Excellent paper, thanks for this, it helps explain why human noses differ between populations, but it doesn't explain why humans evolved external noses in the first place (vs chimps, gorillas etc.).

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.
The answer isn’t so difficult if we simply consider humans like other mammals.
An external nose is seen in, for instance, elephant seals, hooded seals, tapirs, elephants, swine, and among primates the mangrove-dwelling proboscis monkeys. Various functions – often mutually compatible – have been proposed, such as sexual display (male hooded and elephant seal, proboscis monkey), manipulation of food (elephants, tapirs, swine), snorkel (elephants, proboscis monkey) and nose-closing aid during diving (most of these animals). These mammals spend a lot of time at the edge between land and water. Possible functions of an external nose in incipient aquatics are obvious: nose closure, snorkel, keeping the water out, digging in wet soil for food, etc. Afterwards, these external noses can also become used for other functions, such as sexual display (visual as well as auditive) in hooded and elephant seals and proboscis monkeys.
What does this have to do with human evolution?
The earliest known Homo fossils outside Africa – such as Mojokerto on Java, and Dmanisi in Georgia – are about 1.8 million years old. The easiest way to spread to other continents, and to islands such as Java and Flores, is along the seacoasts and from there inland along rivers. During the Glacials of Pleistocene epoch (the “Ice Ages”, from about 1.8 to 0.01 million years ago) most seacoasts were about hundred metres below the present-day sea level, so we don’t know whether or when Homo populations lived there, but seacoasts 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 brain-specific nutrients such as poly-unsaturated fatty acids (for instance, docosahexaenoic acid or DHA). If Pleistocene Homo spread along the coasts, beachcombing, wading and diving for seafoods as Sea Gypsies, Ama divers and Polynesian islanders still do, this could explain why Homo erectus got larger brains (DHA) and larger noses (parttime diving). Apparently, at the coasts they used stone tools to crack open shellfish, crabs and coconuts and to butcher stranded whales (Dungo V site, Baia Farta, Angola), at the riversides they butchered bovids that were trampled and drowned when crossing rivers during the trek, and in wetlands they killed or injured and let bleed to death large herbivores that were hindered in their movements in the mud.
This littoral intermezzo – wading, swimming, beachcombing, long-distance walking and running along the water – could help understand 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.
("Oi, big nose!" New Scientist 2782 p 69 Lastword 16 Oct 2010 Marc Verhaegen)