[ti:Study Explains How Life Survived the 'Snowball Earth' Period]
[by:www.21voa.com]
[00:00.00]更多听力请访问21VOA.COM
[00:00.04]Life on Earth
[00:01.68]faced an extreme test of survivability
[00:06.08]during the Cryogenian Period,
[00:09.40]which began 720 million years ago.
[00:14.96]The planet was frozen over
[00:17.72]most of the 85 million-year period.
[00:22.72]Scientists say Earth
[00:25.32]would have looked like
[00:26.84]a shiny white snowball in space
[00:30.56]during the deep freeze.
[00:33.80]But life somehow survived during this time
[00:37.56]called "Snowball Earth."
[00:40.52]A recent study
[00:42.24]offers a deeper understanding as to why.
[00:47.16]Fossils identified as seaweed
[00:50.56]were found in black shale,
[00:53.00]a kind of rock,
[00:54.56]in central China's Hubei Province.
[00:58.48]The scientists said
[01:01.24]the fossils are a sign
[01:03.44]that livable water environments
[01:06.56]were more widespread at the time
[01:09.60]than they once believed.
[01:12.88]Nature Communications
[01:14.76]published the research this month.
[01:18.64]The findings support the idea
[01:21.48]that the planet was more of a "Slushball Earth"
[01:25.44]with melting snow.
[01:28.40]This enabled the earliest forms of complex life
[01:33.00]to survive in areas once thought
[01:36.36]to have been frozen solid.
[01:39.72]The fossils date from the second
[01:42.28]of the two times during the Cryogenian Period
[01:46.24]when ice sheets stretched
[01:48.84]from the poles toward the equator.
[01:52.68]This period, called the Marinoan Ice Age,
[01:56.64]lasted from about
[01:58.36]651 million to 635 million years ago.
[02:06.32]Huyue Song of the China University of Geosciences
[02:11.52]was the study's lead investigator and writer.
[02:16.64]The researcher said the most important finding
[02:20.56]was that ice-free,
[02:22.80]open water conditions existed in place
[02:27.16]during the last part
[02:29.12]of the Marinoan Ice Age.
[02:32.48]"More extensive areas of habitable oceans
[02:36.72]better explain where
[02:39.00]and how complex organisms
[02:42.44]such as multicellular seaweed survived," Song said.
[02:48.64]The findings demonstrate
[02:50.60]that the world's oceans
[02:52.44]were not completely frozen.
[02:55.68]It means areas of habitable refuge existed
[03:00.72]where multicellular organisms could survive,
[03:04.76]the scientist added.
[03:07.64]Earth formed about 4.5 billion years ago.
[03:13.24]The first single-celled organisms
[03:16.24]appeared in the first billion years
[03:19.08]of the planet's existence.
[03:22.08]Multicellular organisms arrived later,
[03:26.08]maybe two billion years ago.
[03:29.44]But it was only after the Cryogenian period
[03:33.48]that warmer conditions returned.
[03:37.00]That helped drive a quick expansion
[03:40.60]of different life forms
[03:42.60]about 540 million years ago.
[03:47.56]Scientists are trying to better understand
[03:51.00]the start of "Snowball Earth."
[03:54.36]They believe a greatly reduced amount
[03:57.56]of the sun's warmth
[03:59.48]reached the planet's surface
[04:02.12]as its radiation bounced off
[04:05.32]the white ice sheets.
[04:08.24]Seaweed and fossils
[04:10.44]of some other multicellular organisms
[04:13.84]were identified in the black shale.
[04:17.64]This seaweed was a photosynthetic organism
[04:21.96]living on the seafloor
[04:24.12]in a shallow sea environment lit by sunlight.
[04:30.12]"The fossils were preserved
[04:32.04]as compressed sheets of organic carbon,"
[04:35.60]said researcher Qin Ye,
[04:38.44]also of China University of Geosciences.
[04:44.04]Multicellular organisms including red algae,
[04:48.56]green algae and fungi
[04:51.64]appeared before the Cryogenian
[04:54.72]and survived "Snowball Earth."
[04:58.96]The Cryogenian freeze was much worse
[05:02.68]than the most recent Ice Age
[05:05.08]that humans survived.
[05:07.40]It ended about 10,000 years ago.
[05:12.76]I'm Caty Weaver. 更多听力请访问21VOA.COM