Can You Hear Me Now?

27 September 2007

Sure, this Miocene whale ibone looks sick, but the reception’s crap.

The ice cream truck trolls down the street promising sweet frozen treats with doppler distorted midified Scott Joplin, as my neighbor hums along. The message is the medium, compressed and rarefied.

When our fishapod ancestors first flopped out onto land, they were up against some serious obstacles. One might think that breathing would have presented the greatest difficulty. But, in fact, our lobe-finned predecessors were probably quite comfortable with air breathing, much as modern lungfish are today.

But life on land poses significant impediments to even the most gifted aquatic air breathers. Air is approximately 800 times less dense than water–our ancestors must have felt freaking fat awriggling on the shore. Then there is the dessication issue. Chapped and clumsy, the earliest tetrapods must have seen dry land as a nice place to visit but not an ideal home.

[hopefully you are forgiving all of the rampant anthropomorphizing so far…much more follows]

The low density of air also makes it a piss-poor conductor of sound waves. The speed of sound in water is almost 1500 meters per second, compared to a sluggish 344 meters per second in air. Fish have a sophisticated sensory organ that is highly sensitive to compression waves (i.e. sound) among other things. However, this lateral line system is entirely useless on land and has been lost in most living tetrapods (but retained in some amphibians).

Early vertebrates also exapted their vestibular organs (used to determine position and orientation in the water) to detect acoustic waves. Many fish have small bony structures in their skulls, otoliths(click that link, it’s rad) that are sensitive to sound vibrations in the water. Some fish even use their swim bladder to aid in hearing. As the waves move from the liquid water/fish medium into the gassy bladder they refract up toward the brain. The swim bladder itself turns out to be an exapted lung!

All of these structures are great for hearing sound in the water, but all are probably worthless for detecting the subtle vibrations of our ethereal atmosphere. Dry land must have been a world of eerie silence for the first terrestrial vertebrates.

But, as might be suspected, tetrapods got to solving this problem right quick. Well, assuming you take a few tens of millions of years to be quick.

The challenge is catching aerial vibrations and funneling them down into that same vestibular bony labyrinth that was co-opted by early swimming vertebrates. As noted, air isn’t very dense, so you need some delicate tissue in order to do this.

When Bell designed his and telephone, he faced precisely the same challenge. Morse had already worked out how to translate a physical signal into an electrical one. But a tapping finger is considerably more palpable than a whispered, or even shouted, word.

Following the lead of others, Bell first experimented with a vibrating reed that used a magnet to translate physical vibrations into electrical impulses. After some modest success, he ultimately hitched his reed to a thin membrane which was much more sensitive to the subtle vibrations of the human voice (think kazoo).

Incidentally, the Italian emigrant Antonio Meucci did precisely the same thing when designing a remote communication device, the “telettrofono” for his invalid wife about twenty years before Bell.

The most recent claimant to the title of first membrane-based telecommunication device predates Bell and Meucci by only about 260 million years. Writing in a recent PLOS (open access snap!) paper, entitled “Impedance-Matching Hearing in Palezoic Reptiles: Evidence of Advanced Sensory Perception at an Early Stage of Amniote Evolution” Johannes Müller and Linda Tsuji document the earliest reported organism with what we might call an “aerial ear.”

Middle ear reconstruction of Macroleter poezicus. Figure 3 from Müller and Tsuji 2007.

For what it’s worth, Macroleter is a non-pareiasaurian parareptile from the Permian Mezen River Basin of Russia. Along with some its fellow bomb ass parareptilian cousins, Macroleter appears to have constructed one of the first ‘tympanic’ ear by stretching a skin membrane (pink above) across the back end of the skull. The vibrations detected by this ear were piped into the skull via the stapes (yellow above) which might be viewed as analogous to Bells vibrating reed by the generous reader.

Interestingly, the authors relate this specialized hearing structure to other adaptations related to a “dim-light” (i.e. nocturnal) lifestyle, most notably an enlarged eye socket. Even more intriguingly, they suggest that these adaptations may be related to survival of terrestrial organisms across the Permo-Triassic extinction event.

While Macroleter and co. may have been among the first to develop the tympanic ear, they were hardly the only ones.

frog-tymp.JPG

We’re exapting all the way to the tangled bank.

REFERENCES

Müller, J. and L Tsuji; 2007. Impedance-Matching Hearing in Paleozoic Reptiles: Evidence of Advanced Sensory Perception at an Early Stage of Amniote Evolution. PLoS ONE 2(9): e889 doi:10.1371/journal.pone.0000889

Wikipedia Invention of the Telephone: en.wikipedia.org/wiki/Invention_of_the_telephone

2 Responses to “Can You Hear Me Now?”

  1. laelaps Says:

    Now you’re just making me look bad, Neil. Excellent work on this one (I’ve been planning to cover ear evo for a while, although admittedly more in the context of the mammalian ear from jaw bones). Keep up the good work!

  2. Neil Says:

    Thanks Brian. I had originally planned on going into mammalian ears (I mean, talk about exaptation), but I rushed this one to make the Boneyard deadline. You may have noticed I ended rather abruptly…so take the ball and run!


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