Our geography teacher says the earth is only one of the planets of the giant solar system.Unlike other planets, life exists on the earth, but life's origin is still a puzzle.Different religions and cultures have separate ideas.The teacher says it may be divided into four stages.

　　The first stage is about how the universe began.A widely accepted theory can explain it.After the "Big Bang" broke out, atom began to form and combine to create stars and other bodies.The second stage is how the earth formed.Between 4.5 and 3.8 billion years ago, clouds of dust floating in space settled into a solid globe.The earth became violent and it often exploded.The atmosphere and water appeared.Water is a fundamental matter for life to exist on the earth.It also dissolves harmful gases and acids.The third stage is about plants' and animals' appearance.At first, plants in the water appeared and multiplied.They produced oxygen, which encouraged animals in the water to exist.Later, green plants on the land caused animals on the land to appear.They first laid eggs and became babies.Later, mammals gave birth to babies directly.The last stage is that humans became the most important animals on the earth.We put too much carbon dioxide in the atmosphere to cause global warming.We must watch out how things will go and make laws to prevent the earth from becoming too hot.Now that we have taken measures, we will succeed in time.So let's cheer up and work together.

　　我们的地理老师说地球只是广阔的太阳系中的一颗行星而已。不像其他的行星，地球上有生命存在，但生命的起源依旧是一个谜。不同的宗教和文化有各自的看法。老师说它可以被分为四个阶段。

　　第一个阶段是关于宇宙是怎样诞生的。一个被普遍接受的理论可以解释它。"大爆炸"发生后，原子开始形成并结合成恒星和其他天体。第二个阶段是地球是怎样形成的。大约在38～45亿年前，漂浮在太空中的尘埃形成一个固体的球状物。地球活动剧烈，而且经常爆炸。大气层和水出现了。在地球上水是生命存在的最基本的物质。它同时也溶解有害气体和酸。第三个阶段是关于植物和动物的出现。首先，水生植物出现并繁殖。他们产生氧气，这使水生动物存在。后来，陆生植物使陆生动物出现。它们首先产卵，然后成为幼仔。后来，哺乳动物直接生幼仔。最后一个阶段是人类成为地球上最重要的动物。我们把大量的二氧化碳释放到大气层里，导致了全球变暖。我们必须密切注意事情的进展，制定法律阻止地球变得太热。既然我们已采取措施，我们最终会成功的。因此让我们振作起来一起努力吧。

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