I Accept the Theory of Evolution
The recurrent laryngeal nerve (RLN) originates near the top of the brain and connects to the muscles of the larynx (voice box). This nerve allows us to speak and to swallow.
To connect the brain to the larynx, the nerve needs to be 30 centimetres long (at most). In reality, the RLN is more than 90 centimetres long. The RLN travels down the spinal cord all the way to the upper chest. From there, the RLN exits the spinal cord a little below the shoulder blade. The RLN then loops under the aorta (your body’s main artery) and travels back up to the neck, where it finally reaches the larynx (because the RLN intertwines like that with the large heart vessels, surgeons must be very careful not to damage it when they are operating on the heart).
So why is the RLN so messed up?? As always, the answer lies in our evolutionary history. The nerve originated in ancient fish, so all modern vertebrates have it. In fish, the nerve connects the brain to the gills, which were the ancestors of the larynx. However, fish don’t really have necks, their brains are tiny, they don’t have lungs, and their hearts are more like muscular hoses than pumps like ours. Consequently, a fish’s central circulatory system, located mostly in the space behind its gills, is quite different to ours.
In fish, the nerve makes the short trip from the spinal cord to the gills in a predictable and efficient route. Along the way, however, it weaves through some of the major vessels that exit the fish heart, the equivalent of the branching aortas of mammals. This weaving works fine for fish anatomy and allows for a compact and simple arrangement of nerves and vessels in a very tight space. Unfortunately, this paved the way for an anatomical absurdity that would begin to develop as fish evolved into the tetrapods that would eventually give rise to humans.
During the course of vertebrate evolution, the heart began to move farther back as body forms took on a distinct chest and neck. From fish to amphibians to reptiles to mammals, the heart inched farther and farther away from the brain. But the gills did not. The RLN should not have been affected by the changing position of the heart—except that it was intertwined with the vessels. In the slow and gradual evolution from the biology of a fish to the biology of mammal, there was no way for the nerve to magically jump from one side of the aortic arch to the other The RLN got stuck and was forced to grow into a large loop structure in order to travel from the brain to the neck.
(Giraffes, with their long necks, have it even worse that us. Their RLN is 480 centimetres longer than it needs to be!)
To connect the brain to the larynx, the nerve needs to be 30 centimetres long (at most). In reality, the RLN is more than 90 centimetres long. The RLN travels down the spinal cord all the way to the upper chest. From there, the RLN exits the spinal cord a little below the shoulder blade. The RLN then loops under the aorta (your body’s main artery) and travels back up to the neck, where it finally reaches the larynx (because the RLN intertwines like that with the large heart vessels, surgeons must be very careful not to damage it when they are operating on the heart).
So why is the RLN so messed up?? As always, the answer lies in our evolutionary history. The nerve originated in ancient fish, so all modern vertebrates have it. In fish, the nerve connects the brain to the gills, which were the ancestors of the larynx. However, fish don’t really have necks, their brains are tiny, they don’t have lungs, and their hearts are more like muscular hoses than pumps like ours. Consequently, a fish’s central circulatory system, located mostly in the space behind its gills, is quite different to ours.
In fish, the nerve makes the short trip from the spinal cord to the gills in a predictable and efficient route. Along the way, however, it weaves through some of the major vessels that exit the fish heart, the equivalent of the branching aortas of mammals. This weaving works fine for fish anatomy and allows for a compact and simple arrangement of nerves and vessels in a very tight space. Unfortunately, this paved the way for an anatomical absurdity that would begin to develop as fish evolved into the tetrapods that would eventually give rise to humans.
During the course of vertebrate evolution, the heart began to move farther back as body forms took on a distinct chest and neck. From fish to amphibians to reptiles to mammals, the heart inched farther and farther away from the brain. But the gills did not. The RLN should not have been affected by the changing position of the heart—except that it was intertwined with the vessels. In the slow and gradual evolution from the biology of a fish to the biology of mammal, there was no way for the nerve to magically jump from one side of the aortic arch to the other The RLN got stuck and was forced to grow into a large loop structure in order to travel from the brain to the neck.
(Giraffes, with their long necks, have it even worse that us. Their RLN is 480 centimetres longer than it needs to be!)
56-60, F
