「遠古之根」：科學家辨認出已知最早的陸地居民化石

原文網址：www.sciencedaily.com/releases/2016/03/160302082245.htm

'A load of old rot': Fossil of oldest known land-dweller identified

「遠古之根」：科學家辨認出已知最早的陸地居民化石

A fossil dating from 440 million years ago is not only the oldest example of a fossilised fungus, but is also the oldest fossil of any land-dwelling organism yet found. The organism, and others like it, played a key role in laying the groundwork for more complex plants, and later animals, to exist on land by kick-starting the process of rot and soil formation, which is vital to all life on land.

這具4億4千萬年前的化石不只是現存樣品中最古老的真菌化石，同時也是迄今發現的陸居生物化石中年代最久遠者。這種生物和它的同類開啟了陸地上的生物腐朽及土壤形成作用，這對所有陸生生物來說都至關重要。因此在讓後世的高階植物及動物隨後得以在陸地安身立命的基礎建設上，它們扮演了關鍵性的一環。

This early pioneer, known as Tortotubus, displays a structure similar to one found in some modern fungi, which likely enabled it to store and transport nutrients through the process of decomposition. Although it cannot be said to be the first organism to have lived on land, it is the oldest fossil of a terrestrial organism yet found. The results are published in the Botanical Journal of the Linnean Society.

這種稱作Tortotubus的遠古拓荒者身上有一種構造跟某些現代真菌的十分相似，可能可以幫助它們在進行分解作用時儲存和運輸養分。雖然無法斷定它們就是在陸地上生活的第一種生物，但卻是目前為止所能找到的最古老陸生生物化石。這項研究結果刊登於《林奈學會植物學雜誌》(Botanical Journal of the Linnean Society)。

"During the period when this organism existed, life was almost entirely restricted to the oceans: nothing more complex than simple mossy and lichen-like plants had yet evolved on the land," said the paper's author Dr Martin Smith, who conducted the work while at the University of Cambridge's Department of Earth Sciences, and is now based at Durham University. "But before there could be flowering plants or trees, or the animals that depend on them, the processes of rot and soil formation needed to be established."

「在這種生物存活的年代，幾乎所有的生命都侷限在海洋當中。陸地上尚未演化出比簡單的苔癬和地衣類更複雜的生物。」本篇論文的作者 Martin Smith教授說。目前就職於杜倫大學的他於劍橋大學地球科學系任職時進行了此篇研究。「在開花植物和樹木，還有以它們維生的動物出現之前，(陸地上)必須要先建立起一套分解作用和土壤形成作用。」

Working with a range of tiny microfossils from Sweden and Scotland, each shorter than a human hair is wide, Smith attempted to reconstruct the method of growth for two different types of fossils that were first identified in the 1980s. These fossils had once been thought to represent parts of two different organisms, but by identifying other fossils with 'in-between' forms, Smith was able to show that the fossils actually represented parts of a single organism at different stages of growth. By reconstructing how the organism grew, he was able to show that the fossils represent mycelium -- the root-like filaments that fungi use to extract nutrients from soil.

Smith研究了一群來自瑞典和蘇格蘭的極端微小化石，當中的每一具寬度都比毛髮還要細小。他企圖重建兩種在1980年代首度辨認出來的化石的生長模式。過去一度認為這兩種化石分屬於兩種不同生物的結構，但在辨識出一些跟兩者皆頗為相似的其他化石後，Smith得以下結論說這些化石其實是同一生物在不同生長階段的身體部位。在重建了這種生物的生長模式後，他認為這些化石代表了菌絲，一種真菌用來從土壤中吸收養分的根狀纖毛構造。

It's difficult to pinpoint exactly when life first migrated from the seas to the land, since useful features in the fossil record that could help identify the earliest land colonisers are rare, but it is generally agreed that the transition started early in the Palaeozoic era, between 500 and 450 million years ago. But before any complex forms of life could live on land, there needed to be nutrients there to support them. Fungi played a key role in the move to land, since by kick-starting the rotting process, a layer of fertile soil could eventually be built up, enabling plants with root systems to establish themselves, which in turn could support animal life.

由於化石紀錄中可以用來辨認出最早移居至陸地的生物為何者的證據相當稀少，因此很難精確指出生命首度從海洋遷往陸地的時間點。但學者一般認同在古生代早期生命開始往陸地邁進，約在5億至4億5000萬年前。而在任何複雜的生命形式能在陸地生活之前，陸上必須要先有養分才能夠餵養它們。在爬上陸地的過程當中，真菌扮演了關鍵腳色，這是因為它們能使分解作用開始運作，最終可以形成一層肥沃的土壤。具備根系的植物得以依此落腳維生，連帶使動物能夠生長。

Fungi play a vital role in the nitrogen cycle, in which nitrates in the soil are taken up by plant roots and passed along food chain into animals. Decomposing fungi convert nitrogen-containing compounds in plant and animal waste and remains back into nitrates, which are incorporated into the soil and can again be taken up by plants. These early fungi started the process by getting nitrogen and oxygen into the soil.

真菌在氮循環中為不可或缺的一環。在此循環當中，土壤裡的硝酸鹽會被植物根部汲取，並隨著食物鏈轉移至動物身上。能行使分解作用的真菌會將含氮的動植物排泄物和遺骸轉化回硝酸鹽，當它們重新回到土壤後便能再次被植物利用。這些早期的真菌藉由將獲取土壤中的氮和氧而開啟了這整個過程。

Smith found that Tortotubus had a cord-like structure, similar to that of some modern fungi, in which the main filament sends out primary and secondary branches that stick back onto the main filament, eventually enveloping it. This cord-like structure is often seen in land-based organisms, allowing them to spread out and colonise surfaces. In modern fungi, the structure is associated with the decomposition of matter, allowing a fungus colony to move nutrients to where they are needed -- a useful adaptation in an environment where nutrients are scarce and unevenly distributed.

Smith發現 Tortotubus具有跟某些現代真菌相當類似的繩索狀構造。這是種由主菌絲發散出的分支及更細的分支回貼在主菌絲身上，最終將其層層包覆而形成的構造。這種繩索狀構造通常見於陸生生物，這有助於它們附著在固體表面並往外擴散。在現生的真菌身上，此構造通常跟物質的分解有關。真菌菌落可以利用它們將養分移送至有需求的部分，這在營養稀少且不均勻分布的環境中是種相當實用的適應構造。

In contrast with early plants, which lacked roots and therefore had limited interaction with activity beneath the surface, fungi played an important role in stabilising sediment, encouraging weathering and forming soils.

原始植物由於缺少根部，因此它們難以跟地表之下的環境有所互動；相較起來，真菌在穩定環境、促進風化作用和形成土壤的過程中，皆具有更加重要的地位。

"What we see in this fossil is complex fungal 'behaviour' in some of the earliest terrestrial ecosystems -- contributing to soil formation and kick-starting the process of rotting on land," said Smith. A question, however, is what was there for Tortotubus to decompose. According to Smith, it's likely that there were bacteria or algae on land during this period, but these organisms are rarely found as fossils.

「我們從這些化石中看到在某些最原始的陸地生態系系統中，已經有相當複雜的真菌作用在進行著。它們的貢獻使土壤形成，並開啟了陸上的分解作用。」 Smith說。然而，有個問題是： Tortotubus究竟在陸地上分解著什麼物質。據 Smith說，在那段時期陸上可能已經有細菌或藻類存活，但這些生物很難以化石的形式發現。

Additionally, the pattern of growth in Tortotubus echoes that of the mushroom-forming fungi, although unambiguous evidence of mushrooms has yet to be found in the Palaeozoic fossil record. "This fossil provides a hint that mushroom-forming fungi may have colonised the land before the first animals left the oceans," said Smith. "It fills an important gap in the evolution of life on land."

此外，Tortotubus的生長模式也反映出當時可能已有會形成蕈菇的真菌了，雖然迄今我們仍尚未在古生代化石紀錄中確切發現蕈菇存在的證據。「這些化石暗示著會形成蕈菇的真菌定居在陸地的時間，也許比第一隻動物離開海洋還要來得早。」 Smith說。「這填補了陸地生命演化史上的一大空缺。」

The research was supported by Clare College, Cambridge.

本研究由劍橋大學克萊爾學院贊助。

引用自：University of Cambridge. "'A load of old rot': Fossil of oldest known land-dweller identified." ScienceDaily. ScienceDaily, 2 March 2016.