Breathing space

日期:2019-02-26 03:04:01 作者:单于诋 阅读:

By Kurt Kleiner AIR breathing developed independently many times in the history of evolution. But a subtle biological mechanism that allows lungs to function seems to have developed only once, and is still used by all air-breathing vertebrates, say researchers in Australia. Lungs need to expand and contract, but if the surface tension of liquid in the lungs is too high, the tissue sticks together and breathing is impossible. So in air-breathing vertebrates, the surface tension is lowered by a coating of proteins and lipids, called the surfactant system, on cells lining the lungs. Christopher Daniels and student Lucy Sulivan of the University of Adelaide compared the surfactant systems of 18 vertebrates, including fish, lizards, chickens and humans, and found that all make use of a key protein called surfactant protein A. They conclude in this month’s Journal of Molecular Evolution that the surfactant system developed once in an ocean-dwelling ancestor about 350 million years ago, and has been used again and again as vertebrates developed lungs and crawled out of the water. “It’s similar to the insulin system and haemoglobin—it’s one of the things you have to have for the organism to do well,” says Daniels. “Since it evolved, it hasn’t changed.” Daniels thinks the surfactant system may have developed first in the gut, as a way to regulate surface tension between the organs there. Even today, there are similar surfactant proteins in the guts of rats, he says. Because the lungs and the gut are closely related in the developing embryo, Daniels believes a genetic mutation may have shifted the surfactant to the lungs and made air breathing possible. Allan Smits of Quinnipiac College in Hamden, Connecticut, who also studies pulmonary surfactants, says Daniels’s research on the surfactant system is convincing. “Clearly air breathing couldn’t have developed without it,