LAND vertebrates can breathe through their noses thanks to an anatomical rearrangement of fish-style nostrils. That same rearrangement may explain why cleft lips and cleft palates are common birth defects in humans.
The nasal passages of land vertebrates differ dramatically from their fish ancestors. In fishes, the nose is independent of the mouth and throat. Water enters the nasal sac through one pair of nostrils and exits through a second pair. By contrast, land vertebrates 鈥� technically known as tetrapods, because of their four limbs 鈥� have nasal passages that open to the outside world through a pair of external nostrils, and to the throat through a pair of internal nostrils or choanae.
Many biologists suspect the choanae evolved from one pair of fish nostrils that migrated over millions of years to a new position inside the throat. To do that, however, the nostrils would have had to cross through the line of teeth at some point, a move that sceptics regarded as unlikely.
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Their doubts should vanish, thanks to a careful reconstruction of several fossilised skulls of the most primitive known ancestor of tetrapods, a fish known as Kenichthys campbelli, from Yunnan, China. In Kenichthys, the second pair of nostrils opens neither externally nor internally, but directly into a gap in the row of teeth (Nature, vol 432, p 94).
鈥淚t鈥檚 as if we were to have a nostril located on the upper jaw margin between the canine and the adjacent incisor,鈥� says Per Ahlberg of Uppsala University in Sweden, who did the study with Min Zhu of the Chinese Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. In short, Kenichthys is a perfect intermediate, says John Maisey, a vertebrate palaeontologist at the American Museum of Natural History in New York.
Developing human embryos have a gap in the same place in the upper jaw, which later fuses. If it fails to fuse, the result is a cleft palate or cleft lip. Most likely, then, these birth defects arise from the same developmental process that gave us the ability to breathe through our noses, says Ahlberg.
And as for eyes鈥�
The eyes of vertebrates and invertebrates have a common origin. That鈥檚 the implication of the discovery of both vertebrate and invertebrate-type light-sensing cells in a marine worm labelled a 鈥渓iving fossil鈥�.
The eyes of squid and mammals were once held up as textbook examples of evolution鈥檚 ability to reach the same solution from two different starting points. But recent evidence has hinted that vertebrate and invertebrate eyes share a common history in the distant evolutionary past. For example, the gene PAX6 plays a very similar role in eye development in both groups.
Now a research team led by Detlev Arendt at the European Molecular Biology Laboratory in Heidelberg, Germany, has shown that a marine ragworm called Platynereis uses light-gathering cells found in both invertebrates and vertebrates. The invertebrate types are on its head while the vertebrate types are buried in the brain (Science, vol 306, p 869).
The finding is significant because Platynereis is thought to be the nearest living thing to the common ancestor of invertebrates and vertebrates, suggesting both cell types were present before the groups split around 570 million years ago. Arendt believes that Platynereis uses the light sensing cells in its brain to train its circadian clock.
James Randerson