前寒武紀的生命也許比之前認為得還要更加活躍

原文網址：www.sciencedaily.com/releases/2017/05/170519084411.htm

前寒武紀的生命也許比之前認為得還要更加活躍

在地球遙遠的過去曾有一段時期，淺海充斥著許多不知該如何分類的謎樣軟體生物，稱為「埃迪卡拉花園」(Garden of the Ediacaran)。科學家將這段從6.35億年至5.4億年前的時期想像成演化史上一片寧靜，近乎與世無爭的插曲。但是新的跨領域研究提出，生活在此時的生物也許比專家之前認為得還要更有活力。

科學家早就發現要把這些前寒武紀的物種安置在演化樹上是件極端困難的任務。這是因為它們存活在生物演化出形成甲殼或骨骼的能力之前，使得它們沒有留下多少它們曾經存活過的化石證據，而它們能否到處移動的證據就更加稀少了。因此，專家一般的結論是認為近乎所有的埃迪卡拉生物都不會運動，它們固著在海床上的某一處來度過整個成年生涯――除了某些類似水母漂在水中的動物為可能的例外。

這項新發現的關注對象為埃迪卡拉生物中最為謎樣的其中一屬，稱作帕文柯利納蟲(Parvancorina)。此種硬幣大小的生物特徵為在背上具有幾道隆起，形狀就跟小小的船錨一樣。一組國際研究團隊分析了水如何在帕文柯利納蟲的身體附近流動之後，他們總結這些古代生物必定可以自由運動：精確來說，它們必定擁有調整自身方向的能力，來正面迎向它們身處的水流方向。這或許使它們成為已知物種中，最早擁有科學家稱為「向流性」(rheotaxis)能力的物種。

領導此研究，美國范德堡大學地球和環境科學的助理教授Simon Darroch表示：「我們的分析結果顯示水流從頭部流往尾部產生的阻力，比從一側流往另一側的阻力要小上許多。處在以強力水流為特徵的淺海環境中，這項結果意謂著帕文柯利納蟲可以從調整自身方向來面對水流的行為中，得到許多好處。」

他們的分析技術借用了工程學中的計算流體力學(CFD)，結果還顯示出當帕文柯利納蟲面朝水流時，它的身體形狀可以產生數道渦流，流向它身上的某些特定位置。Darroch表示：「如果像我們猜測的一樣，帕文柯利納蟲是懸浮物攝食生物的話，則此特性具有相當益處。因為渦流可以集中懸浮有機物質，讓帕文柯利納蟲很容易就能吃掉它們。」

在5月17日刊登於期刊《皇家學會生物學通訊》(Royal Society journal Biology Letters)，題名為「Inference of facultative mobility in the enigmatic Ediacaran organism Parvancorina」的論文中，有詳細的分析結果描述。

這項發現也佐證了另一篇5月30日刊登在期刊《科學報導》(Scientific Reports)，由澳洲的研究學者進行的獨立研究。他們分析澳洲南部的埃迪卡拉化石場址，發現帕文柯利納蟲的化石排列方向，傾向於對齊當時的主要水流方向。他們確認排列一致的現象並非被動產生，而是該物種在生活史上的某個時刻會主動表現出趨流反應(rheotactic response)。

這是計算流體力學第二次被用在埃迪卡拉化石的研究。2015年，同一組研究團隊利用相同技術來分析三腕蟲(Tribrachidium heraldicum)身體周圍的水流模式。此種盤子狀的生物背上具有三道螺旋狀的凸脊。在那篇研究中，他們的分析結果支持了三腕蟲是已知最早的懸浮物攝食生物，年代可追溯至5.55億年前。

Darroch表示：「我們決定停止去嘗試找出這些物種在演化樹中的定位，而是改去嘗試瞭解演化的力量如何形塑它們。我們想要知道它們古怪的外型會如何影響攝食、繁殖和移動方式。由於它們生活在淺海環境，所以強力的海流在其演化過程中必定會扮演相當重要的腳色。因此在解開這些問題時，計算流體力學便成為相當完美的工具。」

根據團隊成員，牛津大學自然史博物館的研究員Imran Rahman表示，計算流體力學之前是用來分析從核反應爐到飛機，各種建築物和機械的設計並最佳化其效能，但要到最近幾年才有人把它拿來研究化石紀錄。他說：「計算流體力學具有改變我們了解古代生物如何攝食及運動的潛力，所以我很期待未來數年會有更多古生物學家開始運用這項方法。」

共同作者，多倫多大學密西沙加分校的地球科學助理教授Marc Laflamme補充：「仔細想想我們簡直就是在重現古代海洋。我們將這些挑戰我們的認知超過50年以上，滅絕許久的生物數位化後，讓它們生活在我們的虛擬海洋中，以更加認識它們的日常生活。甚至在10年前，像這樣的研究工作都還不可能實行，而我相信其代表了當代古生物學正在進步的方向。」

Darroch表示：「我們確定出一種埃迪卡拉生物可以到處移動的事實，顯示我們對此時期的想像可能在根本層面就犯錯了。或許可以活動的埃迪卡拉生物比我們認為得還要多出許多，而我們打算應用此技術到其他埃迪卡拉生物的化石身上，以證實我們的猜測。」

Life in the Precambrian may have been much livelier than previously thought

The Garden of the Ediacaran was a period in the ancient past when Earth's shallow seas were populated with a bewildering variety of enigmatic, soft-bodied creatures. Scientists have pictured it as a tranquil, almost idyllic interlude that lasted from 635 to 540 million years ago. But a new interdisciplinary study suggests that the organisms living at the time may have been much more dynamic than experts have thought.

Scientists have found It extremely difficult to fit these Precambrian species into the tree of life. That is because they lived in a time before organisms developed the ability to make shells or bones. As a result, they didn't leave much fossil evidence of their existence behind, and even less evidence that they moved around. So, experts have generally concluded that virtually all of the Ediacarans -- with the possible exception of a few organisms similar to jellyfish that floated about -- were stationary and lived out their adult lives fixed in one place on the sea floor.

The new findings concern one of the most enigmatic of the Ediacaran genera, a penny-sized organism called Parvancorina, which is characterized by a series of ridges on its back that form the shape of a tiny anchor. By analyzing the way in which water flows around Parvancorina's body, an international team of researchers has concluded that these ancient creatures must have been mobile: specifically, they must have had the ability to orient themselves to face into the current flowing around them. That would make them the oldest species known to possess this capability, which scientists call rheotaxis.

"Our analysis shows that the amount of drag produced with the current flowing from front to back is substantially less than that flowing from side to side," said Simon Darroch, assistant professor of earth and environmental sciences at Vanderbilt University, who headed the study. "In the strong currents characteristic of shallow ocean environments, that means Parvancorina would have benefited greatly from adjusting its position to face the direction of the flow."

The analysis, which used a technique borrowed from engineering called computational fluid dynamics (CFD), also showed that when Parvancorina faced into the current, its shape created eddy currents that were directed to several specific locations on its body. "This would be very beneficial to Parvancorina if it was a suspension feeder as we suspect because it would have concentrated the suspended organic material making it easier to consume," Darroch said.

Details of the analysis are described in a paper titled "Inference of facultative mobility in the enigmatic Ediacaran organism Parvancorina" published online May 17 by the Royal Society journal Biology Letters.

These conclusions are reinforced by an independent study performed by a team of Australian researchers publishedMarch30 in the journal Scientific Reports. Analyzing an Ediacaran site in South Australia, they found that the Parvancorina fossils were preferentially aligned in the direction of the prevailing current and determined that this alignment was not passive but represented a rheotactic response at some point in the organism's life history.

This is only the second time that CFD has been applied to study Ediacarans. In 2015, the same team of researchers applied this technique to analyze flow patterns around an organism called Tribrachidium heraldicum. This is a disk-shaped organism characterized by three spiraling ridges on its back. In this case, their analysis supported the conclusion that it was the oldest known suspension feeder, dating back to 555 million years.

"We decided to stop trying to figure out where these species fit in the tree of life and to try to determine how they were shaped by evolutionary forces," said Darroch. "We wanted to understand how their weird architectures affected how they ate, reproduced and moved. Because they lived in a shallow sea environment, strong currents must have played a major role in their evolution. So computational fluid dynamics is the perfect tool for addressing this question."

According to team member Imran Rahman, research fellow at the Oxford University Museum of Natural History, CFD has been used to analyze the design and optimize the performance of a wide variety of structures and machines, ranging from nuclear reactors to aircraft, but it is only in the last few years that they have begun applying it to study the fossil record: "CFD has the potential to transform our understanding of how ancient organisms fed and moved, so I would anticipate that many more paleontologists will start making use of the method in coming years."

"When you sit back and think about it, we are virtually recreating ancient oceans, and populating them with digital representations of long extinct organisms that have defied our understanding for over 50 years in order to gain insight on how they lived their day to day lives," added co-author Marc Laflamme, assistant professor of earth science at the University of Toronto Mississauga. "This kind of work would not have been feasible even a decade ago, and I believe it represents the direction that modern paleontology is forging."

"The fact that we have now established that one Ediacaran species could move around suggests that our picture of this period may be fundamentally wrong," said Darroch. "There may have been a lot more movement going on than we thought and we intend to apply this technique to other Ediacaran fossils to determine if that was the case."

原始論文：Simon A. F. Darroch, Imran A. Rahman, Brandt Gibson, Rachel A. Racicot, Marc Laflamme. Inference of facultative mobility in the enigmatic Ediacaran organism Parvancorina. Biology Letters, 2017; 13 (5): 20170033 DOI: 10.1098/rsbl.2017.0033

引用自：Vanderbilt University. "Life in the Precambrian may have been much livelier than previously thought." ScienceDaily. ScienceDaily, 19 May 2017.