5.5億年前的化石顯示了動物留下的最早足跡
由維吉尼亞理工大學的地球科學家共同領導的科學團隊，做出了一項關於演化的重大發現：他們找到了大約五億年前，可能是動物在地球表面留下的第一道痕跡。
這道由夷陵帶刺蟲(Yilingia spiciformis)留下的痕跡化石年代為5.5億年。包括維吉尼亞理工大學的肖樹華在內的科學團隊，於中國發現了這道足跡。圖片來源：維吉尼亞理工大學
維吉尼亞理工大學理學院的地球科學教授肖樹華表示，他們挖出的這些化石極為有力地證明大約5.5億年前原始動物就可以自由運動。肖樹華以及中國科學院南京地質古生物研究所的陳哲、周傳明和袁訊來在中國夷陵附近的層狀岩石裡發現這些化石，其中含有一種原始動物的遺體以及牠們留下的痕跡。團隊將其命名為Yilingia spiciformis，意為夷陵帶刺蟲。
成果發表於最近一期的《自然》(Nature)。肖樹華和其團隊在2013至2018年於中國南部長江三峽區域進行的一連串挖掘工作，也找到了可能是蟲子留下的痕跡。它們和此次發現位於同樣的岩石單位，年代也都大約為埃迪卡拉紀，比恐龍、甚至是盤古超大陸都還古老許多。差別在於這次的化石保存了留下痕跡的動物，不像之前的沒有保存動物遺體只能進行猜測。
「這項發現顯示5.5億年前已經演化出具有體節並能夠自由運動的動物。」肖樹華表示，「不論是從字面上或是比喻來說，運動使動物在地球上『凡走過必留下痕跡』。包括我們人類與大多數動物在內的兩側對稱動物(bilateran)身上都可以找到運動這項特徵。對於地球來說，動物、尤其是人類具有不同凡響的影響力：牠們可以改變地球的面貌。歸根究柢這和動物發展出自由運動的能力息息相關。」
這種像馬陸的動物寬約四分之一英吋到一英吋(約0.6公分到2.5公分)，最長有四英寸(約10公分)。牠們在海床的泥巴上時而爬行時而停下，形成的痕跡長度可達23英吋(約60公分)。這種細長的動物約有50個體節，可以分成左右兩側，並能區分出背部腹部以及頭部尾部。
出現具有體節並能夠往特定方向運動的兩側對稱動物，是動物演化史早期的重大事件。科學家推測該事件發生在6.35億年前至5.39億年前的埃迪卡拉紀，但肖樹華和其團隊發現這些化石之前，並沒有確切的化石證據可以支持這項推測。他們發現的樣品中有一具特別重要，因為裡頭的動物在死前不久才留下痕跡，並一起保存了下來。
值得注意的是，此發現可能是第一筆跡象顯示有動物可以進行決策。肖樹華說這些痕跡顯示牠們試著靠近或遠離某個東西，或許是有先進的中樞神經系統在背後做出指示。可以自由運動的動物對地球表面的環境與生態造成衝擊，最終促成了寒武紀的底質和農藝革命(Cambrian
substrate and agronomic revolution)。
肖樹華也是維吉尼亞理工大學全球變遷中心的附屬會員。他說：「我們是地球上最具影響力的動物。人類留下了相當大量的足跡，不僅是在運動時產生，還源自於其他許多更具影響力的活動，這都跟我們能夠自由移動有關。動物的運動能力是在什麼時候、以什麼方式形成，提供了一種重要的脈絡讓我們從地質與演化的角度，看待人類對地球表面造成的影響。」
蘇格蘭愛丁堡大學地球科學院的教授Rachel
Wood並未參與此研究。她說：「這項驚人的發現找到了極為重要的化石。我們現在有證據顯示寒武紀之前就已經出現具有體節，而且可以在海床上到處移動的動物；更重要的是，我們可以指出實際製造這些痕跡的動物是什麼。保存這類證據的化石相當稀有，它們使我們對動物演化過程中的一大進展有更多理解。」
研究經費來自於中國科學院、中國國家自然科學基金委員會、美國國家科學基金會和國家地理學會。
Fossils dating back 550 million years
among first animal trails
In a remarkable evolutionary discovery, a
team of scientists co-led by a Virginia Tech geoscientist has discovered what
could be among the first trails made by animals on the surface of the Earth
roughly a half-billion years ago.
Shuhai Xiao, a professor of geosciences with the
Virginia Tech College of Science, calls the unearthed fossils, including the
bodies and trails left by an ancient animal species, the most convincing sign
of ancient animal mobility, dating back about 550 million years. Named Yilingia spiciformis – that translates
to spiky Yiling bug, Yiling being the Chinese city near the discovery site –
the animal was found in multiple layers of rock by Xiao and Zhe Chen, Chuanming
Zhou, and Xunlai Yuan from the Chinese Academy of Sciences’ Nanjing Institute
of Geology and Palaeontology.
The findings are published in the latest issue of Nature. The trials are from the same
rock unit and are roughly the same age as bug-like footprints found by Xiao and
his team in a series of digs from 2013 to 2018 in the Yangtze Gorges area of
southern China, and date back to the Ediacaran Period, well before the age of
dinosaurs or even the Pangea supercontinent. What sets this find apart: The
preserved fossil of the animal that made the trail versus the unknowable
guesswork where the body has not been preserved.
“This discovery shows that segmented and mobile
animals evolved by 550 million years ago,” Xiao said. “Mobility made it
possible for animals to make an unmistakable footprint on Earth, both literally
and metaphorically. Those are the kind of features you find in a group of
animals called bilaterans. This group includes us humans and most animals.
Animals and particularly humans are movers and shakers on Earth. Their ability
to shape the face of the planet is ultimately tied to the origin of animal
motility.”
The animal was a millipede-like creature a
quarter-inch to an inch wide and up to 4 inches long that alternately dragged
its body across the muddy ocean floor and rested along the way, leaving trails
as long as 23 inches. The animal was an elongated narrow creature, with 50 or
so body segments, a left and right side, a back and belly, and a head and a
tail.
The origin of bilaterally symmetric animals — known
as bilaterians — with segmented bodies and directional mobility is a monumental
event in early animal evolution, and is estimated to have occurred the
Ediacaran Period, between 635 and 539 million years ago. But until this finding
by Xiao and his team, there was no convincing fossil evidence to substantiate
those estimates. One of the recovered specimens is particularly vital because
the animal and the trail it produced just before its death are preserved
together.
Remarkably, the find also marks what may be the first
sign of decision making among animals – the trails suggest an effort to move
toward or away from something, perhaps under the direction of a sophisticated
central nerve system, Xiao said. The mobility of animals led to environmental
and ecological impacts on the Earth surface system and ultimately led to the
Cambrian substrate and agronomic revolutions, he said.
“We are the most impactful animal on Earth,” added
Xiao, also an affiliated member of the Global Change Center at Virginia Tech.
“We make a huge footprint, not only from locomotion, but in many other and more
impactful activities related to our ability to move. When and how animal
locomotion evolved defines an important geological and evolutionary context of
anthropogenic impact on the surface of the Earth.”
Rachel Wood, a professor in the School of GeoSciences
at University of Edinburgh in Scotland, who was not involved with the study,
said, “This is a remarkable finding of highly significant fossils. We now have
evidence that segmented animals were present and had gained an ability to move
across the sea floor before the Cambrian, and more notably we can tie the
actual trace-maker to the trace. Such preservation is unusual and provides
considerable insight into a major step in the evolution of animals.”
The study was supported by the Chinese Academy of
Sciences, the National Natural Science Foundation of China, the U.S. National
Science Foundation, and the National Geographic Society.
原始論文：Zhe Chen,
Chuanming Zhou, Xunlai Yuan, Shuhai Xiao. Death march of a segmented
and trilobate bilaterian elucidates early animal evolution. Nature,
2019; DOI: 10.1038/s41586-019-1522-7
引用自：Virginia Tech. "Geosciences’ Shuhai Xiao finds fossils
dating back 550 million years, among earliest known displays of animal
mobility."
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