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
12
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 altered in Paragraph 1 is closest in meaning to________.
选项
A、transformed
B、protected
C、made use of
D、adapted to
答案
A
解析
本题为词义题。altered意为“改变,转化”,A项transformed“转换,转变,与之意思最为相近,故选。B项“保护”、C项“使用,利用”、D项“适应”,均不符合要求,可都排除。
转载请注明原文地址:https://www.tihaiku.com/zcyy/3304300.html
相关试题推荐
A17-yearU.S.studyhasfinallyansweredoneofthemostpressingquestion
A17-yearU.S.studyhasfinallyansweredoneofthemostpressingquestion
ThecoffeegiantisfacinguncomfortablequestionsinLosAngeles,afterbec
Questionsofeducationarefrequentlydiscussedasiftheyborenorelation
Questionsofeducationarefrequentlydiscussedasiftheyborenorelation
Questionsofeducationarefrequentlydiscussedasiftheyborenorelation
Questionsofeducationarefrequentlydiscussedasiftheyborenorelation
Questionsofeducationarefrequentlydiscussedasiftheyborenorelation
She’scute,noquestion.Symmetricalfeatures,flawlessskin,lookstobe22
She’scute,noquestion.Symmetricalfeatures,flawlessskin,lookstobe22
随机试题
[originaltext]M:Hi,thisisPaulfromEngland.Firstquestionis:Doyougive
(1)Innovation,theelixirofprogress,hasalwayscostpeopletheirjobs.I
[originaltext]W:CanIbringyouanythingfromthestore,Tom?M:Yeah,couldy
以下关于BIM技术在设计阶段的应用说法错误的是()A.空间形式的思考是B.IM技
(2020年真题)关于按照单一项目的收益分配方式和按照基金整体的收益分配方式,表
下列关于我国银行监管机构防范金融风险的措施,说法错误的是()。A.严格执法,深入
212.党员服务队建设与管理应遵循()原则。 A.坚持“你用电、我用心”
中国古代社会的死刑复奏制度是指奏请皇帝批准执行死刑判决的制度。关于这一制度,下列
根据《仲裁法》的规定,下列各项中,属于仲裁裁决的法定撤销情形的有()。A.裁决
建设项目管理的发展趋势中,属于第四代项目管理的是()。 A.传统的项
最新回复
(
0
)