動(dòng)物們是如何在“雪球地球”中生存下來(lái)的?

動(dòng)物們是如何在“雪球地球”中生存下來(lái)的?

As humans heat up the Earth with greenhouse gases, we're recreating an ancient climate unlike anything our species has ever seen. This is drawing more attention to the history of Earth's climates, especially warm periods like the Pliocene Epoch, which many scientists consider a model for where we're headed.

隨著人類(lèi)用溫室氣體使地球升溫，我們正在重建一種我們從未見(jiàn)過(guò)的遠(yuǎn)古氣候。這引起了人們對(duì)地球氣候歷史的更多關(guān)注，尤其是像上新世這樣的溫暖時(shí)期，許多科學(xué)家認(rèn)為上新世是我們前進(jìn)方向的一個(gè)模型。

An artist's impression of 'Snowball Earth.' (Image: NASA)

At the same time, though, researchers are also shedding new light on other, very different periods in Earth's past. These too can reveal key details about our planet, and even ourselves, despite bearing little resemblance to the world we know today.

然而，與此同時(shí)，研究人員也對(duì)地球歷史上其他非常不同的時(shí)期進(jìn)行了新的研究。這些也可以揭示我們的星球，甚至我們自己的關(guān)鍵細(xì)節(jié)，盡管與我們今天所知的世界幾乎沒(méi)有相似之處。

One such period is the the Cryogenian, which lasted from about 720 million to 635 million years ago. That's when Earth experienced the most extreme ice age in its history, including a global freeze known as "Snowball Earth."

其中一個(gè)時(shí)期是低溫期，從大約7.2億年前持續(xù)到6.35億年前。那時(shí)，地球經(jīng)歷了其歷史上最極端的冰河時(shí)代，包括被稱為“雪球地球”的全球凍結(jié)。

Somehow, though, it was also when the first signs of complex animals appeared in the fossil record, left by creatures who set the stage for a golden age of animal life that continues today. In a new study, researchers examined the chemistry of Cryogenian rocks to learn more about this unfamiliar world — including why it was able to not only support animal life, but also seemingly launch it to new heights.

然而，也正是在那個(gè)時(shí)候，復(fù)雜動(dòng)物的最初跡象出現(xiàn)在化石記錄中，這些生物為延續(xù)至今的動(dòng)物生命的黃金時(shí)代奠定了基礎(chǔ)。在一項(xiàng)新的研究中，研究人員檢測(cè)了低溫巖石的化學(xué)成分，以進(jìn)一步了解這個(gè)陌生的世界——包括為什么它不僅能夠支持動(dòng)物生命，而且似乎還能把它推向新的高度。

Let it snow

下雪

Ice sheets now cover parts of Greenland and Antarctica, but during the Cryogenian Period, they may have covered vast swaths of Earth's surface. (Photo: Laura Tenenbaum [CC BY 2.0]/NASA)

The planet's surface became fully or almost fully frozen during the Cryogenian, with enormous ice sheets stretching down to the tropics. (There is still some debate about the extent of this freeze, though.) Most landmasses were united in the supercontinent Rodinia, but thanks to the global glacier, Earth's entire surface may have been effectively solid. The average surface temperature probably didn't go far above freezing, and some research suggests temperatures were much colder, possibly falling below minus 50 degrees Celsius (minus 58 Fahrenheit).

在低溫時(shí)代，地球表面完全或幾乎完全凍結(jié)，巨大的冰原一直延伸到熱帶地區(qū)。(不過(guò)，對(duì)于凍結(jié)的程度仍有一些爭(zhēng)議。)大部分的大陸塊都在超級(jí)大陸羅迪尼亞統(tǒng)一，但是由于全球冰川的作用，地球的整個(gè)表面可能已經(jīng)變得非常堅(jiān)固。地球表面的平均溫度可能不會(huì)超過(guò)冰點(diǎn)很多，一些研究表明，溫度要低得多，可能會(huì)降到零下50攝氏度(零下58華氏度)以下。

There were actually two big freezes during the Cryogenian, known as the Sturtian and Marinoan glaciations, separated by a brief intermission of heat, melting ice and erupting volcanoes. This was a wild time for our planet, which was see-sawing between extremes of ice and fire, but also an important one. That's because, despite seeming like a terrible time to be alive, the Cryogenian Period apparently helped spark the dawn of complex animals — including our own ancestors.

事實(shí)上，在冰川期有兩次大的冰期，被稱為斯圖爾特期和馬里諾期冰川期，其間有短暫的熱間歇、冰的融化和火山的噴發(fā)。對(duì)我們的星球來(lái)說(shuō)，這是一個(gè)瘋狂的時(shí)代，在極端的冰和火之間搖擺不定，但也是一個(gè)重要的時(shí)刻。這是因?yàn)?，盡管低溫時(shí)代對(duì)人類(lèi)來(lái)說(shuō)似乎是一個(gè)可怕的時(shí)代，但它顯然幫助激發(fā)了復(fù)雜動(dòng)物的黎明——包括我們自己的祖先。

If you're wondering how animals survived on Snowball Earth, you're not alone. It would've been incredibly difficult for animals to survive on the ice sheets, but also in the seawater below, since a global coating of ice would severely hinder the oceans' ability to absorb oxygen. Scientists have long puzzled over this apparent paradox, but the new study, published this week in the Proceedings of the National Academy of Sciences, is the latest in a growing body of research that's finally offering answers.

如果你想知道動(dòng)物是如何在雪球地球上生存下來(lái)的，你并不孤單。對(duì)動(dòng)物來(lái)說(shuō)，要在冰原上以及下面的海水中生存是非常困難的，因?yàn)槿蚋采w的冰層會(huì)嚴(yán)重阻礙海洋吸收氧氣的能力。長(zhǎng)期以來(lái)，科學(xué)家們一直對(duì)這一明顯的悖論感到困惑，但本周發(fā)表在《美國(guó)國(guó)家科學(xué)院院刊》上的這項(xiàng)新研究，是越來(lái)越多最終給出答案的研究中的最新成果。

Explosion of animal life

動(dòng)物生命的大爆發(fā)

The Cambrian Explosion was a huge turning point in Earth's history, thanks to an evolutionary boom that yielded increasingly big and complex animals. (Illustration: Dotted Yeti/Shutterstock)

Life on Earth began long before the Cryogenian, but it was mostly single-celled microbes. Even when multicellular animals did arise, they were simple, often stationary creatures, calmly filtering seawater or grazing on mats of microbes. These early animals did not yet have innovations like eyes, legs, jaws or claws, and in a world without predators, they didn't really need them.

地球上的生命早在低溫時(shí)代之前就開(kāi)始了，但大多數(shù)是單細(xì)胞微生物。即使多細(xì)胞動(dòng)物真的出現(xiàn)了，它們也是簡(jiǎn)單的，通常是靜止的生物，平靜地過(guò)濾海水，或以微生物為食。這些早期的動(dòng)物還沒(méi)有像眼睛、腿、下巴或爪子這樣的創(chuàng)新，在一個(gè)沒(méi)有捕食者的世界里，它們并不真正需要這些。

According to the new study, though, even a frozen ocean might not have been as inhospitable to these ancient organisms as we tend to think.

然而，根據(jù)這項(xiàng)新研究，即使是冰凍的海洋也不會(huì)像我們通常認(rèn)為的那樣不適合這些古代生物生存。

A 'glacial oxygen pump'

“冰川氧氣泵”

The edge of the Venable Ice Shelf juts into the ocean between two peninsulas from Antarctica. (Photo: Operation IceBridge/NASA Goddard Space Flight Center [CC BY 2.0]/Flickr)

The study's authors looked at iron-rich rocks known as ironstones from Australia, Namibia and California, all of which date back to the Sturtian glaciation. These rocks were deposited in a range of glacial environments, the researchers found, providing a well-rounded picture of what marine conditions were like at the time.

該研究的作者觀察了澳大利亞、納米比亞和加利福尼亞的富含鐵的巖石，這些巖石都可以追溯到斯特爾特冰川時(shí)代。研究人員發(fā)現(xiàn)，這些巖石是在一系列冰川環(huán)境中沉積下來(lái)的，為當(dāng)時(shí)的海洋環(huán)境提供了一幅完整的圖像。

Glaciers are created by snow, which slowly becomes compressed into glacier ice as it accumulates. The snow holds air bubbles, including oxygen, that become trapped in the ice. Those bubbles move down through the ice over time, eventually escaping with meltwater from the underside of the glacier. In certain places, that might have provided just enough oxygen to help early marine animals survive Snowball Earth.

冰川是由雪形成的，隨著雪的累積，它慢慢地被壓縮成冰川冰。雪中含有氣泡，包括氧氣，氣泡被困在冰中。隨著時(shí)間的推移，這些氣泡順著冰層向下移動(dòng)，最終與冰川底部的融水一起逃逸。在某些地方，這可能提供了足夠的氧氣幫助早期海洋動(dòng)物在雪球地球上生存。

Winter wonderland

冬季仙境

By studying 'snowball' and 'slushball' phases of Earth's history, scientists hope to learn more about other worlds that seem to have frozen oceans, like the exoplanet Kepler-62f. (Illustration: NASA Ames/JPL-Caltech/Tim Pyle)

In fact, Snowball Earth might have been more than just a hardship for those creatures to overcome. There are hints that specific conditions of the Cryogenian might have helped pave the way for the Cambrian Explosion. "The fact that the global freeze occurred before the evolution of complex animals suggests a link between Snowball Earth and animal evolution," Lechte says. "These harsh conditions could have stimulated their diversification into more complex forms."

事實(shí)上，雪球地球可能不僅僅是這些生物要克服的困難。有跡象表明，低溫期的特定條件可能為寒武紀(jì)大爆發(fā)鋪平了道路。“全球變冷發(fā)生在復(fù)雜動(dòng)物進(jìn)化之前，這一事實(shí)表明，雪球地球和動(dòng)物進(jìn)化之間存在聯(lián)系，”萊克特說(shuō)。“這些嚴(yán)酷的條件本可以刺激它們多樣化，形成更復(fù)雜的形式。”

"These large and nutritious organisms at the base of the food web provided the burst of energy required for the evolution of complex ecosystems," Brocks said. And it was only in these complex environments, he added, "where increasingly large and complex animals, including humans, could thrive on Earth."

布羅克斯說(shuō):“這些位于食物網(wǎng)底部的大型營(yíng)養(yǎng)生物為復(fù)雜生態(tài)系統(tǒng)的進(jìn)化提供了所需的能量。”他補(bǔ)充說(shuō)，只有在這些復(fù)雜的環(huán)境中，“包括人類(lèi)在內(nèi)的越來(lái)越大、越來(lái)越復(fù)雜的動(dòng)物才能在地球上茁壯成長(zhǎng)。”