'Reverse Evolution' of Perching Toe Sheds Light on Dinosaur Gait

It's not exactly a novel idea that birds evolved from dinosaurs. A wealth of evidence saying as much was published last year alone. With this in mind, scientists have recently managed to successfully "reverse evolve" the perching toe of chicken embryos, shedding light on the gait of dinosaurs so long ago.

The findings were published in the journal Scientific Reports.

A unique adaptation in the foot of birds is the presence of a thumb-like opposable toe, which allows them to grasp and perch - aptly named the perching toe. However, in their dinosaur ancestors, this toe was small and non-opposable, and did not even touch the ground - somewhat resembling dogs and cats that have been declawed. But understanding how this unique trait evolved over time has eluded scientists.

That is, until a team from the University of Chile realized that the embryonic development of birds provides a parallel of this evolutionary history.

To reveal the underlying mechanisms of the perching toe, the researchers conducted reverse evolution in bird embryos. They found that the toe starts out like their dinosaur ancestors, but then its base (the metatarsal) becomes twisted, making it opposable. Specifically, this crucial twisting takes place shortly after the embryonic musculature of the toe is in place.

In addition, compared to other digits, this toe matured at a much later stage and retained many rapidly dividing stem cells for a much longer period. Such immature cartilage is highly plastic and easily transformed by muscular activity.

"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," Gunter Wagner, an evolutionary geneticist and professor at Yale University, who was not involved in the study, said in a news release.

Only a few experiments are known to recover dinosaur traits in birds (such as a dinosaur-like shank and tooth-like structures). And now, the undoing of the perching toe can be added to that short list.

But this study is not only significant in that it was able to retrieve this dinosaur-like toe, but also because it helps provide insight into its "true developmental mechanics," and the forces of embryonic muscular activity involved.

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