動物們是如何在“雪球地球”中生存下來的?

動物們是如何在“雪球地球”中生存下來的?

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.

隨著人類用溫室氣體使地球升溫，我們正在重建一種我們從未見過的遠古氣候。這引起了人們對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期，許多科學家認為上新世是我們前進方向的一個模型。

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

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."

其中一個時期是低溫期，從大約7.2億年前持續(xù)到6.35億年前。那時，地球經(jīng)歷了其歷史上最極端的冰河時代，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，也正是在那個時候，復(fù)雜動物的最初跡象出現(xiàn)在化石記錄中，這些生物為延續(xù)至今的動物生命的黃金時代奠定了基礎(chǔ)。在一項新的研究中，研究人員檢測了低溫巖石的化學成分，以進一步了解這個陌生的世界——包括為什么它不僅能夠支持動物生命，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，對于凍結(jié)的程度仍有一些爭議。)大部分的大陸塊都在超級大陸羅迪尼亞統(tǒng)一，但是由于全球冰川的作用，地球的整個表面可能已經(jīng)變得非常堅固。地球表面的平均溫度可能不會超過冰點很多，一些研究表明，溫度要低得多，可能會降到零下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)。對我們的星球來說，這是一個瘋狂的時代，在極端的冰和火之間搖擺不定，但也是一個重要的時刻。這是因為，盡管低溫時代對人類來說似乎是一個可怕的時代，但它顯然幫助激發(fā)了復(fù)雜動物的黎明——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.

如果你想知道動物是如何在雪球地球上生存下來的，你并不孤單。對動物來說，要在冰原上以及下面的海水中生存是非常困難的，因為全球覆蓋的冰層會嚴重阻礙海洋吸收氧氣的能力。長期以來，科學家們一直對這一明顯的悖論感到困惑，但本周發(fā)表在《美國國家科學院院刊》上的這項新研究，是越來越多最終給出答案的研究中的最新成果。

Explosion of animal life

動物生命的大爆發(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代之前就開始了，但大多數(shù)是單細胞微生物。即使多細胞動物真的出現(xiàn)了，它們也是簡單的，通常是靜止的生物，平靜地過濾海水，或以微生物為食。這些早期的動物還沒有像眼睛、腿、下巴或爪子這樣的創(chuàng)新，在一個沒有捕食者的世界里，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)這項新研究，即使是冰凍的海洋也不會像我們通常認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)境中沉積下來的，為當時的海洋環(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。隨著時間的推移，這些氣泡順著冰層向下移動，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大爆發(fā)鋪平了道路。“全球變冷發(fā)生在復(fù)雜動物進化之前，這一事實表明，雪球地球和動物進化之間存在聯(lián)系，”萊克特說。“這些嚴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:“這些位于食物網(wǎng)底部的大型營養(yǎng)生物為復(fù)雜生態(tài)系統(tǒng)的進化提供了所需的能量。”他補充說，只有在這些復(fù)雜的環(huán)境中，“包括人類在內(nèi)的越來越大、越來越復(fù)雜的動物才能在地球上茁壯成長。”