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What is the evolutionary reason behind sharing the same path for food and air?

For example, we have a nose and lungs, yet they are linked by the pharynx, which is shared with the mouth and oesophagus.

Wouldn't it make more evolutionary sense for the nose + lungs to have their own "path", while the mouth and stomach have a separate system? This would prevent "choking", allow for breathing while eating/drinking, etc.

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Note that this does not go for all animals. For instance, the respiratory system in insects is based on a network of "tubes" (tracheae) that penetrate the entire body. This is ventilated through spiracle valves which are distributed over the body of the insect, so in insects the paths of food and oxygen are separated. –  fileunderwater Sep 9 '13 at 7:45

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While it might make more logical sense to have separate passageways for air and food/water, this did not happen in evolutionary history due to the peculiarities of lung development. Vertebrate lungs develop as an outpouching of the gut tube, which itself has a very long evolutionary history (probably homologous among all deuterostomes).

In the image below, the trachea has budded off the esophagus. Further lung development will involve the differentiation of main, secondary, and tertiary bronchi as well as the alveoli, etc.

enter image description here

Additional images and description at http://www.uoguelph.ca/zoology/devobio/210labs/lung1.html

Air-filled organs like lungs and swim bladders actually have a broad distribution going way down into fishes. Human (and other mammalian) lungs are likely homologous with ancestral air-filled organs of fishes. We can't be certain, but there is no reason to think that mammalian lungs are a de novo evolutionary innovation. So our airway and food-ways are tied together developmentally and are constrained by evolutionary history.

As for breathing and eating (swallowing) at the same time, most mammals can do this, and baby humans can as well.

Like most anatomical systems that seem as though they could have evolved better, this system works most of the time and has worked well enough for hundreds of millions of years.

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Incidentally, providing two paths to the lungs could be useful if one is blocked (e.g., from a bad cold) and might allow different specializations to be applied to the different paths to achieve a better balance of trade-offs (e.g., support for panting [higher cross sectional area passage] might conflict with support for smelling [narrower passage]?). –  Paul A. Clayton Sep 10 '13 at 16:42

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