At this point, about 4.5 billion years ago, an object the size of Mars crashed into Earth, 在这个时刻，大约在45亿年以前，一个火星大小的物体撞上了地球，

blowing out enough material to form a companion sphere, the Moon. 炸飞了足够的材料来形成一颗伴星——月球。

Within weeks, it is thought, the flung material had reassembled itself into a single clump¹, 据认为，不出几个星期，被炸飞的材料已经重新聚成一团；

and within a year it had formed into the spherical² rock that companions us yet. 不出一年，它变成了那个现在还陪伴着我们的岩石球体。

Most of the lunar material, it is thought, came from the Earth's crust, not its core, 据认为，构成月球的大部分材料来自地壳，不是地核，

which is why the Moon has so little iron while we have a lot. 这就是月球上极少有铁的原因，而地球上铁却很多。

The theory, incidentally, is almost always presented as a recent one, 顺便说一句，这个理论几乎总是被说成是最近提出的，

but in fact it was first proposed in the 1940s by Reginald Daly of Harvard. 而事实上，它最初由哈佛大学的雷金纳德·戴利于20世纪40年代提出。

The only recent thing about it is people paying any attention to it. 关于这个理论，惟一最近的事儿就是人们已经不大重视它了。

When Earth was only about a third of its eventual³ size, 当地球还是它最终大小的大约三分之一的时候，

it was probably already beginning to form an atmosphere, mostly of carbon dioxide, nitrogen, methane⁴, and sulfur⁵. 它很可能已经开始形成大气，主要由二氧化碳、氮、甲烷和硫组成。

Hardly the sort of stuff that we would associate with life, 我们几乎不会把这些东西与生命联系起来；

and yet from this noxious⁶ stew⁷ life formed. 然而，在这有毒的混杂物中，生命形成了。

Carbon dioxide is a powerful greenhouse gas. 二氧化碳是一种强有力的温室气体。

This was a good thing because the Sun was significantly dimmer back then. 它是一样好东西，因为当时太阳已经弱多了。

Had we not had the benefit of a greenhouse effect, the Earth might well have frozen over permanently⁸, 要是我们没有受益于温室效应，地球很可能已经永久被冰雪覆盖。

and life might never have gotten a toehold. But somehow life did. 生命也许永远找不到一块立足之地。但是，生命以某种方式出现了。

For the next 500 million years the young Earth continued to be pelted⁹ relentlessly¹⁰ by comets, 在之后的5亿年里，年轻的地球继续受到彗星、

meteorites¹¹, and other galactic debris¹², 陨石和银河系里其他碎块的无情撞击。

which brought water to fill the oceans and the components¹³ necessary for the successful formation of life. 这个过程产生了蓄满海洋的水，产生了成功形成生命所必不可少的成分。

It was a singularly hostile environment and yet somehow life got going. 这是个极不友好的环境，然而生命还是以某种方式开始了。

Some tiny bag of chemicals twitched¹⁴ and became animate¹⁵. 有一小袋化学物质抽动一下，变成了活的。

We were on our way. 我们快要来到这个世界上了。

Four billion years later people began to wonder how it had all happened. 40亿年以后，人们开始想，这一切到底是怎么发生的？

And it is there that our story next takes us. 下面，我们就来讲讲这个故事。