Evrim Yazgin
Cosmos science journalist
New analysis linking dinosaurs and Sonic the Hedgehog might explain how “proto-feathers” became the complex appendages, bird feathers.
It’s widely accepted that birds are dinosaurs. Modern feathered friends are the evolutionary descendants of the family of dinosaurs known as theropods – bipedal dinosaurs which includes Velociraptor and Tyrannosaurus rex.
But when, how and why feathers first emerged continues to be debated.
The first proto-feathers in dinosaurs would have existed at least 200 million years ago. Some palaeontologists even suggest that feathers may have their origins further back to a common ancestor between dinosaurs and pterosaurs 240 million years ago.
Proto-feathers – structures that mark the evolutionary starting point of feathers – would have been simple, cylindrical filaments. They wouldn’t have had the barbs, barbules and follicles of modern feathers.
Proto-feathers would have provided thermal insulation and ornamentation before evolving over millions of years into more complex structures that enabled flight.
The new study published in PLOS Biology looked at molecular signalling pathways between cells, in the development of hair, scales and feathers in embryos of modern animals.
One group are called Hedgehog signalling pathways because of the resemblance of the signalling molecule’s resemblance to a hedgehog. These pathways are critical in cell differentiation in embryos.
Mammals have 3 types of Hedgehog pathways – Desert (Dhh), Indian (Ihh), and Sonic (Shh), of which Sonic is the best studied. Dhh and Ihh are named after species of hedgehog, while Shh is named after the Sega video game character Sonic the Hedgehog.
These pathways have also been observed in other parts of the animal kingdom.
“Since the Shh pathway plays a crucial role in feather development, we wanted to observe what happens when it is inhibited,” says co-author Rory Cooper. Cooper is a researcher at the University of Sheffield, UK, who completed this work as part of Michel Milinkovitch’s Lab for Artificial & Natural Evolution (LANE) at the University of Geneva in Switzerland.
Cooper and Milinkovitch had previously shown that stimulating the Shh pathway into chicken embryos led to foot scales transforming into feathers.
This time they blocked the Shh pathway on the 9th day of embryonic development – just before the development of feather buds on the wings.
They found that the buds formed unbranched – resembling what scientists believe the early stages of proto-feathers would have looked like.
By the 14th day of development, normal feather formation had resumed in the embryos. The chicks hatched with patches of naked skin, but follicles were automatically reactivated eventually producing chickens with normal plumage.
“Our experiments show that while a transient disturbance in the development of foot scales can permanently turn them into feathers, it is much harder to permanently disrupt feather development itself,” Milinkovitch says.
“Clearly, over the course of evolution, the network of interacting genes has become extremely robust, ensuring the proper development of feathers even under substantial genetic or environmental perturbations. The big challenge now is to understand how genetic interactions evolve to allow for the emergence of morphological novelties such as proto-feathers.”
