Swedish-Chinese research team uncovers the history of the nose
Our ancestors had two nostrils, one front and one back, but no opening on the palate or in the throat. They could smell, but not breathe with their nose. How did our nose evolve? Per Ahlberg, Uppsala university, and Zhu Min, department of Vertebrate Paleontology in Beijing, China, has now found a fossil that explains the history of the nose.
Have you ever wondered, taking a deep breath of fresh autumn air and sensing how the smell of wet leaves tickles your nose, just how it came about that you can do so? We humans take it for granted that our nose forms a passage between the world around us and our windpipe, but this hasn’t always been the case. We land-based vertebrates or “tetrapods” (mammals, birds, reptiles, and amphibians) originally descend from fish, and fish cant breathe through their nose. On the side of a fish’s head there are two nostrils, one front and one back, that form the opening to a little sac containing the olfactory organs: water flows in through the front nostril and out through the back one, but there is no connection whatsoever to the throat. In other words, fish can smell with their nose, but not breathe.
We tetrapods may have only one external nostril on each side of our head, but we do 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 about is that the front nostril in fish corresponds to our single outer nostril: the question is 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.
The answer to that question is of importance to a greater issue, namely, how large-scale evolution actually functions. This is the part of evolution that is perhaps most difficult to understand. Its easy for us to imagine how evolution deals with small steps, like developing a blue tit and a great titmouse from a common ancestor (this merely involves small details in plumage and size), but how does a nostril move from the face to the palate? Some scientists have claimed that it is impossible for an outer nostril to gradually migrate to the palate, since a “cord” of nerves and blood vessels runs just inside the row of teeth that the nostril would have to sever during its migration. The “cord” is in evidence both in fish and tetrapods, so it appears that nothing has happened to it during our evolution.
In a pinch like this, it is natural to turn to fossils to see what they can tell us, but thus far it hasnt been of much use. Among fossil “coelacanth” fish, which are the closest relatives of tetrapods, some forms have two outer nostrils but no choana, just like modern fish. Other fossil coelacanths, which are even closer to tetrapods, already have a single outer nostril per side and a fully developed choana. In other words, the fossils show that the choana emerged just before our ancestors crawled up onto land, but they tell us nothing about how the choana came to be formed.
Now two researchers, Per Ahlberg from the Department of Physiology and Developmental Biology, Uppsala University, and Zhu Min from the Department of Vertebrate Paleontology in Beijing, China, are able to present a unique fossil that clarifies once and for all how our inner nostril came into being. Their results are being published this week in the internationally respected scientific journal Nature.
The fossil is a small coelacanth fish, Kenichthys campbelli, which comes from China and is roughly 400 million years old. The scientists have found skulls of several individuals: some of the bones have fallen apart, but they are so well preserved that it is possible to see exactly how they fit together. Judging from their features, Kenichthys belongs in our ancestral tree between the fossil coelacanths who have no choana and those that have this inner nostril.
Kenichthys has a back nostril that is located right on the lip, separating the two-toothed upper jawbones, the maxillan and the premaxillan. (It is as if we were to have a nostril in a gap between our front teeth and our canine teeth.) In other words, it constitutes a perfect halfway point in the nostrils migration from the face to the palate, and moreover this halfway point is the precisely the one that some scientists have regarded as an anatomical impossibility. Unfortunately the “cord” of nerves and blood vessels has not been preserved in Kenichthys, but since it normally runs from the maxillan to the premaxillan, it must either have been cut off or relegated to another position. What was considered impossible was apparently possible after all.
At this stage we still don´t know how evolution managed to re-forge the contact between the maxillan and the premaxillan and re-establish the “cord” after the nostril had migrated past. Zhu and Ahlberg hope that future developmental biological research will be able to identify the molecular mechanisms that govern 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 rather common problem in human developmental biology that is termed “cleft jaw”.
During the development of the human fetus, the upper jaw is formed when two processes, which will form the maxillan and the premaxillan respectively, meet under the outer nostril and grow together. In this way they separate the outer nostril from the choana. If this coalescing process does not function, a so-called “cleft jaw” is formed, that is, a furrow that runs right up into the nasal sac. This “complicated” relationship between the upper jaw and the nasal sac does not exist in fish: after all, they have no nostril on the palate, so the upper jaw is quite simply formed below the nostrils without any problematic process of coalescence. It seems as if our complicated developmental process is a “memory” of how the back nostril made its way to the palate between the maxillan and the premaxillan, and then reconnoitered again behind it. Somewhat drastically it could be said that the intermediate form Kenichthys lived with a cleft jaw all its life. Thus a really old fossil fish can elucidate not only an important aspect of our early evolution but also the reasons behind one of the most common problems of human developmental biology.
Anneli Waara | alfa