easier to to find them because you know where to look and when to look, but before people didn’t know and they overlooked them.”
Eventually, he and Di-Poï identified placodes in several species of snake, lizard and crocodile.
“They obviously inherited this from a common ancestor,” Milinkovitch said.
“That makes sense, ecologically speaking, when you think about, ‘what is the innovation of amniotes?'” he continued, using the term to describe creatures like reptiles, birds and mammals, whose fetuses develop in membrane-bound amniotic sac that allows their mothers to lay fertilized eggs on land (or nurture them inside the uterus, as most mammals do).
Unlike amphibians and lobe-finned fish, amniotes aren’t anchored to water by the need to lay their eggs there. That meant it was worth investing in adaptations that allowed us to live entirely terrestrial lives, like skin or scales that keep us from drying out. Hundreds of millions of years after reptiles, birds, and mammals diverged from this original amniote, we united by the outcomes of this innovation.
“They are extremely different morphologically, but if you look past that you can see the homology,” Milinkovitch said. “That’s the beauty of it.”