地球土著还是外星来客?

　　Life on Earth may have its origins in outer space, according to NASA research. Scientists have analysed meteorites that formed billions of years ago before falling to Earth。

　　The carbon-rich fragments were found to contain chemicals similar to one of the key components of DNA, the building blocks of life。

　　Tests show that the presence of these chemicals cannot be explained away by Earthly contamination, suggesting DNA’s origins may lie in outer space。

　　The study/s lead author, Dr Michael Callahan, of the Nasa Goddard Space Flight Centre in Maryland, said: "With meteorites and comets impacting the early Earth, it appears that they did deliver some very important ingredients."

　　Previous studies found meteorites harbour the other two of the three basic ingredients of life. These are amino acids, the "beads" that form proteins when strung together, and chemicals needed to allow cells to create a membrane barrier. The chemicals in DNA are the third。

　　The find comes from US scientists, predominantly NASA researchers, who analysed the chemical make-up of 12 meteorites。

　　Writing in the journal Proceedings of the National Academy of Sciences, the researchers said their find has "far-reaching implications"。