Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Did our ancestors breathe through their ears?

19.01.2006


A fossil fish skull from Latvia that researchers from Uppsala University, Sweden, describe in this weeks issue of Nature shows that the earliest land animals probably breathed through their ears.



"It looks as if the first step in the evolution of the middle ear had nothing to do with hearing. Our forebears developed ears in order to breathe through them," says Professor Per Ahlberg.

The human sense of hearing is based on the interaction of two different organs: the inner ear and the middle ear. The inner ear contains sensory cells that capture sound vibrations and send them on as nerve impulses to the brain. The middle ear is an ingenious mechanical audio amplifier that captures the weak sound vibrations in the air with a membrane (the eardrum), amplifies them with a leverage system (ear bones) and sends them on to the inner ear. Without the middle ear, the inner ear would not function.


All vertebrates have inner ears, but the middle ear exists only in land animals. Fish dont need middle ears since sound vibrations are stronger in water and easily pass through the body of a fish. The construction of the middle ear differs, however, among different groups of land animals: mammals have an eardrum and three ear bones (hammer (malleus), anvil (incus), and stirrup (stapes)), while birds, reptiles, and frogs have only one ear bone (stirrup) that connects the eardrum directly to the inner ear. But it is questionable whether the eardrums in mammals, reptiles, and frogs are identical or whether they arose independently of each other.

A comparison with fish muddies the picture even further: instead of middle ears, fish have a little gill, the blow-hole, that isnt covered by an eardrum but rather forms an open canal between the throat and the outside of the head. The equivalent of the stirrup, the hyomandibula, supports the gill lid but has no contact with the inner ear. Neither the hyomandibula nor the blow-hole plays any role in hearing.

These differences make it difficult to understand how the middle ear arose. How could evolution change both the structure and function of the fishes‚ blow-hole so radically? Did the earliest land animals have a sound amplifying middle ear at all? The earliest fossil land vertebrates or tetrapods, like Acanthostega from Greenland (that lived roughly 360 million years ago), had a stirrup that was in contact with the inner ear, but it was large and clumsy and appears not to have been connected to the eardrum. They also had a couple of round Œoutlets‚ in the rear edge of the skull: in modern frogs the corresponding outlet is the fastening point for the eardrum, but in fish it is the site of the outer opening of the blow-hole. This combination of characteristics has led to the hypothesis that the earliest land animals still had open blow-holes and perhaps breathed through them.

The Uppsala scientists‚ new data strongly support this hypothesis. The information comes from the skull of a Panderichthys from Latvia, the fossil fish that is closest to the emergence of land animals. It has been known that Panderichthys had a hyomandibula, and it was generally assumed that its blow-hole was of the normal fish type. But this is not the case: in actual fact the hole is similar to the middle ear‚ of a tetrapod like Acanthostega. Since the hyomandibula of the Panderichthys had no contact with the inner ear, its blow-hole could hardly have had a sound-amplification function.

"Thus the transformation of the form of the blow-hole must have been caused by another driving force than the improvement of hearing," says Per Ahlberg.

Compared with closely related fish, the blow-hole in Panderichthys has a considerably larger diameter and is furthermore both shorter and straighter. It looks like an adaptation to active breathing (of either water or air) through the blow-hole, compared with ordinary‚ fish in which only a small portion of breathing water passes through this hole. A similar adaptation can be seen in modern rays, which have a very large blow-hole.

Since the middle ear‚ in the earliest tetrapods has the same form as the blow-hole‚ in Panderichthys, it seems likely that they retained the breathing function. But in tetrapods the gill lid is gone and the hyomandibula is transformed into a primitive stirrup. The fact that the stirrup has contact with the inner ear indicates that a rudimentary hearing function had also been added.

"We can speculate about how this came about. The blow-hole of a fish can be closed by a valve muscle on top. If an early tetrapod did the same thing, a truly enclosed middle ear was temporarily created, where the stirrup, which probably supported the wall of the middle ear, could forward vibrations from the middle ear to the inner ear. When the hearing function eventually became more important, the blow-hole was permanently closed by an eardrum," reasons Per Ahlberg.

The article is being published in Nature on January 19.

Anneli Waara | alfa
Further information:
http://www.uu.se
http://www.nature.com

More articles from Life Sciences:

nachricht Link Discovered between Immune System, Brain Structure and Memory
26.04.2017 | Universität Basel

nachricht Researchers develop eco-friendly, 4-in-1 catalyst
25.04.2017 | Brown University

All articles from Life Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Making lightweight construction suitable for series production

More and more automobile companies are focusing on body parts made of carbon fiber reinforced plastics (CFRP). However, manufacturing and repair costs must be further reduced in order to make CFRP more economical in use. Together with the Volkswagen AG and five other partners in the project HolQueSt 3D, the Laser Zentrum Hannover e.V. (LZH) has developed laser processes for the automatic trimming, drilling and repair of three-dimensional components.

Automated manufacturing processes are the basis for ultimately establishing the series production of CFRP components. In the project HolQueSt 3D, the LZH has...

Im Focus: Wonder material? Novel nanotube structure strengthens thin films for flexible electronics

Reflecting the structure of composites found in nature and the ancient world, researchers at the University of Illinois at Urbana-Champaign have synthesized thin carbon nanotube (CNT) textiles that exhibit both high electrical conductivity and a level of toughness that is about fifty times higher than copper films, currently used in electronics.

"The structural robustness of thin metal films has significant importance for the reliable operation of smart skin and flexible electronics including...

Im Focus: Deep inside Galaxy M87

The nearby, giant radio galaxy M87 hosts a supermassive black hole (BH) and is well-known for its bright jet dominating the spectrum over ten orders of magnitude in frequency. Due to its proximity, jet prominence, and the large black hole mass, M87 is the best laboratory for investigating the formation, acceleration, and collimation of relativistic jets. A research team led by Silke Britzen from the Max Planck Institute for Radio Astronomy in Bonn, Germany, has found strong indication for turbulent processes connecting the accretion disk and the jet of that galaxy providing insights into the longstanding problem of the origin of astrophysical jets.

Supermassive black holes form some of the most enigmatic phenomena in astrophysics. Their enormous energy output is supposed to be generated by the...

Im Focus: A Quantum Low Pass for Photons

Physicists in Garching observe novel quantum effect that limits the number of emitted photons.

The probability to find a certain number of photons inside a laser pulse usually corresponds to a classical distribution of independent events, the so-called...

Im Focus: Microprocessors based on a layer of just three atoms

Microprocessors based on atomically thin materials hold the promise of the evolution of traditional processors as well as new applications in the field of flexible electronics. Now, a TU Wien research team led by Thomas Müller has made a breakthrough in this field as part of an ongoing research project.

Two-dimensional materials, or 2D materials for short, are extremely versatile, although – or often more precisely because – they are made up of just one or a...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Expert meeting “Health Business Connect” will connect international medical technology companies

20.04.2017 | Event News

Wenn der Computer das Gehirn austrickst

18.04.2017 | Event News

7th International Conference on Crystalline Silicon Photovoltaics in Freiburg on April 3-5, 2017

03.04.2017 | Event News

 
Latest News

Scientist invents way to trigger artificial photosynthesis to clean air

26.04.2017 | Materials Sciences

Ammonium nitrogen input increases the synthesis of anticarcinogenic compounds in broccoli

26.04.2017 | Agricultural and Forestry Science

SwRI-led team discovers lull in Mars' giant impact history

26.04.2017 | Physics and Astronomy

VideoLinks
B2B-VideoLinks
More VideoLinks >>>