A Chinese fish fossil with a nasal cavity running from the outside of its face into its throat, as in all modern land vertebrates, may prove to be a crucial evolutionary link between sea and land animals.

Tetrapods, or four-legged land vertebrates, have long been thought to have originated from sea creatures, and most of the physical modifications involved in this change have been accounted for.

But the origin of internal nostrils has eluded scientists for decades.

The 395 million-year-old fossil may have solved the riddle of how our nasal cavity adopted its present shape, according to a paper by Chinese scientist Zhu Min and Swedish scientist Per E. Ahlberg published Thursday in Nature.

The fish, Kenichthys campbelli, found in Yunnan Province in 2000, actually has nostrils that open in the middle of its upper teeth, almost as if it has a cleft palate, and the external nostrils gradually migrate through the palate towards the throat.

Most modern fish have four nostrils, whereas their air-breathing counterparts on land have two external nostrils that lead to "internal nostrils" or choana. Scientists had not previously found any fossil evidence for their formation.

"This is a debate that has lasted for about a century, but it is practically settled by the new data," notes Philippe Janvier in an accompanying commentary.

Tetrapods breathe with lungs, and only with the help of internal nostrils can air enter the lungs when the mouth is closed or taking in food.

(China Daily November 5, 2004)