從澳洲的化石裡找到所有動物的祖先

原文網址：https://news.ucr.edu/articles/2020/03/23/ancestor-all-animals-identified-australian-fossils

從澳洲的化石裡找到所有動物的祖先

最早的兩側對稱動物是一種生活在5.5億年前、像是蠕蟲的生物

By Holly Ober

由加州大學河濱分校的地質學家領導的團隊，發現了一類動物家族樹上最初的祖先。這株家族樹不僅包括了大部分我們所熟知的動物，也包含了人類在內。

藝術家繪製的Ikaria wariootia復原圖。圖片來源：Sohail Wasif/UCR

這篇論文今日(2020.3.23)發表在《美國國家科學院院刊》(Proceedings of the National Academy of Sciences)，其中被科學家命名為Ikaria wariootia、像是蠕蟲的小生物是最早的兩側對稱動物(bilaterian)。兩側對稱動物可以區分出正面背面且左右兩邊對稱，還有一條腸子連接身體前後兩端。

最初的多細胞生物，像是海綿和藻蓆等具有各式各樣的外型。這些合稱為埃迪卡拉生物群的生物包含了最古老的複雜多細胞生物化石。不過它們絕大多數都跟我們今日周遭的動物沒有直接關係，像是狀如荷葉的狄更遜水母(Dickinsonia)就沒有嘴巴或腸子這類多數動物都有的基本特徵。

發展出兩側對稱是動物演化史上相當重要的一步，這讓動物可以為了特定目的而移動，也是一種組織身體結構時普遍且成功的方式。從蠕蟲、昆蟲、恐龍到人類在內的眾多動物，都是圍繞著相同的兩側對稱藍圖來建構身體。

演化生物學家研究現生動物的基因之後，推測所有兩側動物最古老的共同祖先應該是一種簡單的小型生物，只有原始的感覺器官。但他們認為這種生物要成為化石保存下來，而且我們還要辨認出來，就算不是無法辦到，也相當有難度。

澳洲南部的尼爾皮納有在5.5億年前的埃迪卡拉紀形成的沉積岩。15年來，科學家一致認為在此發現的挖掘痕跡化石是兩側對稱動物製造的。但是沒有任何跡象可以確切指出製造這些洞穴的生物是什麼，使得科學家只能憑空猜測它們的身分。

最近剛從加州大學河濱分校獲得博士學位的Scott Evans以及地質學教授Mary Droser注意到某些洞穴附近有橢圓形的印痕。在NASA地外生物學計畫的補助之下，他們利用立體雷射掃描儀分析這些規律出現、形狀一致的圓柱體，結果發現它們具有頭尾分明的構造，而且還能依稀看出肌肉組織形成的紋路。這些動物的長度為2-7釐米、寬度為1-2.5釐米，最大者的外型就跟米粒一樣——正好是製造這些洞穴的生物該有的尺寸。

Ikaria wariootia在岩石上的印痕。圖片來源：Droser Lab/UCR

「之前我們就認為那段期間應該會存在著兩側對稱動物，但也一直都明白要辨識出它們絕非易事。」Evans表示。「我們一看到3D掃描的結果，就知道我們有了重大發現。」

研究人員還包括加州大學聖地牙哥分校的Ian Hughes和南澳博物館的James Gehling。他們將這種動物命名為Ikaria wariootia，以彰顯最初擁有這片土地的原住民。其中屬名來自Ikara，這是群山包圍的威爾潘納盆地在Adnyamathanha語中的稱呼，意思是「聚會地點」。種名則來自於從費蓮達山脈流至Nilpena Station的Warioota溪。

「Ikaria挖出的洞穴位在其他洞穴化石的下方，代表它是具有這種複雜程度的化石中最古老的。」Droser表示。「狄更遜水母和其他大型生物也許在演化上走到了死胡同。但我們知道同時間還有許多小生物，而Ikaria也許就是我們在尋找的最早的兩側對稱動物。」

利用3D雷射儀掃描分析Ikaria wariootia的印痕得出的影像。圖片來源：Droser Lab/UCR

雖然Ikaria的體型較小，但對比於同期的其他化石卻更加複雜。它會在海床表層富含氧氣的沙子裡挖洞來尋找有機物，代表它有原始的感知能力。Ikaria印痕化石前後兩端的深度與弧度有明顯差異，支持了它在洞穴移動時是有方向性的。

這些洞穴裡也保存了交叉的V型紋路，意味著Ikaria和蛆蟲一樣，是靠著全身肌肉收縮的「蠕動」方式來前進。此外，洞穴中的沉積物曾遭到搬運，以及Ikaria是以埋藏在沙中的有機物維生的證據，都指向Ikaria可能擁有嘴巴、肛門和腸子。

「這些都和演化生物學家預測的一樣。」Droser表示。「我們找到的證據和他們的預測竟然可以如此契合，真的是令人相當激動。」

Ancestor of all animals identified in Australian fossils

A wormlike creature that lived more than 555 million years ago is the earliest bilaterian

A team led by UC Riverside geologists has discovered the first ancestor on the family tree that contains most familiar animals today, including humans.

The tiny, wormlike creature, named Ikaria wariootia, is the earliest bilaterian, or organism with a front and back, two symmetrical sides, and openings at either end connected by a gut. The paper is published today in Proceedings of the National Academy of Sciences.

The earliest multicellular organisms, such as sponges and algal mats, had variable shapes. Collectively known as the Ediacaran Biota, this group contains the oldest fossils of complex, multicellular organisms. However, most of these are not directly related to animals around today, including lily pad-shaped creatures known as Dickinsonia that lack basic features of most animals, such as a mouth or gut.

The development of bilateral symmetry was a critical step in the evolution of animal life, giving organisms the ability to move purposefully and a common, yet successful way to organize their bodies. A multitude of animals, from worms to insects to dinosaurs to humans, are organized around this same basic bilaterian body plan.

Evolutionary biologists studying the genetics of modern animals predicted the oldest ancestor of all bilaterians would have been simple and small, with rudimentary sensory organs. Preserving and identifying the fossilized remains of such an animal was thought to be difficult, if not impossible.

For 15 years, scientists agreed that fossilized burrows found in 555 million-year-old Ediacaran Period deposits in Nilpena, South Australia, were made by bilaterians. But there was no sign of the creature that made the burrows, leaving scientists with nothing but speculation.

Scott Evans, a recent doctoral graduate from UC Riverside; and Mary Droser, a professor of geology, noticed miniscule, oval impressions near some of these burrows. With funding from a NASA exobiology grant, they used a three-dimensional laser scanner that revealed the regular, consistent shape of a cylindrical body with a distinct head and tail and faintly grooved musculature. The animal ranged between 2-7 millimeters long and about 1-2.5 millimeters wide, with the largest the size and shape of a grain of rice — just the right size to have made the burrows.

“We thought these animals should have existed during this interval, but always understood they would be difficult to recognize,” Evans said. “Once we had the 3D scans, we knew that we had made an important discovery.”

The researchers, who include Ian Hughes of UC San Diego and James Gehling of the South Australia Museum, describe Ikaria wariootia, named to acknowledge the original custodians of the land. The genus name comes from Ikara, which means “meeting place in the Adnyamathanha language. It’s the Adnyamathanha name for a grouping of mountains known as Wilpena Pound.  The species name comes from Warioota Creek, which runs from the Flinders Ranges to Nilpena Station.

“Burrows of Ikaria occur lower than anything else. It’s the oldest fossil we get with this type of complexity,” Droser said. “Dickinsonia and other big things were probably evolutionary dead ends. We knew that we also had lots of little things and thought these might have been the early bilaterians that we were looking for.”

In spite of its relatively simple shape, Ikaria was complex compared to other fossils from this period. It burrowed in thin layers of well-oxygenated sand on the ocean floor in search of organic matter, indicating rudimentary sensory abilities. The depth and curvature of Ikaria represent clearly distinct front and rear ends, supporting the directed movement found in the burrows.

The burrows also preserve crosswise, “V”-shaped ridges, suggesting Ikaria moved by contracting muscles across its body like a worm, known as peristaltic locomotion. Evidence of sediment displacement in the burrows and signs the organism fed on buried organic matter reveal Ikaria probably had a mouth, anus, and gut.

“This is what evolutionary biologists predicted,” Droser said. “It’s really exciting that what we have found lines up so neatly with their prediction.”

原始論文：Scott D. Evans, Ian V. Hughes, James G. Gehling, and Mary L. Droser. Discovery of the oldest bilaterian from the Ediacaran of South Australia. PNAS, March 23, 2020 DOI: 10.1073/pnas.2001045117

引用自：University of California - Riverside. "Ancestor of all animals identified in Australian fossils."