首页
登录
职称英语
Why has life flourished on Earth? This question has a two-part, answer. Firs
Why has life flourished on Earth? This question has a two-part, answer. Firs
游客
2023-12-25
24
管理
问题
Why has life flourished on Earth? This question has a two-part, answer. First, Earth has been a cradle for life because of its position relative to the Sun. Second, once life began on Earth, simple early life-forms (photosynthetic bacteria) slowly but inexorably altered the environment in a manner that not only maintained life but also paved the way for later, complex life-forms. These changes allowed later organisms to evolve and thrive. Humans and other higher organisms owe their life-supporting environment to these early life-forms.
Earth’s earliest atmosphere contained several gases: hydrogen, water vapor, ammonia, nitrogen, methane, and carbon dioxide, but no oxygen. Gas mixtures
emitted
from present-day volcanoes resemble this early atmosphere, suggesting its origin from volcanic eruptions. In Earth’s earliest atmosphere, methane and carbon dioxide occurred at much higher levels than at present—a circumstance that was favorable for early life. Methane and carbon dioxide are greenhouse gases that warm atmospheres by
retarding
loss of heat to space. These two gases kept Earth warm during the Sun’s early history, when the Sun did not bum as brightly as it now does. (An early dim period, with later brightening, is normal for stars of our Sun’s type.)
Earth’s modern atmosphere, which is 78 percent nitrogen gas, 21 percent oxygen, and about 1 percent argon, water vapor, ozone, and carbon dioxide, differs dramatically from the earliest atmosphere just described. The modern atmosphere supports many forms of complex life that would not have been able to exist in Earth’s first atmosphere because the oxygen level was too low. Also, if atmospheric methane and carbon dioxide were as abundant now as they were in Earth’s earliest atmosphere, the planet’s temperature would likely be too hot for most species living today. How and when did the atmosphere change?
The answer to this riddle lies in the metabolic activity of early photosynthetic life-forms that slowly but surely transformed the chemical composition of Earth’s atmosphere. Some of these early organisms were photosynthetic relatives of modem cyanobacteria (blue-green bacteria). In the process of photosynthesis, carbon dioxide gas combined with water yields oxygen. In Earth’s early days, all over the planet countless photosynthetic bacteria performed photosynthesis. Together, these ancient bacteria removed massive amounts of carbon dioxide from Earth’s atmosphere by converting it to solid organic carbon. These ancient bacteria also released huge quantities of oxygen into the atmosphere. Other ancient bacteria consumed methane, greatly reducing its amount in the atmosphere. When our Sun later became hotter, the continued removal of atmospheric carbon dioxide and methane by early bacteria kept Earth’s climate from becoming too hot to
sustain
life. Modern cyanobacteria still provide these valuable services today. [br] The word retarding in Paragraph 2 is closest in meaning to________.
选项
A、regulating
B、slowing
C、restoring
D、directing
答案
B
解析
本题为词义题。retarding意为“阻碍;使放缓速度”,文中指甲烷和二氧化碳通过减缓热量丧失使大气变暖,因此B项slowing“减缓,使降速”与之意义最为接近。A项“管理;调整”、C项“使恢复”、D项“把(某物)对准;针对”,均不符合要求,可都被排除。
转载请注明原文地址:https://tihaiku.com/zcyy/3304303.html
相关试题推荐
ThecoffeegiantisfacinguncomfortablequestionsinLosAngeles,afterbec
Questionsofeducationarefrequentlydiscussedasiftheyborenorelation
Questionsofeducationarefrequentlydiscussedasiftheyborenorelation
Questionsofeducationarefrequentlydiscussedasiftheyborenorelation
Theactresswasvery______attheinsultingquestionraisedbyheropponentatth
She’scute,noquestion.Symmetricalfeatures,flawlessskin,lookstobe22
She’scute,noquestion.Symmetricalfeatures,flawlessskin,lookstobe22
She’scute,noquestion.Symmetricalfeatures,flawlessskin,lookstobe22
She’scute,noquestion.Symmetricalfeatures,flawlessskin,lookstobe22
Thestatesmanwasevidently______thejournalist’squestionsandglaredathimf
随机试题
Conversation:Thefollowingisanarticleaboutwebstation.Afterreadingi
Astudybytracingthehistoryoftheemergenceandspreadof335infectiousdis
[originaltext]M:Idon’tliketinsbookverymuch.W:NeitherdoI.Q:Whatdoe
Manychildrenfirstlearnthevalueofmoneybyreceivinganallowance.The
《宪法》第44条规定,国家依照法律规定实行全体劳动者的退休制度。()
按照《证券公司合规管理实施指引》的要求,证券公司董事会、监事会或监事、经营管理主
现行药典三部收载A.药材及饮片、植物油脂和提取物、成方制剂和单味制剂 B.
麻醉中药罂粟壳每张处方不得超过()常用量,且不得单包A3D B5D C7D
目前,我国进入了全面建成小康社会的决定性阶段,党中央在全面把握国际经济格局调整和
某国对外开放程度和外贸依存度都较高,国际融资尤其是短期融资规模相对较大。该国货币
最新回复
(
0
)