Hairy Humans and Scaly Lizards Have More in Common Than You Think

Lizard scales and feathers seem like they're a long way from human hair. But it turns out that fur, feathers, and scales all evolved from one common ancestor, no matter how different these adornments seem: they can all be traced back to a lizardlike creature that roamed the Earth some 310 million years ago.

"In my lab, we try to understand the biological and physical mechanisms responsible for the emergence of the complexity and diversity of life," evolutionary geneticist Michel Milinkovitch of the University of Geneva told me over the phone. He's an author on a new study, published Friday in Science Advances, that describes an evolutionary link between fur, feathers, and scales.

This question has actually been hotly debated for years. Fur, feather, and scales are all "very different structures," said Milinkovitch, who's also affiliated with the Swiss Institute of Bioinformatics. "It's difficult to see how you could modify something ancestral into something as different as hair, and a feather."

And, unlike the teeth or bones of ancient creatures, these soft appendages aren't well-preserved in the fossil record, so it's difficult to trace their evolution in that way.

To complicate the picture even more, when mammals and birds grow in embryo, their hair and feathers develop from a similar primordial structure called a "placode"—a thickening of the epidermis with columnar cells that leads to the expression of certain genes. Biologists have wondered for a long time how exactly to explain this connection, because birds and mammals evolved from different forks on the evolutionary tree.

And reptile scales were thought not to come from an anatomical placode at all. That left some thinking that birds and mammals must have independently developed placodes as they evolved into the feathery, furry creatures that populate our world today.

In 2015, a Yale University team published work showing that certain molecular signatures are expressed during the development of alligator scales, hair, and feathers. That got everyone riled up again about whether these signatures could suggest a common origin, or just that the same genes were being used to develop very different traits.

"It isn't crazy to think that some genes are being used to make [similar] structures, but don't come from the same ancestor," Milinkovitch told me. It happens in nature all the time: Hox genes, which are found in many creatures, from humans to fruit flies, can lay out the organization of different parts in different creatures, he explained.

In this new study, Milinkovitch and his co-author, Nicolas Di-Poï, demonstrate that scales in reptiles actually do develop from a placode that has the same anatomical and molecular signatures as those in birds and mammals. It's just that nobody had apparently ever seen it before. They discovered this by using cutting-edge molecular and other techniques to analyze the skin of crocodile, snake, and lizard embryos as they developed.

The authors also looked at the bearded dragon, which comes in three types: a normal, "wild-type" animal; another with one copy of a natural genetic mutation, inherited from its mother or father, that has smaller scales; and a third that is completely naked and without scales, because it has inherited two copies of the mutation, one from each of its parents.

In comparing their genomes, they found that naked lizards have a mutation on a gene called ectodysplasin-A (EDA). When that same gene is disrupted in humans and mice, it can cause problems with development of teeth, nails, and hair, Milinkovitch said.

"We realize now that lizards have anatomical placodes we didn't see before," Milinkovitch told me. (As for why other scientists hadn't seen these previously, "It's very transitory and you have to look at the right place at the right time," he said. "Now that we know where to look, and when, it's not too difficult.")

This research tells us something about where all of us hairy, furry mammals come from—from humans and hedgehogs to dolphins, which are born with whiskers on their snout, Milinkovitch noted—and it's also a reminder of an ancient connection we share with lizards and birds, no matter how distant and tenuous that link seems today.

It helps explain the incredible diversity of our world.

"We are trying to understand what I like to call the emergence of beauty," Milinkovitch said. "Because when things are complex and diverse, they are beautiful."