研究爭議十足的化石指出生命可能在40億年前就已經出現在地球上
1992年，當時的研究人員發現了可能是由地球最原始生物留下的跡象：被包覆在澳洲岩石裡年代為35億年的細微曲線。然而，從那時起科學家就持續爭論著這些印痕是否真的代表了遠古微生物；就算是，它們的年代是否真的有這麼久遠。現今一項對這些微體化石的詳盡分析結果認為這些結構確實代表了一群遠古微生物，由於它們可能相當複雜，因此地球上的生命必定要比他們早5億年前左右就已經誕生了。
未參與這項新研究工作的澳洲伯斯科廷大學的地質生物學家Birger Rasmussen表示，研究成果暗示這群原始微生物擁有令人驚訝的複雜程度，它們能夠進行光合作用並利用其他化學作用來獲取能量。同樣沒有參與這項成果的美國堪薩斯大學勞倫斯市校區的地質生物學家Alison Olcott Marshall也表示：「這項研究或許會引發一股風潮，刺激其他研究人員對這些岩石進行更新的研究以找出數據來支持或者反對這項論點。」
美國加州大學洛杉磯分校的William Schopf是發現這些澳洲微體化石的古生物學家。在此新研究中，他和美國威斯康辛大學麥迪遜分校的地質學家John Valley組成了研究團隊。Valley的專長是運用二次離子質譜儀(SIMS)這種分析技術來測定樣品中不同形式的碳之間的比例，藉此他們可以判定樣品是否為有機物。
為了找到具有足夠化石樣品來讓SIMS進行分析的岩石薄片，Schopf在顯微鏡底下工作了4個月之久。最後他找到的樣品中具有11副微體化石，從它們的形狀和大小差異Schopf認為其中有5個不同的微生物種類。同時他也提供了不含這些可能為化石的岩石樣品作為比對。
新的證據支持這些曲線代表了原始生命。Credit: J.
William Schopf, UCLA
Schopf、Valle和其他研究人員在今日發表於《美國國家科學院院刊》(Proceedings of the National Academy of
Sciences)的文章中表示他們對這些材料的分析結果得到了幾種不同的碳同位素比例。其中兩種微體化石的碳同位素比例跟一種現存的細菌一樣，它可以利用陽光來合成進行活動所需的碳化合物――這種原始光合作用並不需要氧氣。另為兩種微體化石的碳同位素比例則跟一種古菌相同，其仰賴甲烷做為它的能量來源――並且在多細胞生物的形成過程中具有重要地位。最後一種微體化石的碳同位素類型則指出這種生物的的代謝過程中會產生甲烷。
Schopf表示有這麼多種不同類型的碳同位素強化了它們確實為化石的論點。如果這些曲線是由任何一種無機作用造成，則應該會留下一致的碳同位素訊號。而在地球歷史中的那個時間點微生物已經如此多樣，也代表地球生命的起源應該可以追溯至距今40億年以前。雖然已經有其他研究人員找到的生命訊號至少可以追溯到如此遙遠以前，但這些發現甚至比Schopf的還要更具爭議性。
Rasmussen同意「新成果更加鞏固了這些細微構造為生物來源的說法」，但他也擔心這些微體化石的保存情況可能相當差勁。Olcott
Marshall完全不認為這些岩石中的痕跡是化石，而是一種地質作用的產物。她的評論更加尖銳：「這種分析技術造成的誤差相當大。」因此她認為這些數據並沒有辦法確切證明岩石中含有不同類型的微生物。
但是SIMS的專家則相當讚賞這項成果。「這是個十分縝密且設計良好的實驗。」未參與此研究的美國華盛頓大學西雅圖分校的化學家Lara Gamble表示，「他們相當努力確保所有的數據都有經過嚴格校正。」
Rasmussen希望後續工作會分析更多的微體化石。「由於我們正在關注的這些構造確實有可能是生命留下的最古老痕跡，所以值得我們進行研究來證明它們。」他說，「精進我們辨識地球上遠古生命訊號的技術相當重要，因為我們也正在放眼火星以及更遙遠的星球尋找生命。」
Life may have originated on Earth 4 billion years ago, study of
controversial fossils suggests
In 1992, researchers discovered evidence of what was then
potentially the earliest life on Earth: 3.5-billion-year-old microscopic
squiggles encased in Australian rocks. Since then, however, scientists have
debated whether these imprints truly represent ancient microorganisms, and even
if they do, whether they’re really that old. Now, a comprehensive analysis of
these microfossils suggests that these formations do indeed represent ancient
microbes, ones potentially so complex that life on our planet must have
originated some 500 million years earlier.
The new work indicates
these early microorganisms were surprisingly sophisticated, capable of
photosynthesis and of using other chemical processes to get energy, says Birger
Rasmussen, a geobiologist at Curtin University in Perth, Australia, who was not
involved with the work. The study “will probably touch off a flurry of new
research into these rocks as other researchers look for data that either
support or disprove this new assertion,” adds Alison Olcott Marshall, a
geobiologist at the University of Kansas in Lawrence who was not involved in
the effort.
In the new study, William
Schopf, a paleobiologist at the University of California, Los Angeles—and the
discoverer of the Australian microfossils—teamed up with John Valley, a
geoscientist at the University of Wisconsin in Madison. Valley is an expert in
an analytical technique called secondary ion mass spectrometry (SIMS), which
can determine the ratio of different forms of carbon in a sample—key to gauging
whether it’s organic.
Schopf spent 4
months working with microscopes to find a thin slice of the rock that contains
the fossils with specimens accessible enough to study with SIMS; that sample
contained 11 microfossils whose diversity of shapes and sizes suggested they
represented five species of microbes. He also provided samples of rock
containing no putative fossils for comparison.
New evidence
supports that these “squiggles” represent early life. Credit: J. William
Schopf, UCLA
The analysis
detected several distinct carbon ratios in the material, Schopf, Valley, and
colleagues report today in the Proceedings of the National Academy of
Sciences. Two types of microfossils had the same carbon ratio as modern
bacteria that use light to make carbon compounds that fuel their activities—a
primitive photosynthesis that did not involve oxygen. Two other types of
microfossils had the same carbon ratios as microbes known as archaea that
depend on methane as their energy source—and that played a pivotal role in the
development of multicellular life. The ratio of a final type of microfossil
indicated that this organism produced methane as part of its metabolism.
That there are
so many different carbon ratios strengthens the case that these are real
fossils, Schopf says. Any inorganic processes that could have created the
squiggles would be expected to leave a uniform carbon ratio signature, he says.
The fact that microbes were already so diverse at this point in Earth’s
history also suggests that life on our planet may date back to 4 billion years
ago, he says. Other researchers have found signs of life dating back at
least that far, but those findings are even more controversial than Schopf’s.
“The new
results add weight to the idea that the microstructures are biological,”
Rasmussen agrees. But he is concerned that the microfossils may have been badly
preserved. Olcott Marshall, who thinks the rock impressions are not fossils at
all, but the product of geological processes, is even more critical: “The
errors produced by this analytical technique are so large” that the data are
not clear enough to say there are different types of microbes in rock, she says.
But SIMS
experts praise the work. “It was a really careful, well thought out
experiment,” says Lara Gamble, a chemist at the University of Washington in
Seattle who was not involved in the study. “They put in a lot of effort to try
to make sure everything was calibrated properly.”
Rasmussen
hopes there will follow-up work that analyzes more microfossils. “It’s worth
getting this right, given that we are looking at some of the oldest possible
traces of life,” he says. “Honing our skills at recognizing ancient
biosignatures on Earth is important as we cast our eyes to Mars and beyond.”
原始論文：J. William
Schopf, Kouki Kitajima, Michael J. Spicuzza, Anatoliy B. Kudryavtsev, John W.
Valley. SIMS analyses of the oldest known assemblage of microfossils
document their taxon-correlated carbon isotope compositions. Proceedings
of the National Academy of Sciences, 2017; 201718063 DOI: 10.1073/pnas.1718063115
引用自：Elizabeth
Pennisi. Life may have originated on Earth 4 billion years ago, study of
controversial fossils suggests. Science, 2017; 358(6370). doi:10.1126/science.aar7944
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