Hearing came before ears existed (as sight must have come before eyes)

Of course all our senses lie in the brain and not in the sensors receiving the input from our surroundings. So while our ears, eyes, skin and taste-buds detect certain physical characteristics and convert them into electrical signals, it is our brains which interpret the electrical signals they receive as being sound or colour or taste or heat. Our sense organs are merely transducers, converting some physical characteristic into an electrical signal. So how did the brain evolve and develop the interpretation “software” for these signals before the sensors had even been developed?

Two new papers show that Lungfish and salamanders can hear, despite not having an outer ear or tympanic middle ear. 

  1. C. B. Christensen, H. Lauridsen, J. Christensen-Dalsgaard, M. Pedersen, P. T. Madsen. Better than fish on land? Hearing across metamorphosis in salamanders. Proceedings of the Royal Society B: Biological Sciences, 2015; 282 (1802): 20141943 DOI: 10.1098/rspb.2014.1943
  2. K. Knight. Lungfish hear air-borne sound. Journal of Experimental Biology, 2015; 218 (3): 329 DOI: 10.1242/%u200Bjeb.119487

Aarhus University Press Release:

Lungfish and salamander ears are good models for different stages of ear development in these early terrestrial vertebrates. Two new studies published in the renowned journals Proceedings of the Royal Society B and The Journal of Experimental Biology show that lungfish and salamanders can hear, despite not having an outer ear or tympanic middle ear. The study therefore indicates that the early terrestrial vertebrates were also able to hear prior to developing the tympanic middle ear. …….

……. However, available palaeontological data indicate that the tympanic middle ear most likely evolved in the Triassic period, approximately 100 million years after the transition of the vertebrates from an aquatic to a terrestrial habitat during the Early Carboniferous. The vertebrates could therefore have been deaf for the first 100 million years on land. ….. 

…… They studied the hearing of lungfish and salamanders by measuring auditory nerve signals and neural signals in the brainstem as a function of sound stimulation at different frequencies and at different levels. Surprisingly, the measurements showed that not only the terrestrial adult salamanders, but also the fully aquatic juvenile salamanders – and even the lungfish, which are completely maladapted to aerial hearing – were able to detect airborne sound despite not having a tympanic middle ear. By studying the animals’ sense of vibration, the researchers were able to demonstrate that both lungfish and salamanders detect sound by sensing the vibrations induced by sound waves. …..

My experience in the engineering world suggests that there must be a connection  – a feedback loop – between the “software” interpreting the signals in a brain and the development of the transducers generating the signals. For example, rotating equipment (turbines, compressors or pumps) are routinely plastered with pressure and temperature and stress (really just pressure) sensors. But the 4 – 20 mA signals they generate have to be interpreted by software in a brain. Over the last 40 years I have observed that simple interpretation software has led to improved (more focused and more accurate) sensors which has in turn given even more sophisticated software.

And so it must have also been with our senses. Primitive brains must have interpreted some “sound waves” picked up incidentally as “sound”. Some feedback loop must have then provided the impetus for the evolution of a “sound detector”. The improved sensor would then have increased the sophistication of the interpretation in the brain and given rise to further development of the sensors. Today our ears detect pressure waves of frequency between 20 and 20,000 Hz and convert them into electrical nerve signals interpreted by the brain as sound. Evolution is really not about pro-active selection of advantageous characteristics but of deselection of those not fit enough to aid survival. Evolution has nothing to do with the selection of the “best” or even of the “fittest” characteristics but is all about deselection of those having an insufficient fit. Of course in a competitive environment between individuals, those with “advantageous characteristics” would surely have helped in the culling – directly or indirectly – of the less fit. But that begs the question as to why we cannot hear ultrasound? Was the ability to hear ultrasound of no survival benefit? Was it too much for the “software”? Or was the audible range just a compromise between range on the one hand and intricacy of the sensor on the other?

There must have been a similar start to the development of sight. The incidental or accidental detection of certain frequencies of electromagnetic radiation must have led to a feedback loop between the interpretation software in the brain and the development of suitable sensors. And now our eyes detect electromagnetic radiation of frequency between 430 and 790 terraherz (TH) and convert them into electric signals which are sent to the brain for interpretation. We find benefit in cameras which can “see” uv and infrared light. But it is not an ability that has evolved in our eyes.

I begin to think that in considering evolution we must distinguish between external forces which direct the death of unfit species (environmental changes mainly) and the internal forces within the individuals of a species which leads to “deficient” individuals being “deselected”. And the feedback loop between the brain and our sensory organs – which is no doubt still operating – is probably one such internal force.

To put it crudely, our ears and our eyes are as good as they are because those individuals who had worse ears or eyes could not survive to reproduction. And our ears and eyes are not any better than they are because being any better does not contribute to any increased survival and reproduction.

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