But now researchers examining an ancient (and still living) species of fish have found the common genetic link between fins and hands. They published their findings earlier this month in the Proceedings of the National Academy of Sciences.
Researchers examined the switches controlling genes related to the development of fins in the gar fish and hands in mice. Geneticists inserted three gar gene switches into developing mice and found the activity of the switches to be "nearly indistinguishable" to the mice's hand gene switches, said lead author Andrew Gehrke, a graduate student at the University of Chicago.
"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.
Now, researchers can point to the genetics of living fish to explain the aquatic origins of hands, wrists and digits, and can say that the appearance of hands likely came from subtle variations to an ancient genetic network.
Indeed, this new study follows recent fossil findings regarding the origins of limbs. Ten years ago, University of Chicago organismal biologist Neil Shubin discovered the fossils of the 375-million-year-old Tiktaalik roseae, a water-to-land transitional fish that had limb-like fins. (Shubin is also a co-author on the study with Gehrke 's study).
That fish is thought to be one of the keys to explaining the move from water to land for fish.
Earlier this year, the theory on how fish developed limbs was further illuminated when researchers announced findings after examining the back half of Tikataalik's fossil. That extinct animal actually had large, strong and mobile hind fins and an enlarged pelvis, suggesting that limbs in the front and rear evolved simultaneously. That went against the "front-wheel-drive" hypothesis that held limbs first evolved from front fins, while back fins stayed small , maybe even after fish transitioned to land.
"These are four-wheel-drive animals, not just front-wheel-drive only," paleontologist Ted Daeschler of the Academy of Natural Sciences said in January.
And here's Shubin explaining the importance of discovering the large back-fins in the Tiktaalik: