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.

Eva Horvath-Papp, Leicester, UK

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.

Marc Verhaegen, Putte, Belgium

If we believe zoologist Desmond Morris, the shape of the human nose is one of many unique features of the human anatomy that lack obvious purpose, but that most probably exist to respond to stimulation in one way or another to facilitate sexual pair bonding.

In The Naked Ape he writes: "One [anatomist] has referred to it as a mere 'exuberant variation of no functional significance'. It is hard to believe that something so positive and distinctive in the way of primate appendages should have evolved without a function. When one reads that the side walls of the nose contain a spongy erectile tissue that leads to nasal enlargement and nostril expansion by vaso-congestion during sexual arousal, one begins to wonder."

In other words, apparently we have big noses to give our partners something else to notice or to play with.

Neil Fairweather, Risley, Cheshire, UK