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A gene responsible for the development of fins in a primitive fish also helped shape the hands, feet and wings of every land animal alive today.
Researchers studying the Australian lungfish Neoceradotus found one of its fin-sprouting genes also guides the growth of digits in land vertebrates—those creatures with backbones.
The finding, to be detailed in an upcoming issue of the Journal of Experimental Zoology, adds to growing evidence that digits in humans and other land creatures are the equivalent of fin bones in fish. It is yet another example of evolution tweaking what already works to generate novel traits.
"People have found comparable genes and gene-expression patterns in the fins of ray-fin fishes and also sharks, so it seems like the pattern goes very, very deep in vertebrate history," said study team member Zerina Johanson, a paleontologist at the Natural History Museum in London.
The Australian lungfish is the only living member of a group of fish called lobe-fins, which is considered the closest living relatives of land animals. It is a so-called "living fossil," because it has survived virtually unchanged since first appearing in the fossil record 100 million years ago.
The development of fingers and toes in embryos of land animals is closely linked to a gene called Hoxd13. This gene orchestrates a series of developmental steps involving the sequential release of certain proteins that affects the outer part of the limb and the digits but not the arm bones. It was once thought that digit development was unique to tetrapods, creatures that have, or once had, fingers and toes.
The new findings suggest this is not the case. Johanson and her colleagues found that the genes involved in creating the Australian lungfish's fins made proteins in a nearly identical pattern as in tetrapods, by acting on the small fin bones but not the rest of the limb.
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"Because of the similarities, we can say that fish fins have similar structures to tetrapod digits, [and that] tetrapod digits are no longer unique to the group," Johanson told LiveScience.
And because whales and birds descended from creatures with fingers and toes—hoofed mammals and dinosaurs, respectively—their flippers and wings are also evolutionarily linked to fish fins, she added.