顯示動物存在的最古老證據

原文網址：https://news.ucr.edu/articles/2018/10/15/oldest-evidence-animals-found-ucr-researchers

顯示動物存在的最古老證據

分子化石證據指出在寒武紀一億年前的古代海床上就已經有海綿了

Sarah Nightingale

加州大學河濱分校的研究人員發現目前為止顯示動物存在的最古老證據，年代可以追溯至著名的寒武紀大爆發(動物化石突然大量出現的事件)的一億年前。

在遠古岩石中發現的26-mes也能由現生的尋常海綿Rhabdastrella globostellata製造出來。圖片來源：Paco Cárdenas

這項研究由加州大學河濱分校地球科學系的教授Gordon Love領導，結果發表在《自然―生態學與演化》(Nature Ecology & Evolution)。第一作者為Love研究團隊的博士生Alex Zumberge。

研究人員尋找的目標並非常見的實體化石。他們一路追蹤顯示動物曾經存在的古老分子標記，稱為生物指標(biomarker)，直到6.6億至6.35億年前的新元古代。在阿曼、西伯利亞、印度的岩石和石油中，他們發現一種只有海綿這種最原始的動物才會製造的類固醇化合物。

「第一批海綿可能非常小、沒有骨骼、甚至不會留下任何保存良好或者容易辨識的實體化石紀錄，因此分子化石在找尋最古老的動物時便成為相當重要的工具。」Zumberge表示。「我們持續尋找獨特且穩定的生物標記可以用來指出海綿或者其他原始動物的存在，而非複雜多細胞生物出現之前統治地球數十億年的單細胞生物。」

他們發現的生物標記是一種類固醇化合物――26-methylstigmastane (26-mes)。就目前所知，它具有的獨特構造只有生存於現代的海綿――尋常海綿(demosponge)中的某些種類可以製造。

Zumberge說：「這種類固醇生物標記是第一個證據顯示至少在遙遠的6.35億年前，古代海裡就已經有許多尋常海綿，也就是多細胞生物生長了。」

這項成果的基礎是2009年Love團隊進行的研究。他們當時發表第一個有力證據指出新元古代就已經有動物存在。該證據是阿曼南部岩石中另一種不同的類固醇生物標記，稱為24-異丙基膽甾烷 (24-isopropylcholestane，24-ipc)。然而，並非只有尋常海綿會製造24-ipc。在現代某些藻類中也能發現到，使得24-ipc生物標記的效力具有爭議。這次發現另一種新的古代生物標記――專屬於尋常海綿的26-mes，使得研究人員更有信心認為兩種分子化石都是由古代海床上的尋常海綿所製造。

研究另一個重點是確定現代的尋常海綿中，哪些種類可以製造這種特殊的類固醇構造，進而留下獨特的生物標記。研究人員發現現生的尋常海綿中，某些分類群偏好製造26-mes，另外一些則偏好24-ipc。

「結合新元古代尋常海綿留下的甾烷紀錄後，顯示出古代岩石中會同時出現24-ipc和26-mes，這不可能是由尋常海綿的單獨一個支系或者已經滅絕的一個幹群(stem-group)產生。」Love表示。「反之，製造這種特殊固醇類的能力可能出現在尋常海綿演化樹十分基部的地方，而今日許多尋常海綿的類群也擁有這項能力。」

Oldest evidence for animals

Molecular fossil evidence suggests sponges lived on the ancient ocean floors 100 million years before the Cambrian period

Researchers at the University of California, Riverside, have found the oldest clue yet of animal life, dating back at least 100 million years before the famous Cambrian explosion of animal fossils.

The study, led by Gordon Love, a professor in UCR’s Department of Earth Sciences, was published today in Nature Ecology & Evolution. The first author is Alex Zumberge, a doctoral student working in Love’s research group.

Rather than searching for conventional body fossils, the researchers have been tracking molecular signs of animal life, called biomarkers, as far back as 660-635 million years ago during the Neoproterozoic Era. In ancient rocks and oils from Oman, Siberia, and India, they found a steroid compound produced only by sponges, which are among the earliest forms of animal life.

“Molecular fossils are important for tracking early animals since the first sponges were probably very small, did not contain a skeleton, and did not leave a well-preserved or easily recognizable body fossil record,” Zumberge said. “We have been looking for distinctive and stable biomarkers that indicate the existence of sponges and other early animals, rather than single-celled organisms that dominated the earth for billions of years before the dawn of complex, multicellular life.”

The biomarker they identified, a steroid compound named 26-methylstigmastane (26-mes), has a unique structure that is currently only known to be synthesized by certain species of modern sponges called demosponges.

“This steroid biomarker is the first evidence that demosponges, and hence multicellular animals, were thriving in ancient seas at least as far back as 635 million years ago,” Zumberge said.

The work builds from a 2009 study by Love’s team, which reported the first compelling biomarker evidence for Neoproterozoic animals from a different steroid biomarker, called 24-isopropylcholestane (24-ipc), from rocks in South Oman. However, the 24-ipc biomarker evidence proved controversial since 24-ipc steroids are not exclusively made by demosponges and can be found in a few modern algae. The finding of the additional and novel 26-mes ancient biomarker, which is unique to demosponges, adds extra confidence that both compounds are fossil biomolecules produced by demosponges on an ancient seafloor.

The study also provides important new constraints on the groups of modern demosponges capable of producing unique steroid structures, which leave a distinctive biomarker record. The researchers found that within modern demosponges, certain taxonomic groups preferentially produce 26-mes steroids while others produce 24-ipc steroids.

“The combined Neoproterozoic demosponge sterane record, showing 24-ipc and 26-mes steranes co-occurring in ancient rocks, is unlikely attributed to an isolated branch or extinct stem-group of demosponges,” Love said. “Rather, the ability to make such unconventional steroids likely arose deep within the demosponge phylogenetic tree but now encompasses a wide coverage of modern demosponge groups.” 

原始論文：J. Alex Zumberge, Gordon D. Love, Paco Cárdenas, Erik A. Sperling, Sunithi Gunasekera, Megan Rohrssen, Emmanuelle Grosjean, John P. Grotzinger, Roger E. Summons. Demosponge steroid biomarker 26-methylstigmastane provides evidence for Neoproterozoic animals. Nature Ecology & Evolution, 2018; DOI: 10.1038/s41559-018-0676-2

引用自：University of California - Riverside. " Oldest evidence for animals found by UCR researchers."