A unique adaptation in the foot of birds is the presence of a thumb-like opposable toe, which allows them to grasp and perch. However, in their dinosaur ancestors, this toe was small and non- opposable, and did not even touch the ground, resembling the dewclaws of dogs and cats. Remarkably, the embryonic development of birds provides a parallel of this evolutionary history: The toe starts out like their dinosaur ancestors, but then its base (the metatarsal) becomes twisted, making it opposable. Brazilian researcher Joâo Botelho, working at the lab of Alexander Vargas at the University of Chile, decided to study the underlying mechanisms. Botelho observed that the twisting occurred shortly after the embryonic musculature of this toe was in place.
“This is one of the clearest examples of how indirect the morphological consequences of genetic change are mediated”, Gunter Wagner, evolutionary geneticist and professor at Yale.
Bird embryos move a lot inside the egg during development, and the onset of movement at this toe coincided with the twisting of its base. Botelho also demonstrated that in this toe, genes of cartilage maturation were expressed at a much later stage than other digits: It retains many rapidly dividing stem cells for a much longer period. Such immature cartilage is highly plastic and easily transformed by muscular activity.
These observations suggested the toe is twisted as a result of mechanical forces imposed on it by the embryonic musculature. Definitive proof, however, would come from experiments. When Botelho applied Decamethonium bromide, a pharmacological agent capable of paralyzing embryonic musculature, the result was a non-opposable toe with a straight, non-twisted base identical to that of their dinosaur ancestors. Only a few experiments are known to recover dinosaur traits in birds (such as a dinosaur-like shank and tooth-like structures). The undoing of the perching digit is thus an important new addition, and the results have now been published in Scientific Reports, an open-access journal of the Nature Publishing Group.
The significance of this experiment, however, goes beyond the fact that a dinosaur-like toe is being retrieved. Evolutionary research often centers on mutations, but the development and evolution of the perching toe cannot be understood without the forces of embryonic muscular activity. The study is described as “true developmental mechanics” by Gunter Wagner, an evolutionary geneticist and professor at Yale. “This is one of the clearest examples of how indirect the morphological consequences of genetic change are mediated. The experiments prove that interactions about organ systems channel the directions of organismal evolution.”