The recurrent laryngeal nerve (RLN) is a branch of the vagus nerve that supplies muscles of the larynx. There are two – right and left, in the human body.
The nerves emerge from the vagus nerve at the level of the arch of aorta, and then travel up the side of the trachea to the larynx. The right and left nerves are not symmetrical. The left nerve loops under the arch, and the right nerve travels upwards.
The extreme detour of the recurrent laryngeal nerves, about 4.6 metres (15 ft) in the case of giraffes, is evidence of evolution. The nerve’s route would have been direct in the fish-like ancestors of modern tetrapods, traveling from the brain, past the heart, to the gills (as it does in modern fish).
Over the course of evolution, as the neck extended, and the heart became lower in the body, the laryngeal nerve was caught on the wrong side of the heart.
Natural selection gradually lengthened the nerve by tiny increments to accommodate, resulting in the circuitous route now observed.
Adapted from Recurrent_laryngeal_nerve, Wikipedia
Why Evolution Is True, by Jerry A. Coyne, Viking Adult, 2009
One of nature’s worst designs is shown by the recurrent laryngeal nerve of mammals. Running from the brain to the larynx, this nerve helps us speak and swallow. The curious thing is that it is much longer than it needs to be. Rather than taking a direct route from the brain to the larynx, a distance of about a foot in humans, the nerve runs down into our chest, loops around the aorta and a ligament derived from an artery, and then travels back up to connect to the larynx. It winds up being three feet long. In giraffes the nerve takes a similar path, but one that runs all the way down that long neck and back up again: a distance fifteen feet longer than the direct route! When I first heard about this strange nerve, I had trouble believing it. Wanting to see for myself, I mustered up my courage to make a trip to the human anatomy lab and inspect my first corpse. An obliging professor showed me the nerve, tracing its course with a pencil down the torso and back up to the throat.
This circuitous path of the recurrent laryngeal nerve is not only poor design, but might even be maladaptive. That extra length makes it more prone to injury. It can, for example, be damaged by a blow to the chest, making it hard to talk or swallow. But the pathway makes sense when we understand how the recurrent laryngeal nerve evolved. Like the mammalian aorta itself, it descends from those branchial arches of our fishlike ancestors. In the early fishlike embryos of all vertebrates, the nerve runs from top to bottom alongside the blood vessel of the sixth branchial arch; it is a branch of the larger vagus nerve that travels along the back from the brain. And in adult fish, the nerve remains in that position, connecting the brain to the gills and helping them pump water.
During our evolution, the blood vessel from the fifth arch disappeared, and the vessels from the fourth and sixth arches moved downward into the future torso so that they could become the aorta and a ligament connecting the aorta to the pulmonary artery. But the laryngeal nerve, still behind the sixth arch, had to remain connected to the embryonic structures that become the larynx, structures that remained near the brain. As the future aorta evolved backward toward the heart, the laryngeal nerve was forced to evolve backward along with it. It would have been more efficient for the nerve to detour around the aorta, breaking and then re-forming itself on a more direct course, but natural selection couldn’t manage that, for severing and rejoining a nerve is a step that reduces fitness. To keep up with the backward evolution of the aorta, the laryngeal nerve had to become long and recurrent. And that evolutionary path is recapitulated during development, since as embryos we begin with the ancestral fishlike pattern of nerves and blood vessels. In the end, we’re left with bad design.