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.

Darwin's Theory of Evolution has long maintained that tetrapods,or four-footed land vertebrates, originated from the sea, but scientists cannot agree on how fish adapted themselves to land life in terms of physical modifications. Among the riddles, the origin of internal nostrils has been one of the most hotly debated.

The 395-million-year-old fossil fish may have solved the riddle of how our nasal cavity adopted its present layout, according to apaper by Chinese scientist Zhu Min and Swedish scientist Per E. Ahlberg carried in the latest issue of Nature published on Thursday.

The primitive fish, Kenichthys campbelli, found in Yunnan 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 cleft towards the throat, the paper acknowledges.

Most modern fish have four nostrils on their noses, whereas their air-breathing counterparts on land have two external nostrils that lead to internal nostrils.

Scientists had not found any fossil evidence showing the inner nostrils of fish, or choana, passing through the line of the teeth,an essential intermediate step to form the inner nostrils.

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

Four-footed land vertebrate breathes with lungs, and only with the help of internal nostrils could air enter the lungs when the mouth is closed or taking in foods.