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June 29, 2015

Why Do Mammalian and Reptilian Eardrums Look the Same?

Image by Bogdanov of Wikipedia
From an evolutionary standpoint, reptiles/birds and mammals are as different as one can get among vertebrate animals. Given just how different the two animal classifications are, it is strange why the eardrums look SO similar. So similar that there has been a debate whether the ear in reptiles/birds and mammals evolved from a common ancestor OR... did the ear evolve independently to a similar appearing structure.

Based on Japanese research, the argument is now settled... the ear evolved independently into a similar appearing eardrum, a phenomenon called convergent evolution.

How did they make this determination?

As we have already known, the ear in mammals (i.e. humans) develop from the lower jaw. What was unclear is whether this was also true in reptiles. If there was a common ancestor, the ear would also be found to develop from the lower jaw in reptiles. If the ear does not, that would imply convergent evolution.

So what the scientists did was inhibit the lower jaw formation in fetal mice and chickens.

As expected, no ear structures developed in mice lacking a lower jaw.

But in chickens, although no lower jaw developed, what happened was that TWO upper jaws from which TWO sets of eardrums appeared.

How cool is that?

Reference:
Developmental genetic bases behind the independent origin of the tympanic membrane in mammals and diapsids. Nature Communications 6, Article number: 6853 doi:10.1038/ncomms7853
Fauquier blog
Fauquier ENT

Dr. Christopher Chang is a private practice otolaryngology, head & neck surgeon specializing in the treatment of problems related to the ear, nose, and throat. Located in Warrenton, VA about 45 minutes west of Washington DC, he also provides inhalant allergy testing/treatment, hearing tests, and dispenses hearing aids. Google+ Christopher Chang, MD Bio

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