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“The genetic program to make wrists and digits is really, really ancient. It goes all the way back to the common ancestor of all fish,” Gehrke said. “There are hypotheses that maybe some of these genetic switches evolved later on to create digits and wrists and what we know as hands. But our work shows these genetic switches are very, very old and present in ancient fish, and functioned in a really similar way” in all four-limbed creatures.

But mice have far more genetic switches related to hands and fingers than gar have for their fins. Gehrke said future research will look into what happens when the switches that develop digits in mice are inserted into fish genomes. “Does that produce something more digit-like?” he asked.

It’s not that one ever thought to test this out using genetics before. But previous researchers had focused on Teleost fish, an extremely large and diverse classification that includes nearly all commercial and sport fish (as in, almost every kind of fish that probably comes to your mind). Inserting switches from those fish into mice showed no genetic activity, Gehrke said.

For their research, Gehrke and his colleagues looked at a different kind of fish, gar, whose genome was recently sequenced. This ancient fish split off from Teleost fish long ago, and hasn’t evolved as rapidly as the Teleost fish, meaning it serves as a better representation of a common ancestor, Gehrke said.

“We knew from fossils that wrists and digits first showed up in the water in a fish-like organism, but we didn’t know where those bones came from,” Gehrke said.