From smelling your morning coffee to blowing your nose, myriad activities depend on your nose forming a passage between your surroundings and your windpipe.

This may seem obvious, but it was not always so: humans and other land-based vertebrates, or “tetrapods” (mammals, birds, reptiles, and amphibians), originally descended from fish. Although fish do have nostrils, one pair on each side of the snout, these are used exclusively for smelling water. There is no connection to the throat, so fish cannot breathe through their nose. Humans can, because we have an inner nostril, or “choana,” on each side that opens on the palate in the throat. How did it get there?

Some scientists have claimed that it is impossible for an outer nostril to migrate gradually to the palate without disrupting a cord of nerves and blood vessels that runs just inside the row of teeth. Now a missing link has been found that shows how a nostril progressed down the face and into the mouth over a few million years.

The link is a small lobe-finned fish, Kenichthys campbelli, from Yunnan in China, which is 400 million years old. This was reported in the journal Nature by Per Ahlberg of Uppsala University and Dr. Zhu Min of the Institute of Vertebrate Palaeontology and Palaeoanthropology in Beijing.

The primitive fish shows evolution halfway through rearranging the nasal passage – on each side, one of its nostrils opens in the middle of its upper teeth, said Mr. Ahlberg. “This is a perfect halfway point in the nostril’s migration from the face to the palate.”

There is a connection between the migration of the nostril and the problem of a “cleft palate,” said Mr. Ahlberg. “Our complicated and easily disrupted developmental process is a ‘memory’ of how the back nostril made its way through the upper jaw, which then reconnected behind it.”