The Phrenic Nerve
The Phrenic Nerve controls the contraction of the diaphragm. Similar to fish, it comes out of the brain stem from the base of the skull and goes all the way down the neck and chest cavity to the diaphragm, leaving it exposed and vulnerable. While this works well for fish, it’s a poor setup for mammals, including humans. Subsequently, scientists believe this is a leftover evolutionary trait from fish.