One of my favorite “proofs” of evolution is the recurrent laryngeal nerve (RLN)—the nerve that innervates the larynx from the brain, helping us speak and swallow. It takes a very circuitous course, looping from the brainstem down around the aorta and then back up to the larynx. Here’s its course in humans:
It’s a prime example of “bad design”, that is, of the ham-handedness of any creator that was responsible for designing organisms. Of course, we aren’t designed, but evolved from very different ancestors. That’s why our bodies are full of glitches and kludges, and this nerve is one of them. It’s much longer than it need be, taking a tortuous route several feet longer than the direct path from brain to neck.
I’ve talked about the evolutionary reasons for this many times; you can see the full explanation in WEIT or read about it here. This diagram shows how it worked: the nerve used to line up with a blood vessel, both servicing the gills of our fishy ancestors. When the vessel moved backwards during evoution, the RLN was constrained to remain behind it, still retaining its connection to the larynx, which evolved from a gill arch. The nerve could not “break” to attain the shortest route, for that would not be possible by natural selection: it would interrupt the nerve transmission and be maladaptive. Click to enlarge:
As I also point out in WEIT, this poor design reaches ludicrous heights, so to speak, in giraffes, whose long neck makes the RLN take a 15-foot detour:
Do remember that this nerve consists of a bundle of nerve cells, each of which travels the entire length of the nerve. Thus the giraffe must have nerve cells (including the axons) about fifteen feet long. That is a very long cell!
But there are even longer. In a new paper now in press at Acta Palaeontologica Polnoica (free at the link, reference below), anatomist Mathew J. Wedel simply thought a bit more about the nerve: what would it look like in animals with even longer necks? Those, of course, would include the sauropod dinosaurs. And in some of them, like the gynormous Supersaurus, the recurrent laryngeal nerve, and its included cells, could have been longer than 28 meters (92 feet). Here’s a diagram from Wedel’s paper, which is very clear and well written (he also explains it in a post at his website, Sauropod Vertebra Picture of the Week):
There’s little doubt, by the way, that dinosaurs did have recurrent laryngeal nerves. All living tetrapods do, whether they be amphibians, reptiles, or mammals. This suggests very strongly that the RLN is an ancestral condition in tetrapods, resulting from their mutual evolution from fishy ancestors.
But wait, there’s more! (Does this sound like an ad for Ginsu knives?) Even longer cells can be found in living organisms, in particular the blue whale, Balaenoptera musculus. Wedel speculates, reasonably (I doubt that dissections have been done), that nerves running from the whale’s brain to the flukes of its tail might be 30 meters or more (98 feet) in length.
But there’s still more! The sauropod dinosaur Amphicoelias fragillimus was, arguably, as long as 49-58 meters (161-190 feet) from snout to tail. The nerves innervating the tail could have been only a meter or so shorter than that. Now we’re not sure about the neck length of A. fragillimus: its RLN could have been 38 meters long (124 feet) or more; remember that the nerve has to run the length of the 19-meter neck twice.
So what was the longest cell in the history of life? Our best guess is 40-50 meters (130-160 feet!) for nerves innervating the tail in the longest sauropods:
- Pain signals traveling along the RLN in sauropods could have moved extremely slowly. Wedel notes that “unmyelinated vagal afferent fibers have conduction velocities as low as 0.5 m/s, and some unmyelinated fibers are present even in the recurrent laryngeal nerve of the giraffe. Unless selected for faster response, similar unmyelinated fibers would have taken almost a full minute to relay ‘slow pain’ signals to the brain of Supersaurus!” Wedel notes, though, that an injury to the dinosaur’s throat could have been detected more quickly from damage to nerves in the skin of the throat, whose path to the brain was only a meter long.
- A bigger problem: nerve cells must transport material from the cell body itself to the tips of the axons. It’s done through a process called “axoplasmic streaming”, which carries different molecules at different rates. Neurotransmitters and enzymes, for example, travel 200-400 mm (8-16 inches) per day, but the transport of some proteins is slower: 0.1 -1.0 mm per day! As Wedel notes, “Even at 1 mm/day, slow axoplasmic streaming would take more than four decades to move proteins from the nerve cell body to the axon terminals in the longest neurons of large whales. This, of course, is not feasible, and Wedel suggests that either axoplasmic streaming must be much faster in whales (and in dinosaurs), or there is some other way they transport proteins through nerve cells. Here’s a fertile field for cell biologists!
Regardless, the recurrent laryngeal nerve of the long-necked dinosaurs is not only a “monument of inefficiency,” as Wedel notes in his title, but an even better monument to evolution.
Wedel, M.J. 2011. A monument of inefficiency: the presumed course of the recurrent laryngeal nerve in sauropod dinosaurs. Acta Palaeontologica Polonica, in press. DOI:10.4202/app.2011.0019
From Wedel’s website: