動物演化史：在火熱的開始之後是一陣寒冬

原文網址：www.sciencedaily.com/releases/2017/06/170615084556.htm

動物演化史：在火熱的開始之後是一陣寒冬

動物演化的最初階段進行速度比迄今認為得還要快：新的分析結果提出首批動物門以很快的速度接連出現，時間點為7億年前全球陷入冰河期之前。

化石證據顯示現存的所有動物門幾乎都是在5.4億年前，地球歷史剛進入寒武紀時就已經出現了。人類所知最早的動物化石已經呈現出相當複雜的型態，意謂動物必定早在寒武紀開始之前就出現了。然而，可以確切分類的化石中，只有極少數能確定它們的年代是在前寒武紀時期。為了確定動物演化樹的根源看起來是何種樣貌，生物學家需要對最古老的動物類群――海綿、腔腸動物、櫛水母和絲盤蟲做出更加可信的定年資訊。慕尼黑路德維希-馬克西米利安大學地球和環境科學系，古生物和地質生物學門的Martin Dohrmann博士和Gert Wörheide教授，運用了以分子時鐘為基礎的新策略，來研究早期動物演化的年代順序，並對最古老動物類群的出現時間做出新的預測。他們的結果刊登於期刊《科學報告》(Scientific Reports)。

分子時鐘的原理是基於所有生物體內的基因體，都會隨著時間經過而累積突變。因此，兩條生物譜系之間的基因差異程度，應該會跟它們從共同祖先分歧出來已經過了多久有關。Dohrmann解釋：「我們研究的基礎結合了現存生物的基因數據，加上從定年良好的化石中取得的資訊。然後再藉助複雜的電腦演算來分析。」為了進行研究，研究人員利用了一個龐大的資料庫，其中含有55個物種身上128種蛋白質的定序結果。這些物種包括了所有動物類群中的代表性物種。研究人員特別聚焦於那些非常久以前就跟其他物種分道揚鑣的生物。

分析證實了一篇前人研究得出的結論，他們將動物的起源定在從10億年前持續至5.4億年前的新元古代(Neoproterozoic Era)。然而，令他們大感驚訝的是，分析結果也呈現出最早的動物門和所有兩側對稱動物的祖先，都是在短短5000萬年中(以地質角度來說)接連出現。Dohrmann表示：「此外，這段發生在演化史早期的多樣化階段，似乎是在讓地球進入雪球地球的極端氣候變遷之前。在7.2億年前至6.35億年前，整個地球都陷入一段極為漫長的冰河期，稱為雪球地球。」為了評估這項新發現的可信程度，研究人員接下來的計畫是利用包含層面更廣的資料庫和改良後的統計方法進行更深入的分析。Wörheide表示：「要得到從最初動物的型態和生態角度來看都相當穩固的結論，我們還需要更加瞭解新元古代時普遍的環境狀態，以及更多可以確切歸屬至特定分類族群的化石。」

Animal evolution: Hot start, followed by cold shock

The initial phases of animal evolution proceeded faster than hitherto supposed: New analyses suggest that the first animal phyla emerged in rapid succession -- prior to the global Ice Age that set in around 700 million years ago.

The fossil record reveals that almost all of the animal phyla known today had come into existence by the beginning of the Cambrian Period some 540 million years ago. The earliest known animal fossils already exhibit complex morphologies, which implies that animals must have originated long before the onset of the Cambrian. However, taxonomically assignable fossils that can be confidently dated to pre-Cambrian times are very rare. In order to determine what the root of their family tree looked like, biologists need reliable dating information for the most ancient animal subgroups -- the sponges, cnidarians, comb jellies and placozoans. Dr. Martin Dohrmann and Professor Gert Wörheide of the Division of Palaeontology and Geobiology in the Department of Earth and Environmental Sciences at Ludwig-Maximilians-Universitaet (LMU) in Munich have now used a new strategy based on the so-called molecular-clock to investigate the chronology of early animal evolution and produce a new estimate for the ages of the oldest animal groups. Their findings appear in the journal Scientific Reports.

The molecular clock approach is based on the principle that mutations accumulate in the genomes of all organisms over the course of time. The extent of the genetic difference between two lineages should therefore depend on the time elapsed since they diverged from their last common ancestor. "Our study is based on a combination of genetic data from contemporary animals and information derived from well dated fossils, which we analyzed with the help of complex computer algorithms," Dohrmann explains. For the study, the researchers used an unusually large dataset made up of the sequences of 128 proteins from 55 species, including representatives of all the major animal groups, focusing in particular on those that diverged very early.

The analysis confirms the conclusion reached in an earlier study, which dated the origin of animals to the Neoproterozoic Era, which lasted from 1000 to 540 million years ago. However, much to their surprise, the results also suggested that the earliest phyla, and the ancestors of all bilateral animal species (the so-called Bilateria), originated within the -- geologically speaking -- short time-span of 50 million years. "In addition, this early phase of evolutionary divergence appears to have preceded the extreme climate changes that led to Snowball Earth, a period marked by severe long-term global glaciation that lasted from about 720 to 635 million years ago," Dohrmann says. In order to assess the plausibility of the new findings, the researchers plan to carry out further analyses using more extensive datasets and improved statistical methods." To arrive at well-founded conclusions with respect to the morphology and ecology of the earliest animals, we also need to know more about the environmental conditions that prevailed during the Neoproterozoic, and we need more fossils that can be confidently assigned to specific taxonomic groups," Wörheide says.

原始論文：Martin Dohrmann, Gert Wörheide. Dating early animal evolution using phylogenomic data. Scientific Reports, 2017; 7 (1) DOI: 10.1038/s41598-017-03791-w

引用自：Ludwig-Maximilians-Universität München. "Animal evolution: Hot start, followed by cold shock." ScienceDaily. ScienceDaily, 15 June 2017.