Sino-Swedish research smells out the truth about the human nose
Our very early ancestors could use their nose to smell but palaeontologists have shown that they could not breathe through it. The big question until recently was, how did our noses evolve? A pair of researchers from Sweden and China found a fossil that probes the history of the human proboscis.
It may seem hard to believe but humans – as land-based vertebrates, or ‘tetrapods’ which include mammals, birds, reptiles, and amphibians – originally descend from fish. Perhaps easier to believe, is the fact that, unlike us, fish cannot breathe through their nose. Something happened during our evolution to turn our noses from smelling to breathing apparatus.
|Complex evolutionary steps gave us human tetrapods the ability to smell and breathe with our noses|
Min Zhu of the Chinese Academy of Sciences’ palaeontology institute and Per Ahlberg of Uppsala University’s Evolutionary Biology Centre (SE) uncovered a fossilised piece of the palaeontological puzzle which clarifies, once and for all, how our inner nostril came into being. Their findings are being reported in Nature (Vol. 432, p.94-97) under the title ‘The origin of the internal nostril of tetrapods’.
Fish have two nostrils on the side of their heads – one in the front and one in back – that form the openings to a little sac containing the olfactory organs. Water flows in through the front nostril and out through the back, but there is no connection to the throat – hence fish can smell but not breathe with their noses. We tetrapods, on the other hand, have an inner nostril or ‘choana’ that opens on the palate or in the throat.
This is what makes it possible for us to breathe through our nose. But how did this inner nostril evolve? One thing all scientists agree on is that the front nostril in fish corresponds to our single outer nostril: the question the Sino-Swedish duo tackled was, whether the back nostril was transformed into our choana by ‘migrating’ to the palate, or whether the choana is a new opening that arose with tetrapods.
This is a difficult part of evolution to understand, contend the scientists. They say it is easy for us to imagine how evolution deals with small steps, like changing colour or size, but how does a nostril move from the face to the palate? Some experts claim it is impossible for an outer nostril to migrate gradually to the palate because a ‘cord’ of nerves and blood vessels runs just inside the row of teeth that the nostril would have to sever during its migration. As the cord is in evidence both in fish and tetrapods, it seems that nothing has happened to it during our evolution.
Insight for developmental biology
To solve this mystery, science turns to the study of fossils, but palaeontologists have failed to find the vital clue. Among the discoveries of fossilised ‘lobe-finned’ fish – the closest relatives of tetrapods – it has been noted that some have two outer nostrils but no choanae, just like modern fish. Other lobe-fins, even closer to tetrapods, show a single outer nostril per side and a fully developed choana – a change emerging just before our ancestors crawled onto land. But these still tell us nothing about how the choanae came to be formed.
Now, with the discovery in China of well-preserved coelacanth fossils of a type called Kenichthys campbelli, which are roughly 395 million years old, the two scientists can fill the gap in our ancestral tree between coelacanths with no choana and those that have an inner nostril. Kenichthys has a back nostril that is located on the lip, separating the upper jawbones of the maxilla and premaxilla. This is a halfway point in the nostril migration from the face to the palate, say the duo, a transition that many regarded as an “anatomical impossibility”.
“At this stage, we still don’t know how evolution managed to re-forge the contact between the maxilla and the premaxilla and re-establish the cord after the nostril had migrated past,“ Zhu and Ahlberg admit. They hope that future developmental biological research will be able to identify the molecular mechanisms governing the formation of this section of the head. One thing is already apparent: there is a connection between the migration of the nostril in our ancestors and the problem in humans known as cleft palate.
In a human embryo, the upper jaw develops from two separate outgrowths that fuse together beneath the nose, creating a bony bridge that separates the nose from the palate and internal nostril. If this fails to happen, the result is a cleft palate. “It seems that this pattern of development contains a ‘memory’ of how our internal nostril migrated onto the palate through a gap in the upper jaw”, says Ahlberg.
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