地球土著還是外星來客?

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"。據英國《每日郵報》8月9日報道：通過對墜落地球之前已存在了數十萬億年隕石進行分析，美國宇航局的科學家們發現地球上的生命或許起源於外太空。

發現的富含碳的隕石碎片中所含的化學元素和構成生命的DNA中的一種關鍵元素極爲相似。

測試還表明，這種化學元素的存在並不是因爲地球污染造成的，因此，DNA的起源很可能存在於外太空。

來自馬里蘭州宇航局戈達德太空飛行中心的領頭研究員邁克爾•卡拉漢博士說：“由於隕石和彗星對早期的地球有碰撞，看來這確實地球撒播了一些重要的成份。”

早先的研究也發現隕石內蘊含構成生命的三種基礎成分中的兩樣。這兩種被稱爲“玻璃粉”的氨基酸物質壓縮時可以形成蛋白質，並形成用於幫助細胞生成黏膜屏障的化學物質。這種存在於DNA中的化學物質就是構成生命的第三種成分。

此發現是以美國宇航局科學家爲主導的研究成果，他們分析研究了12塊隕石的化學構成。

此研究發表在《美國國家科學院院刊》上，研究者們稱其意義深遠。