It’s long been understood that hair and feathers are cut from the same cloth, as both shield fragile skin. But this connection has puzzled scientists for many years: birds and mammals aren’t evolutionary “sisters”—they didn’t split in one fell swoop on the tree of life.
Scientists at the University of Virginia may have figured out an answer to this perplexing conundrum. New research from the university suggests the first vertebrates to colonize dry land carried with them a souvenir from the sea: scales.
This is not the first time that researchers have investigated this fuzzy topic. A vaguely reptilian common ancestor is theorized to have given rise to both mammals and birds. But for decades, scientists could not find the link, leading to the idea that mammals and birds may have separately “invented” these coverings, despite their base similarities.
Then, two years ago, scientists from the University of Geneva and the University of Helsinki stumbled upon a rare, scaleless bearded dragon in a pet market. Curious, the researchers purchased the animal and brought it to their laboratory, where they sequenced its DNA. This uncovered a startling fact: the same developmental structures that give rise to feathers and hair may control the production of reptilian scales. It turned out errors in a gene called ectodysplasin-A, which in mammals and birds contributes to the growth of hair and feathers, respectively, also leaves bearded dragons in the buff. What’s more, when the group looked at lizard, snake, and crocodile embryos through a powerful microscope, they saw what appeared to be the same tiny, raised bumps that mature into hairs and feathers in other species.
However, some scientists were reserved in interpreting the data, with one pathologist assessing the finds as only an “incremental update” in a 2016 interview with Science. One common gene may not be definitive to prove common ancestry, and anatomy with very different developmental origins can often give rise to similar-looking structures; for instance, wings arose separately in insects and birds.
The research from the University of Virginia, published Tuesday in the journal eLife, has the potential to put the debate to rest. Rifling through even older branches of the evolutionary tree, UVA biologist Andrew Aman examined the development of lab-grown zebrafish, a favorite tool among biologists due to the fact that these fish are transparent for much of their adolescence. But their ghostly visage has also potentially led researchers astray.
“Zebrafish skin, including the bony scales, is largely transparent and researchers probably have simply looked past the scales to the internal structures,” Aman explains in an interview with Fariss Samarrai at UVA Today.
But upon closer inspection, Aman and his colleagues found that the genes that dictate the formation of zebrafish scales are identical to those previously identified in mammals, birds, and reptiles, indicating that these three latter groups develop in ways similar to not only each other, but also a far more ancient ancestor, dating back as far as 385 million years. Gratifyingly, ectodysplasin-A, mutations in which had disrobed bearded dragons, was among the genes studied in Aman’s work.
The new research paints a plausible portrait of descent: our marine ancestors, decked out in scales, swam the seas. Somewhere along the way, a few hapless individuals crested the waves and found shelter on land, where weighty scales, which are built around a base made of bone, proved cumbersome. As time went on, lightweight hairs and feathers replaced this heavy marine armor, shedding their reliance on the hard, internal scaffolding. In reptiles, scales stuck around, but, like hair and feathers, were no longer reliant on bone.
Knowing that hair, feathers, and scales sprout from the same basic genetic pathways helps close a gap in our understanding of our own evolutionary roots. But the researchers believe their findings also carry modern-day implications. A better understanding of early growth and regeneration likely will only improve our understanding of human developmental disorders, including birth defects.
Illuminating the common origins between fish scales and human hair equips scientists with an additional model in which to study important diseases. As a perk, zebrafish are already a fixture in research facilities around the world—and that certainly beats combing the world’s pet shops for (un)bearded dragons in the name of science.