It's long been understood (based on fossils and morphology) that vertebrate limbs (i.e., arms and legs) developed from the paired pectoral and pelvic fins of fish. When you look at the fossil record from fish through amphibians to reptiles, you can see those fins gradually become larger and stronger and then turn into true limbs.
Scientists decided to test this theory with some genetic manipulation. The Shh ("Sonic hedgehog", not a joke) gene is critical to limb development, and a regulatory gene called ZRS controls how Shh is expressed. If you shut off ZRS in mice, for example, limbs come out malformed or missing. So they got some fish, and shut off the ZRS gene, expecting the paired fins to disappear. Instead, the single dorsal fin on the back disappeared.
So they went looking around the genome, and they found a copy of ZRS, dubbed sZRS, and shut it off, and lo and behold, the paired fins disappeared, too. sZRS is analogous to a region of the human LMBR1 gene already known to play a role in Shh expression. Anyway, so genetic confirmation of what we already knew, right? Cool, but nothing big.
What's really interesting is the insight it gives to a question many people have about evolution, which is, How do new traits get started? It's easy to see how our arms and legs are related to the legs of, say dogs, and it's not a stretch to imagine the legs of dogs are related to the paired fins of fish. But where did paired fins come from? What this suggests is that the original* purpose of Shh and ZRS was to create the dorsal fin in fish, and at some point, some lucky fish had a transcription error which gave it an extra copy of the ZRS, and it ended up with some extra fins. Those extra fins eventually became all the pectoral and pelvic fins - and arms and legs - we know today.
*For technical pedants: perhaps not original, per se, but the role that the genes played before they played any role in the development of paired fins/limbs. It's entirely possible that either gene - or both - filled some other role before dorsal fins developed.
Scientists decided to test this theory with some genetic manipulation. The Shh ("Sonic hedgehog", not a joke) gene is critical to limb development, and a regulatory gene called ZRS controls how Shh is expressed. If you shut off ZRS in mice, for example, limbs come out malformed or missing. So they got some fish, and shut off the ZRS gene, expecting the paired fins to disappear. Instead, the single dorsal fin on the back disappeared.
So they went looking around the genome, and they found a copy of ZRS, dubbed sZRS, and shut it off, and lo and behold, the paired fins disappeared, too. sZRS is analogous to a region of the human LMBR1 gene already known to play a role in Shh expression. Anyway, so genetic confirmation of what we already knew, right? Cool, but nothing big.
What's really interesting is the insight it gives to a question many people have about evolution, which is, How do new traits get started? It's easy to see how our arms and legs are related to the legs of, say dogs, and it's not a stretch to imagine the legs of dogs are related to the paired fins of fish. But where did paired fins come from? What this suggests is that the original* purpose of Shh and ZRS was to create the dorsal fin in fish, and at some point, some lucky fish had a transcription error which gave it an extra copy of the ZRS, and it ended up with some extra fins. Those extra fins eventually became all the pectoral and pelvic fins - and arms and legs - we know today.
*For technical pedants: perhaps not original, per se, but the role that the genes played before they played any role in the development of paired fins/limbs. It's entirely possible that either gene - or both - filled some other role before dorsal fins developed.
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