化石指出花朵的起源時間比過去認為的還要早5000萬年

原文網址:https://elifesciences.org/for-the-press/23106e89/fossils-suggest-flowers-originated-50-million-years-earlier-than-thought

化石指出花朵的起源時間比過去認為的還要早5000萬年

在發表於開放取用期刊《eLife》的新研究中，科學家描述的一種植物化石顯示1億7400萬年以前的侏儸紀早期就已經有花朵盛開。

含有南京花(Nanjinganthus)化石的粉砂岩石板。圖片來源：中國科學院南京地質古生物研究所

在此研究之前，一般認為被子植物(開花植物)的歷史不超過1億3000萬年。然而，研究作者最新發現並命名為樹狀蕊南京花(Nanjinganthus dendrostyla)的花朵化石，卻對這個廣為接受的植物演化理論提出了質疑，他們認為該物種活著的年代比上述還早了大約5000萬年左右。此外，南京花還有許多特性是這些理論未預料到的。

被子植物是植物王國中非常重要的一類成員，而它們的起源也是演化生物學家之間爭論許久的議題。過去許多理論認為被子植物的歷史不超過1億3000萬年，然而分子時鐘卻指出它們的淵源必定還要更加古老。目前為止，還沒有任何基於化石的證據可以把它們存在的時間更往前推。

「之前研究人員無法確定花朵的起源時間和地點，因為白堊紀時許多花朵就像是憑空出現一樣。」中國科學院南京地質古生物研究所的副教授傅強表示。「要有憑有據地回答這些問題的唯一方法就是研究花朵化石，尤其是在較早的地質年代中出現的。」

團隊研究的樣品採自中國南京地區以含有侏儸紀早期化石而聞名的南象山層岩石露頭中，這64塊石板所保存的264個樣品包含了198朵花。如此大量的化石樣品讓研究人員可以剖開其中一些樣品，運用精密的顯微鏡從不同角度做出不同倍率下花朵化石的高解析度圖像。接著再用這些從不同化石得到的型態與結構上的詳細資訊，重建出樹狀蕊南京花的面貌。

被子植物的關鍵特徵是完全包裹起來的胚珠――此部位在受精之後會形成種子。研究人員將南京花重建出來後，發現它具有由杯狀的花托跟子房頂部一起包覆而成的胚珠/種子。這項發現的重要之處在於此特徵證明了南京花的地位確實為被子植物。雖然之前曾有報告在中國東北找到侏儸紀中晚期的被子植物，但是南京花的構造特徵可以和那些樣品區分開來，使它成為被子植物的一個新屬。

在做出這項發現之後，研究團隊現在想要瞭解被子植物是單系群還是多系群――也就是南京花所屬的幹群產生了之後所有的被子植物；或者南京花最後走到了演化的死胡同，跟之後的大部分物種都沒有什麼關係。

「被子植物的起源對很多植物學家來說是頭痛許久的問題。」論文資深作者，中國科學院南京地質古生物研究所的教授王鑫總結。「我們的發現對植物學界來說是一大進展，並讓我們對被子植物有更深的瞭解。這可以使我們更有效地運用並照料地球上的植物資源。」

Fossils suggest flowers originated 50 million years earlier than thought

Scientists have described a fossil plant species that suggests flowers bloomed in the Early Jurassic, more than 174 million years ago, according to new research in the open-access journal eLife.

Before now, angiosperms (flowering plants) were thought to have a history of no more than 130 million years. The discovery of the novel flower species, which the study authors named Nanjinganthus dendrostyla, throws widely accepted theories of plant evolution into question, by suggesting that they existed around 50 million years earlier. Nanjinganthus also has a variety of ‘unexpected’ characteristics according to almost all of these theories.

Angiosperms are an important member of the plant kingdom, and their origin has been the topic of long-standing debate among evolutionary biologists. Many previously thought angiosperms could be no more than 130 million years old. However, molecular clocks have indicated that they must be older than this. Until now, there has been no convincing fossil-based evidence to prove that they existed further back in time.

“Researchers were not certain where and how flowers came into existence because it seems that many flowers just popped up in the Cretaceous from nowhere,” explains lead author Qiang Fu, Associate Research Professor at the Nanjing Institute of Geology and Paleontology, China. “Studying fossil flowers, especially those from earlier geologic periods, is the only reliable way to get an answer to these questions.”

The team studied 264 specimens of 198 individual flowers preserved on 34 rock slabs from the South Xiangshan Formation – an outcrop of rocks in the Nanjing region of China renowned for bearing fossils from the Early Jurassic epoch. The abundance of fossil samples used in the study allowed the researchers to dissect some of them and study them with sophisticated microscopy, providing high-resolution pictures of the flowers from different angles and magnifications. They then used this detailed information about the shape and structure of the different fossil flowers to reconstruct the features of Nanjinganthus dendrostyla.

The key feature of an angiosperm is ‘angio-ovuly’ – the presence of fully enclosed ovules, which are precursors of seeds before pollination. In the current study, the reconstructed flower was found to have a cup-form receptacle and ovarian roof that together enclose the ovules/seeds. This was a crucial discovery, because the presence of this feature confirmed the flower’s status as an angiosperm. Although there have been reports of angiosperms from the Middle-Late Jurassic epochs in northeastern China, there are structural features of Nanjinganthus that distinguish it from these other specimens and suggest that it is a new genus of angiosperms.

Having made this discovery, the team now wants to understand whether angiosperms are either monophyletic – which would mean Nanjinganthus represents a stem group giving rise to all later species – or polyphyletic, whereby Nanjinganthus represents an evolutionary dead end and has little to do with many later species.

“The origin of angiosperms has long been an academic ‘headache’ for many botanists,” concludes senior author Xin Wang, Research Professor at the Nanjing Institute of Geology and Paleontology. “Our discovery has moved the botany field forward and will allow a better understanding of angiosperms, which in turn will enhance our ability to efficiently use and look after our planet’s plant-based resources.”

原始論文：Qiang Fu, Jose Bienvenido Diez, Mike Pole, Manuel García Ávila, Zhong-Jian Liu, Hang Chu, Yemao Hou, Pengfei Yin, Guo-Qiang Zhang, Kaihe Du, Xin Wang. An unexpected noncarpellate epigynous flower from the Jurassic of China. Elife, 2018; 7

引用自：eLIFE. "Fossils suggest flowers originated 50 million years earlier than thought."