Where else would we put our spectacles?

Perhaps our noses serve a social function. They have erectile tissue in them and flare, turn red, and become slightly more erect when we're excited, angry, or upset. We don't grow hair on our noses, so they're free to convey our feelings.

My daughter Adèle (aged 6) says that without external nose we would not be good-looking. So maybe there is some mate selection going on here, as exemplified by the well known beauty of Cleopatra’s nose…

As stated, the question takes much for granted; in evolutionary  studies the word "why" is extremely treacherous. To propose alternatives to observed functions is generally trivial. Descriptions out of context aren't definitive; a male proboscis monkey might see humans as flat-faced primates. So would most primates; ask any chimp.

And human nose shape often differs ethnically in suggestively adaptive ways, e.g. many long-established subarctic populations tend towards various shapes of snub noses compared to those of temperate zones; such snubness might well reduce the liability to frostbite. Inhabitants of desert regions often have long, arched noses, perhaps better adapted to dry, hot, dusty air.

We could multiply exceptions and speculations; but really, our recent ancestors have been so mobile that most interpretations amount to Just-So stories.

The external human nose is not functionally trivial; imagine inverting or just amputating it and walking through rain or high wind. It reduces respiratory vulnerability in heat, cold, dirt, or pathogen-ridden conditions and contributes to our heat-exchange and water conservation.  

The vulnerability is slight compared to the problems arising even if we flatten the face without amputation. Ask  any vet about the problems in bulldogs and long-haired show cats!

So don't sniff too loudly and don't feed the tapirs.

Jon

One need look no further than the Aquatic Ape Theory (AAT), (or one of the many excellent books by Elaine Morgan), for an explanation as to why our streamlined noses evolved, along with nearly all the other main differences between ourselves and other apes, (eg: hairlessness, bipedal motion, a descended larynx, a diving reflex, tear glands, sweat glands, speech, subcutaneous fat layer, swimming babies, webbed skin fingers and toes (a receding trait), paddle like feet, and so on and so on), which are also many of the similarities we share with other aquatic species.

The AAT suggests that sometime between 5 and 8 million years ago our ape ancestors became cut off from the main continent of Africa and stranded on an island or narrow peninsula, where they were forced to adapt to a semi aquatic, coastal environment, living partially in shallow water and feeding on fish and seafood. By the time they were able to once more join the mainland and begin their migration into Africa, they had become hominids, walking on two legs, and probably naked.

Most of our aquatic features we still retain. Our streamlined noses help us when we dive into, or swim under water, as they deflect water to either side, away from our nostrils. Another species of ape that has a protruding nose - the aptly named 'proboscis monkey' - also spends a lot of time in the water and when they swim they hold their noses above the water, 'like miniature snorkels'*. This monkey is also the only other known species that frequently walks on two legs. Of course, they do this when they are wading through water as they live in swamp land most of the time.

Whales and dolphins' nostrils have evolved into blowholes, and moved to the top of their head, so that they can inhale /exhale more efficiently when they surface. Seals are able to close their nostrils when they dive. Many diving seabirds have lost the use of their nostrils altogether and can only breathe through their mouths.

Francesca Mansfield

Volos, Greece

*cf: The Aquatic Ape Hypothesis by Elaine Morgan