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"。据英国《每日邮报》8月9日报道：通过对坠落地球之前已存在了数十万亿年陨石进行分析，美国宇航局的科学家们发现地球上的生命或许起源于外太空。

发现的富含碳的陨石碎片中所含的化学元素和构成生命的DNA中的一种关键元素极为相似。

测试还表明，这种化学元素的存在并不是因为地球污染造成的，因此，DNA的起源很可能存在于外太空。

来自马里兰州宇航局戈达德太空飞行中心的领头研究员迈克尔•卡拉汉博士说：“由于陨石和彗星对早期的地球有碰撞，看来这确实地球撒播了一些重要的成份。”

早先的研究也发现陨石内蕴含构成生命的三种基础成分中的两样。这两种被称为“玻璃粉”的氨基酸物质压缩时可以形成蛋白质，并形成用于帮助细胞生成黏膜屏障的化学物质。这种存在于DNA中的化学物质就是构成生命的第三种成分。

此发现是以美国宇航局科学家为主导的研究成果，他们分析研究了12块陨石的化学构成。

此研究发表在《美国国家科学院院刊》上，研究者们称其意义深远。