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The Human Nose
THE NOSE KNOWS: Nose shapes in human ancestors and different populations of modern humans are molded by genes and the environment.
credit: KAUSTUBH ADHIKARI
Current concenus:
"Humans, unlike other apes, possess a projecting, external nose whose overall structure is reflected in a series of derived skeletal features including: nasal bone convexity, an internasal angle, lateral nasal aperture eversion, prominence and anterior positioning of a true anterior nasal spine rather than a nasal tubercle, an acutely angled subnasal alveolar clivus, and an expansion of the breadth of the nasal bones relative to that of the piriform aperture. This constellation of features first appears with the emergence of Homo erectus in early Pleistocene Africa. Although nasal projection undoubtedly evolved in the context of craniofacial and dental reduction during hominin evolution, it also likely represented an adaptive response to the need for respiratory moisture conservation in an arid environment. More specifically, it indicates a shift to increasingly prolonged bouts of activity in such dry and open environments. While the adaptive basis for nose form is questioned by some, substantial evidence from clinical studies, mathematical airflow, temperature/humidity modeling, and quantitative studies showing departure from neutral evolutionary expectations support its adaptive basis. Nonetheless, the precise role of the external nose independent of the internal nasal cavity in terms of conditioning respiratory air and ameliorating moisture loss is incompletely understood." [CARTA. link]
The following passage was printed in New Scientist (number 2782, 16 October 2010 p 69) as an answer to a Last Word question:
'"Oi, big nose!"
Why do humans evolve external noses that don’t seem to serve any useful purpose – our smelling sensors are inside the head. Our nose is vulnerable to damage, and the majority of primates and other mammals manage with relatively flat faces. Traditional explanations are that the nose protects against dry air, hot air, cold air, dusty air, whatever air, but most savannah mammals have no external noses, and polar animals such as arctic foxes or hares tend to evolve shorter extremities including flatter noses (Allen’s Rule), not larger as the Neanderthal protruding nose.
Response A:
The answer isn't so difficult if we simply consider humans like other mammals.
An external nose is seen in elephant seals, hooded seals, tapirs, elephants, swine and, among primates, in the mangrove-dwelling proboscis monkeys. Various, often mutually compatible functions, have been proposed, such as sexual display (in male hooded and elephant seals or proboscis monkeys), manipulation of food (in elephants, tapirs and swine), a snorkel (elephants, proboscis monkeys) and as a nose-closing aid during diving (in most of these animals). These mammals spend a lot of time at the margins of land and water. Possible functions of an external nose in creatures evolving into aquatic ones are obvious and match those listed above in many cases. They can initially act as a nose closure, a snorkel, to keep water out, to dig in wet soil for food, and so on. Afterwards, these external noses can also become co-opted for other functions, such as sexual display (visual as well as auditory) in hooded and elephant seals and proboscis monkeys.
But what does this have to do with human evolution?
The earliest known Homo fossils outside Africa such as those at Mojokerto in Java and Dmanisi in Georgia are about 1.8 million years old. The easiest way for them to have spread to other continents, and to islands such as Java, is along the coasts, and from there inland along rivers. During the glacial periods of the Pleistocene the ice age cycles that ran from about 1.8 million to 12,000 years ago most coasts were about 100 metres below the present-day sea level, so we don't know whether or when Homo populations lived there. But coasts and riversides are full of shellfish and other foods that are easily collected and digested by smart, handy and tool-using "apes", and are rich in potential brain-boosting nutrients such as docosahexaenoic acid (DHA).
If Pleistocene Homo spread along the coasts, beachcombing, wading and diving for seafoods as Polynesian islanders still do, this could explain why Homo erectus evolved larger brains (aided by DHA) and larger noses (because of their part-time diving). This littoral intermezzo could help to explain not only why we like to have our holidays at tropical beaches, eating shrimps and coconuts, but also why we became fat and furless bipeds with long legs, large brains and big noses.
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Posted by: Marc Verhaegen, Putte, Belgium.
   
Response B:
We must rid ourselves of the notion that the nose is specifically made for smelling. The nasal passages serve many functions, but are independent of olfaction. The nose carries out other well-known tasks such as the filtration of small particles from the air to
prevent them entering the lungs - which results in the nasal contents children seem so fascinated with.
The nasal cavity also serves to moderate the temperature of incoming air. This is most often appreciated in non-equatorial regions, where air temperature can be well below body temperature and may cause shock to the system. The blood vessels that enrich the nasal passage provide a steady stream of warm blood, which helps to bring the temperature of incoming air closer to body temperature.
This is possibly also the reason why humans of Caucasian descent that originally evolved to live in the colder northern hemisphere tend to have longer, more protruding noses than people living near the equator, where the air temperature varies less, and does not dip close to freezing.
While these theories are quite widely quoted and have merit, perhaps the most interesting explanation comes from the school of thought that humans once had the sea or other bodies of water as their predominant habitat. The"aquatic ape theory" draws parallels with other aquatic mammals such as dolphins and hippopotamuses to explain our significant loss of hair, and abundance of subcutaneous fat compared to our primate brethren.
Outward facing nostrils would be quite a hindrance to inhabiting areas that involved being submerged in water for long periods of time. But by protruding outwards and forcing the nostrils to face the floor, it is possible for a person holding their head upright to bob their head under the water without water entering the airways - air trapped in the nose prevents any water entering. This gives the bearer of an external nose an advantage of 1 or 2 minutes under water without having to resurface.
Furthermore, the outer side of the nose has an excellent shape for swimming forwards. Its streamlined surface ensures that while swimming head down and forwards at a speed, all incoming water flows with the least amount of resistance around the face. Moreover, due to the conical shape of the nose, all water rolls away from the nostrils, diminishing the chance of it entering the airways.'
Post-publication addendum:
The importance of downwards facing nostrils in diving is evident when viewing instances of chimpanzees trying to dive. There have been a few reports of chimps diving, and in all the examples seen, the chimps are grabbing at their face in order to cover their nostrils whilst submerge themselves.
It is quite clear that unless they have a mask or assistance to prevent water entering their nose, they risk drowning from water entering the nose. In all the cases, the chimp in question is seen blocking the nostrils with one hand, severely limiting the amount that can be achieved through the use of hands. They can carry fewer things with only one hand, and may not be able to complete some tasks requiring two hands, such as opening a jar for example, or more close to real life, opening a mollusc or moving rocks in order to retrieve something. Moreover, they would not be able to carry a tool and direct themselves through the propulsion of their other free arm.
Humans on the other hand are the only great ape with a protruding nose, which directs water away from or around the nostrils. It allows forward swimming without water entering the nose and risking drowning. This allows humans to be able to use both hands to complete tasks and to manoeuvre themselves in water. It is clear that chimps would never be able to do free-diving hunting like the Bajau people,both from a physiological standpoint of being able to hold their breath, as well as having hands free for direction, propulsion and carrying.
Posted by: Eva Horvath-Papp, Leicester, UK
How Your Nose Got Its Shape
Climate variation has sculpted our schnozzes since the earliest humans evolved, but environmental pressures can’t explain everything.
The Scientist [link]
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