地球上的生物體型首次變大的原因為何？
地球上某些最早期的複雜生物――同時可能也是最早出現的動物――體型增加的原因並非是為了競爭食物，而是想要盡可能地把後代散佈到遠方。
由劍橋大學主導的這項研究發現五億多年前生活在海洋當中最為成功的物種，是可以把牠們的後代「拋」得最遠，藉此拓展到周圍環境的物種。這項研究發表在期刊《自然―生態學與演化》(Nature Ecology and Evolution)。
在距今6.35億至5.41億年前的埃迪卡拉紀以前，生物的體型相當微小。但在埃迪卡拉紀時首度出現了大型的複雜生物，其中某些種類，像是被稱作脈形類(rangeomorph)的生物，甚至可以長到兩公尺這麼高。脈形類是地球最早出現的複雜生物之一。雖然看起來像是蕨類，但它們卻有可能是最早存在的動物種類之一――不過要完全下定結論對科學家來說仍相當困難。埃迪卡拉生物外觀上並沒有口部以及用來移動的器官或方法，因此科學家認為它們是從周遭的海水當中吸收養分維生。
隨著埃迪卡拉生物越長越大，它們的形貌也變得更加多樣，其中某些發展出類似枝幹的構造來支撐牠們往上生長。
在現代，環境裡的生物(像是森林)彼此之間會為了如陽光之類的資源而產生強烈競爭。因此，高大的樹木或植物會比它們的矮小鄰居有更加明顯的優勢。「我們想要知道埃迪卡拉紀時，類似的因素是否驅動了生物的變化。」這篇論文的主要作者，劍橋大學地球科學系的Emily
Mitchell博士表示，「地球上的生物體型變大是競爭之下造成的結果嗎？」
Mitchell和共同作者，加拿大紐芬蘭紀念大學的Charlotte
Kenchington博士，檢視了從紐芬蘭東南方的迷斯塔肯角(Mistaken
Point)取得的化石，此處為世上保存最多埃迪卡拉化石的場址之一。
之前的研究推論體型增加的動力是為了爭奪在不同水深的養分。然而，目前的研究成果顯示埃迪卡拉紀的海洋更像是可以讓你吃到飽的自助餐。
「當時海洋裡的養分十分充足，因此對於資源的爭奪沒有很激烈，且掠食者也還尚未出現。」劍橋大學默里•愛德華茲學院的研究員Mitchell表示，「因此必然有其他原因使得那段時間的生物變得如此巨大。」
由於埃迪卡拉生物無法移動而且被保存在它們活著當時的所在位置，使得研究人員可以利用化石紀錄來分析整個族群。運用空間分析技術，Mitchell和Kenchington發現物種高度和食物競爭之間沒有關聯。不同類型的生物並沒有佔據水層當中的不同位置以避免資源競爭――也就是不存在所謂的「分層」(tiering)作用。
「如果它們有在競爭食物的話，那我們應該可以發現這些有枝幹的生物會有高度分層的現象。」Kenchington表示，「但是，我們的發現卻正好相反。事實上，分層現象在不具枝幹的生物中比在具有枝幹的生物中還要明顯，因此這些枝幹可能別有用途。」
據研究人員表示，脈形類產生後代的方法是釋放出小型的繁殖體，因此枝幹的可能用途之一便是能把後代散佈得更遠。在最高大的脈形類周圍有著最大群的後代子孫，代表高度帶來的好處並非是可以取得更多食物，而是有更高的機會佔領某一地區。
「雖然較高大的物種可能會傾向出現在流速較快的水中，但這些群集缺乏分層現象，顯示高度在攝取養分這一方面並沒有帶給牠們任何獨特的優勢。」Mitchell表示，「反之，地球上的生物在當時體型變大的主因似乎是為了繁衍後代。」
儘管它們曾經如此成功，脈形類和其他的埃迪卡拉生物卻在大約5.4億年前寒武紀開始時通通消失了。此時的演化有了飛快進展，大多數的主要動物族群都是在此時第一次出現在化石紀錄中。
Why life on Earth first got big
Some of the
earliest complex organisms on Earth – possibly some of the earliest animals to
exist – got big not to compete for food, but to spread their offspring as far
as possible.
The research, led by the University of Cambridge,
found that the most successful organisms living in the oceans more than half a
billion years ago were the ones that were able to ‘throw’ their offspring the farthest,
thereby colonising their surroundings. The results are reported in the journal Nature Ecology and Evolution.
Prior to the Ediacaran period, between 635 and 541
million years ago, life forms were microscopic in size, but during the
Ediacaran, large, complex organisms first appeared, some of which – such as a
type of organism known as rangeomorphs – grew as tall as two metres. These
organisms were some of the first complex organisms on Earth, and although they
look like ferns, they may have been some of the first animals to exist –
although it’s difficult for scientists to be entirely sure. Ediacaran organisms
do not appear to have mouths, organs or means of moving, so they are thought to
have absorbed nutrients from the water around them.
As Ediacaran organisms got taller, their body shapes
diversified, and some developed stem-like structures to support their height.
In modern environments, such as forests, there is
intense competition between organisms for resources such as light, so taller
trees and plants have an obvious advantage over their shorter neighbours. “We
wanted to know whether there were similar drivers for organisms during the
Ediacaran period,” said Dr Emily Mitchell of Cambridge’s Department of Earth
Sciences, the paper’s lead author. “Did life on Earth get big as a result of
competition?”
Mitchell and her co-author Dr Charlotte Kenchington
from Memorial University of Newfoundland in Canada examined fossils from
Mistaken Point in south-eastern Newfoundland, one of the richest sites of
Ediacaran fossils in the world.
Earlier research hypothesised that increased size was
driven by the competition for nutrients at different water depths. However, the
current work shows that the Ediacaran oceans were more like an all-you-can-eat
buffet.
“The oceans at the time were very rich in nutrients,
so there wasn’t much competition for resources, and predators did not yet
exist,” said Mitchell, who is a Henslow Research Fellow at Murray Edwards
College. “So there must have been another reason why life forms got so big
during this period.”
Since Ediacaran organisms were not mobile and were
preserved where they lived, it’s possible to analyse whole populations from the
fossil record. Using spatial analysis techniques, Mitchell and Kenchington
found that there was no correlation between height and competition for food.
Different types of organisms did not occupy different parts of the water column
to avoid competing for resources – a process known as tiering.
“If they were competing for food, then we would
expect to find that the organisms with stems were highly tiered,” said
Kenchington. “But we found the opposite: the organisms without stems were
actually more tiered than those with stems, so the stems probably served
another function.”
According to the researchers, one likely function of
stems would be to enable the greater dispersion of offspring, which
rangeomorphs produced by expelling small propagules. The tallest organisms were
surrounded by the largest clusters of offspring, suggesting that the benefit of
height was not more food, but a greater chance of colonising an area.
“While taller organisms would have been in faster-flowing
water, the lack of tiering within these communities shows that their height
didn’t give them any distinct advantages in terms of nutrient uptake,” said
Mitchell. “Instead, reproduction appears to have been the main reason that life
on Earth got big when it did.”
Despite their success, rangeomorphs and other
Ediacaran organisms disappeared at the beginning of the Cambrian period about
540 million years ago, a period of rapid evolutionary development when most
major animal groups first appear in the fossil record.
原始論文：Emily G.
Mitchell, Charlotte G. Kenchington. The utility of height for the
Ediacaran organisms of Mistaken Point. Nature Ecology &
Evolution, 2018; DOI: 10.1038/s41559-018-0591-6
引用自：University
of Cambridge. "Why life on Earth first got big."
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