研究人員找到古生物學的聖杯：5.58億年前的脂質揭露出已知最古老的動物身分
澳洲國立大學和其他國家的科學家在一具年代久遠的化石中發現脂質分子，從而得出地質紀錄中有證據顯示其為動物的化石最早存活於5.58億年前。
一具狄更遜水母的化石。圖片來源：澳洲國立大學
這種奇特的生物稱為狄更遜水母(Dickinsonia)，牠們可以長到1.4公尺長，橢圓形的身體被一道道橫肋分成許多節。狄更遜水母是埃迪卡拉生物群(Ediacaran Biota)的一員，它們生存的年代為「寒武紀大爆發」(Cambrian explosion，現生動物的濫觴)的2000萬年前。
Ilya Bobrovskiy是澳洲國立大學的博士學者，他在人煙罕至的俄羅斯西北方白海一帶，發現一具保存十分良好的狄更遜水母化石。這具化石的組織仍然保有動物的特有標記――稱為膽固醇的脂質分子。
主要研究人員，澳洲國立大學的副教授Jochen
Brocks表示「寒武紀大爆發」發生的時候，化石紀錄的主要組成開始變成較為複雜的動物以及其他大型生物，像是軟體動物、蠕蟲、節肢動物和海綿。
澳洲國立大學地球科學研究院的副教授Jochen
Brocks說：「我們從化石中發現的脂質分子證明許多大型動物在5.58億年前就已經出現在地球上，比過往認為的時間還要早上數百萬年。」
「超過75年來，科學家一直在爭論狄更遜水母和其他怪異的埃迪卡拉生物群化石究竟是什麼：它們是巨大的單細胞變形蟲、地衣、演化實驗的失敗品或是地球最古老的動物？我們從化石中找到的脂質證實狄更遜水母是人類所知最古老的動物化石，解決了這道歷經數十年、堪稱是古生物學聖杯的謎題。」
5.4億年前的寒武紀大爆發期間出現的大型動物接管了在此之前由細菌主導的世界。Bobrovskiy表示團隊發展出新的方法來研究狄更遜水母的化石，以瞭解此過程的關鍵。
澳洲國立大學地球科學研究院的Bobrovskiy說：「我們必須克服的問題是要找到仍然含有少許有機物的狄更遜水母化石。」
「含有這些化石的岩石，比方說澳洲的埃迪卡拉山，大部分形成之後會承受相當高的溫度壓力作用，接著又遭受風化。古生物學家數十年來都在研究這類岩石，因此才遲遲無法解出狄更遜水母的真實身分。」
古生物學家一般研究的是狄更遜水母化石的結構，但是Bobrovskiy從俄羅斯的遠古岩石中找到的狄更遜水母化石含有脂質分子。他把這些分子萃取出來加以分析後得到了突破性的發現。
Bobrovskiy說：「我搭乘直升機到那個人煙極為罕至、熊和蚊子居住的地方。然後我找到了狄更遜水母的化石，其中的有機物質依然完整。」
「這些化石的發現地點是白海沿岸高60到100公尺的懸崖中間，我必須要用繩子懸掛在岩壁上，然後敲下巨大的砂岩岩塊，把它們往下丟再撿回來清洗乾淨，一直重複此過程直到找到我想要的化石。」
Brocks副教授說可以研究這些遠古生物留下來的分子帶來了重大突破。
「Ilya給我看實驗結果時我幾乎無法相信，」他說。
「不過我也立刻了解到它代表的意義有多麼重大。」
Researchers find Holy Grail of palaeontology – 558 million-year-old fat
reveals earliest known animal
Scientists from The Australian National University (ANU) and
overseas have discovered molecules of fat in an ancient fossil to reveal the
earliest confirmed animal in the geological record that lived on Earth 558
million years ago.
The strange creature called
Dickinsonia, which grew up to 1.4 metres in length and was oval shaped with
rib-like segments running along its body, was part of the Ediacaran Biota that
lived on Earth 20 million years prior to the ‘Cambrian explosion’ of modern
animal life.
ANU PhD scholar Ilya
Bobrovskiy discovered a Dickinsonia fossil so well preserved in a remote area
near the White Sea in the northwest of Russia that the tissue still contained
molecules of cholesterol, a type of fat that is the hallmark of animal life.
Lead senior researcher
Associate Professor Jochen Brocks said the ‘Cambrian explosion’ was when
complex animals and other macroscopic organisms – such as molluscs, worms,
arthropods and sponges – began to dominate the fossil record.
“The fossil fat molecules
that we’ve found prove that animals were large and abundant 558 million years
ago, millions of years earlier than previously thought,” said Associate
Professor Jochen Brocks from the ANU Research School of Earth Sciences.
“Scientists have been
fighting for more than 75 years over what Dickinsonia and other bizarre fossils
of the Edicaran Biota were: giant single-celled amoeba, lichen, failed
experiments of evolution or the earliest animals on Earth. The fossil fat now
confirms Dickinsonia as the oldest known animal fossil, solving a decades-old
mystery that has been the Holy Grail of palaeontology.”
Mr Bobrovskiy said the team
developed a new approach to study Dickinsonia fossils, which hold the key
between the old world dominated by bacteria and the world of large animals that
emerged 540 million years ago during the ‘Cambrian explosion’.
“The problem that we had to
overcome was finding Dickinsonia fossils that retained some organic matter,”
said Mr Bobrovskiy from the ANU Research School of Earth Sciences.
“Most rocks containing
these fossils such as those from the Ediacara Hills in Australia have endured a
lot of heat, a lot of pressure, and then they were weathered after that – these
are the rocks that palaeontologists studied for many decades, which explained why
they were stuck on the question of Dickinsonia’s true identity.”
Palaeontologists normally
study the structure of fossils, but Mr Bobrovskiy extracted and analysed
molecules from inside the Dickinsonia fossil found in ancient rocks in Russia
to make the breakthrough discovery.
“I took a helicopter to
reach this very remote part of the world – home to bears and mosquitoes – where
I could find Dickinsonia fossils with organic matter still intact,” Mr
Bobrovskiy said.
“These fossils were located
in the middle of cliffs of the White Sea that are 60 to 100 metres high. I had
to hang over the edge of a cliff on ropes and dig out huge blocks of sandstone,
throw them down, wash the sandstone and repeat this process until I found the
fossils I was after.”
Associate Professor Brocks
said being able to study molecules from these ancient organisms was a
gamechanger.
“When Ilya showed me the
results, I just couldn’t believe it,” he said.
“But I also immediately saw
the significance.”
原始論文：Ilya
Bobrovskiy, Janet M. Hope, Andrey Ivantsov, Benjamin J. Nettersheim, Christian
Hallmann, Jochen J. Brocks. Ancient steroids establish the Ediacaran
fossil Dickinsonia as one of the earliest animals. Science,
2018; 361 (6408): 1246 DOI: 10.1126/science.aat7228
引用自：Australian National University. "Fat from
558 million years ago reveals earliest known animal."
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