原文網址：http://www.cam.ac.uk/research/news/big-shape-shifting-animals-from-the-dawn-of-time
太初時代的巨大變形動物
在五億年以前，海洋化學成分的重大改變使得第一批大型生物――或許也是某些最早的動物得以出現並繁榮生長，此時間點同時也標記出地球環境的改變讓動物接管了這個世界。
為什麼地球上的動物體型會在某個時刻由小變大？劍橋大學和東京工業大學的研究人員找出了某些最初的大型生物，稱作「脈形類」(rangeomorph)，藉由從周圍環境吸收養分時能改變體型和形狀的能力而可以長到兩公尺高。
刊登在期刊《自然―生態學和演化》(Nature Ecology
and Evolution)的研究結果也有助於解釋地球上的生命如何從成員一度僅有微生物，最後變成能演化出恐龍和藍鯨之類的巨大生物。
脈形類是地球上最初的大型生物之一，牠們出現的時候其他大多數生命形式的體型得用顯微鏡才能看見。某些脈形類的高度僅有數公分，但有些最多可以長到兩公尺高。
這些居住在海中的生物生存於6億3500萬年至5億4100萬年前的埃迪卡拉紀(Ediacaran
period)。牠們柔軟的身體是由許多分枝組成，而每根分枝又分岔出許多更小的旁枝，使牠們的外型呈現出幾何學中的碎形，就像我們今天所看到的肺部血管、蕨類或雪花一樣。
由於沒有任何一種跟脈形類相似的現存生物，使得科學家難以瞭解牠們如何攝食、生長或者繁殖，更別說是牠們跟其他現存動物族群之間可能的關係為何。然而，雖然牠們某種程度上看起來很像植物，科學家相信牠們或許是存活在地球的最早動物之一。
論文第一作者，劍橋大學生命科學系與東京工業大學地球生命研究所的Jennifer Hoyal
Cuthill博士表示：「我們想要知道為什麼這些大型生物會在此特定時間點出現在地球歷史上。牠們以相當巨大的體型突如其來地出現在化石紀錄中，我們猜測牠們跟海洋化學的改變只是剛好在同一時刻發生，或者是有直接因果關係？」
研究人員利用微電腦斷層攝影、攝影測量法以及數學和電腦模型來觀察從加拿大紐芬蘭西南方、英國和澳洲出土的脈形類化石。
化石證據的分析結果首次證明了牠們的體型增長跟養分有關。所有生物都需要養分才能存活並成長，但養分多寡同時也決定了生物的體型大小和外型，此概念稱為「生態表型可塑性」(ecophenotypic plasticity)。Hoyal Cuthill和共同作者Simon Conway
Morris教授提出脈形類不僅表現出高度的生態表型可塑性，此能力也讓牠們在劇烈變化的世界中取得大量優勢。舉例來說，如果上方海水氧濃度提高，底下的脈形類可以迅速「變形」成修長的錐狀外形。
Hoyal Cuthill表示：「在埃迪卡拉紀時，地球海洋似乎出現了重大變化。這可能刺激了生物生長，使地球上的生物體型突然之間開始變大許多。埃迪卡拉紀的海中出現了什麼樣的重大地球化學變化使得生物體型開始變大，要下確切的結論恐怕還言之過早，但我們認為有幾個強力的候選因素，特別是氧濃度的提高，因為動物需要氧氣來進行呼吸作用。」
海洋化學的重大變化接續在稱作Gaskiers冰期的大規模冰河期之後。當海洋的的養分濃度低落時，生物顯然會讓牠們的體型維持在比較嬌小的狀態。但氧氣或是其他種養分突然(地質上來說)增加時，生物就有機會長到比原本大上許多，即使牠們是基因組成完全一樣的生物。這意味著巨大脈形類的突然出現可能是氣候和海洋化學發生的重大變化造成的直接後果。
然而，雖然脈形類對埃迪卡拉紀的環境適應相當良好，海洋化學仍持續變化。到了5億4100萬年前的「寒武紀大爆發」――在這段演化疾速進行的時期中，大多數主要動物類群首次在化石紀錄中登場――環境已經劇烈改變，使得脈形類走向滅亡，之後再也沒有出現跟牠們十分類似的物種了。
Big, shape-shifting
animals from the dawn of time
Major
changes in the chemical composition of the world’s oceans enabled the first
large organisms – possibly some of the earliest animals – to exist and thrive
more than half a billion years ago, marking the point when conditions on Earth
changed and animals began to take over the world.
Why did life on Earth change from small to
large when it did? Researchers from the University of Cambridge and the Tokyo
Institute of Technology have determined how some of the first large organisms,
known as rangeomorphs, were able to grow up to two metres in height, by
changing their body size and shape as they extracted nutrients from their
surrounding environment.
The results, reported in
the journal Nature Ecology and Evolution, could also help explain
how life on Earth, which once consisted only of microscopic organisms, changed
so that huge organisms like dinosaurs and blue whales could ultimately evolve.
Rangeomorphs
were some of the earliest large organisms on Earth, existing during a time when
most other forms of life were microscopic in size. Some rangeomorphs were only
a few centimetres in height, while others were up to two metres tall.
These
organisms were ocean dwellers that lived during the Ediacaran period, between
635 and 541 million years ago. Their soft bodies were made up of branches, each
with many smaller side branches, forming a geometric shape known as a fractal,
which can be seen today in things like lungs, ferns and snowflakes.
Since
rangeomorphs don’t resemble any modern organism, it’s difficult to understand
how they fed, grew or reproduced, let alone how they might link with any modern
group. However, although they look somewhat like plants, scientists believe
that they may have been some of the earliest animals to live on Earth.
“What we wanted
to know is why these large organisms appeared at this particular point in
Earth’s history,” said Dr Jennifer Hoyal Cuthill of Cambridge’s Department of
Earth Sciences and Tokyo Tech’s Earth-Life Science Institute, the paper’s first
author. “They show up in the fossil record with a bang, at very large size. We
wondered, was this simply a coincidence or a direct result of changes in ocean
chemistry?”
The
researchers used micro-CT scanning, photographic measurements and mathematical
and computer models to examine rangeomorph fossils from south-eastern
Newfoundland, Canada, the UK and Australia.
Their analysis
shows the earliest evidence for nutrient-dependent growth in the fossil record.
All organisms need nutrients to survive and grow, but nutrients can also
dictate body size and shape. This is known as ‘ecophenotypic plasticity.’ Hoyal
Cuthill and her co-author Professor Simon Conway Morris suggest that
rangeomorphs not only show a strong degree of ecophenotypic plasticity, but
that this provided a crucial advantage in a dramatically changing world. For
example, rangeomorphs could rapidly “shape-shift”, growing into a long, tapered
shape if the seawater above them happened to have elevated levels of oxygen.
“During the
Ediacaran, there seem to have been major changes in the Earth’s oceans, which
may have triggered growth, so that life on Earth suddenly starts getting much
bigger,” said Hoyal Cuthill. “It’s probably too early to conclude exactly which
geochemical changes in the Ediacaran oceans were responsible for the shift to
large body sizes, but there are strong contenders, especially increased oxygen,
which animals need for respiration.”
This change in
ocean chemistry followed a large-scale ice age known as the Gaskiers
glaciation. When nutrient levels in the ocean were low, they appear to have
kept body sizes small. But with a geologically sudden increase in oxygen or
other nutrients, much larger body sizes become possible, even in organisms with
the same genetic makeup. This means that the sudden appearance of rangeomorphs
at large size could have been a direct result of major changes in climate and
ocean chemistry.
However, while
rangeomorphs were highly suited to their Ediacaran environment, conditions in
the oceans continued to change and from about 541 million years ago the
‘Cambrian Explosion’ began – a period of rapid evolutionary development when
most major animal groups first appeared in the fossil record. When the
conditions changed, the rangeomorphs were doomed and nothing quite like them
has been seen since.
原始論文：Jennifer F.
Hoyal Cuthill, Simon Conway Morris. Nutrient-dependent growth underpinned
the Ediacaran transition to large body size. Nature Ecology &
Evolution, 2017; DOI: 10.1038/s41559-017-0222-7
引用自：University of
Cambridge. "Big, shape-shifting animals from the dawn of time."
