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In fish, the homologous nerve innervates the gills.

The difference is that in fish, the aorta doesn't come out of the heart. The heart pumps blood to the gills, and from the gills the oxygenated blood flows into the fish-aorta, which runs alongside the spine and carries blood to the rest of the body. Any nerve running in a straight line from the brain to the gills would pass between several major gill blood vessels to get there.

So once our ancestors developed lungs, we didn't need the blood from our hearts to run all the way up to where our gills used to be before reaching the aorta. Instead, one of the blood vessels that used to service the gills gets repurposed into our "aortic arch", and stays down close to the heart so it can connect up with the rest of the aorta down there.

So the nerve that in fish (and tetrapod embryos) runs directly from the brain to the gills (or what we've repurposed gill tissue into, various structures around our ears and throats) ends up between the heart and the aortic arch. So since the aortic arch stays down in the torso, and our necks develop in between our skulls and our torsos, the recurrent laryngeal nerve ends up stretched all the way down to where the aortic arch ends up, and then comes all the way back to where the "gills" are.

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Handy side note: aortic aneurysms occuring in the area of this aorta/nerve arrangement often result in the person’s voice becoming hoarse. Along with other symptoms this can help differentiate chest pain associated with heart attacks from chest pain secondary to aneurysm / dissection.

Thanks for your excellent reply!

Coincidentally, the paper you cited also explained how isolated situs inversus may form; I'm really grateful you posted the link.

Feel free to correct my understanding: As my understanding, lung did not evolve from gill; rather, it's a structure unrelated to gill and innervated differently and recurrent laryngeal nerve then repurposed to innervate various structures on mammalian neck instead.

Never knew that. Thanks!

Correct. Lungs evolved completely separately from gills. Our ancestors repurposed gills into inner ears, the cartilage of the trachea, and the hyoid bone, all structures still around our jaws/throat.

(Technically, our even earlier ancestors repurposed parts of their gills into the first jaws, too.)

Early in embryonic development, however, our gill arches and their major blood vessels still appear…and the recurrent laryngeal nerve grows out to that embryonic gill tissue. Later in development, the “gills” stay basically in place and become those other structures in the ear and throat. So instead of innervating gills, the nerve is now innervating…all the things we repurposed our gills into.

Thanks again for your excellent reply!

> As my understanding, lung did not evolve from gill

Correct. And as a side note, lots of people will tell you that lungs evolved from swim bladders, but in fact it appears that the reverse is true. Early lungs developed as a pouch off the digestive tract in early fish living in low oxygen waters, and only later did they develop into the specialized swim bladders that many fish have today.

That's something new for me. Thanks!

Side question though, can fish absorb oxygen with the swim bladder?

Some fish have dual use lung/swimbladders, but a great many common fish have lost the respiration use entirely and the swimbladder often has no open connection at all remaining to the digestive tract. These fish often use other methods if they want to breath air, like the labryinth organ in bettas.

Even fish with totally closed swim bladders can absorb gasses from them into the blood...and push gasses into them from the blood. That's how they inflate and deflate them.

Thanks again for your reply!

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