Why has our nose evolved with the nostrils facing down?

Question

For most animals, the whole bottom of their face sticks out and they have a snout with a mouth and nose at the end. But humans have a flat face and just a nose that sticks out. We used to have the whole face sticking out, but our jaws got smaller and smaller. Because our noses still needed space to do their job, they had to stay sticking out. People who come from very cold areas have bigger noses than people from where it is hot, so they can warm the air up more.

Why has our nose evolved with the nostrils facing down? Is it because we were water animals, and that would help with water coming in?

Answer 1

I have not been able to find any studies that look specifically at the genetic mechanism for nose development. I certainly do not know but I'll offer up two hypotheses.

My first hypothesis is that the elongated nose is a remnant of the elongated lower face of our ancestral species, as you note in your answer. Take a look at this brief YouTube video (1:17 long, the part relevant to this answer begins at 33 seconds) of a time-lapse composite showing the develop of the human face. Our face develops as two separate halves that meet together in the middle. That's what creates the philtrum under our nose.

Watch the especially carefully from the 54 second mark to 1 minute mark. Note that our nostrils are pointing directly forward and the overall region looks somewhat like a pig snout. The mouth is vertical aligned with the front of the nose. The overall face is much more rectangular and the eyes are on the sides of the face.

Then, the face begins to become rounder and flatten. The eyes move to the front of the face and the recesses our our eye sockets recede. The lower face below the nose recedes, becoming flatter. The only part that does not recede is the nose area. The flattening of the lower face may actually cause the nostrils to turn downward. Our nose does not develop outward from a flat face but instead is left behind as the rest of the face flattens. This would be my initial thought as the primary reason for the shape of the nose and direction of the nostrils.

My second hypothesis (and honestly independent of the comment by @Remib) is related to our sense of smell and taste. Although our ability to detect flavors is a complex process, the primary sense involved is smell (Shepherd 2006). Therefore, downward-pointing nostrils are located directly above the mouth so we get a good smell of the food just before it enters the mouth, perhaps enhancing our flavor-sensing ability.

Shephered, G.M. 2006. Smell images and the flavour system in the human brain. Nature 444: 316-321.

Note that these two hypotheses are not mutually exclusive. The downward pointing nostrils may be a remnant of our facial development but provides an (adaptive?) advantage for flavour perception.

Answer 2

Ive found that the study of this feature, the nose is far from complete.Although we see the evolution of other mammals and their faces has a great deal to do with there ability to survive different enviornments I believe it , our snozz is a genetic mutation in which allows us the ability to go from nonaquatic to aquatic with out needing a snorkel. ?.by defying why we have this thing on our face......some so large some so small I dont believe to have much to do with ancestial deviation of the jaw but the defying feature of the human race.....thank you

Answer 3

Marc Verhaegen, Putte, Belgium New Scientist 2782 p 69 Lastword 16 October 2010

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 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.