Margen
In 2009, on a giraffe opened up for the documentary Inside Nature's Giants, the British biologist Richard Dawkins lifted with both hands, in front of the camera, a nerve that runs all the way down the animal's neck, enters the chest, loops underneath the aorta — the great vessel leaving the heart — and climbs back up to the throat. Some 4.6 metres of cable to connect two points a few centimetres apart.
It is called the recurrent laryngeal nerve, and you have two of them. The left one takes the long way round, under the arch of the aorta. The right one settles for looping around the artery that feeds the right arm, higher up, and is therefore a good deal shorter. Both branch off the vagus nerve, the great nervous motorway linking the brain to the internal organs, peel away from it inside the chest and head back up to the larynx. The word recurrent means exactly that: it retraces its steps.
An engineer asked to wire a giraffe would solve it in three seconds. The order leaves the brain and the larynx sits a hand's width from the brain: short cable, next job. Nobody designed this. It was built on top of whatever was already plugged in and working.
The origin lies in an ancestor shaped like a fish. That animal had no neck: brain, heart and gills sat almost on top of one another, and the nerve passed behind one of the gill vessels in a straight line, no detour at all. It worked, so it was inherited.
Then came millions of years of lengthening necks. The heart and its great vessels stayed in the chest; the head moved away. The nerve remained hooked underneath the vessel and stretched: every new centimetre of neck added two, one going down and one coming back up. Unhooking it would have required a generation of animals with no voice, and that generation was never born.
In your body the detour is short and still it shows up in the consulting room. The nerve climbs tucked in behind the thyroid gland, along the groove between the windpipe and the oesophagus. That is why thyroid surgery is a delicate business and why one of its known risks is permanent hoarseness: if the scalpel touches that nerve, the vocal cord on that side stops moving. Surgeons look for it and keep an eye on it throughout the operation, sometimes with a device that warns them when the nerve is close.
In the giraffe, which stands five and a half metres tall, the bill rises to those 4.6 metres. And not because its neck is stuffed with extra bones: the giraffe has seven cervical vertebrae, the same as you. They are just longer. In a human being the left nerve's detour measures about fifteen centimetres. In the giraffe, thirty times that.