How old is the Earth?
地球的年龄有多大？
Well, by counting the number of isotopes in a sample of rock that's undergone radioactive decay,
通过对岩石样品中正经历着放射衰变的同位素的计数，
geologists have estimated the Earth's birthday, when it first formed from a solar nebula, to be 4.6 billion years ago.
地质学家们已经估算出了地球的生日，当它从太阳星云中诞生的时候，大约是在46亿年之前。
But just how long is that really? Here's some analogies that might help you understand.
但是那到底是多久呢？这里有几个类比应该能够帮助我们理解。
For example, let's imagine the entire history of Earth until the present day as a single calendar year.
例如，让我们把地球的整个历史，一直到现在，想象成一个日历年。
On January 1st, the Earth begins to form.
在1月1号的时候，地球诞生了。
By March 3rd, there's the first evidence of single-celled bacteria.
到了3月3号，开始有证据证明单细胞细菌的出现。
Life remains amazingly unicellular until November 11th
在10月11号之前，生物仍惊人地保持着单细胞的状态，
when the first multicellular organisms, known as the Ediacaran fauna, come along.
直到第一种多细胞生物，又被称为埃迪卡拉动物群的出现。
Shortly thereafter, on November 16th at 6:08 p.m. is the Cambrian Explosion of life,
不久之后的11月16号下午6点08分，是寒武纪生物大爆发
a major milestone, when all of the modern phyla started to appear.
这是一个重要的里程碑，所有的现代物种开始出现。
On December 10th at 1:26 p.m., the dinosaurs first evolve but are wiped out by an asteroid just two weeks later.
12月10号下午1点26分，恐龙首先进化，但在仅仅两周以后就被一个小行星彻底抹去。
On December 31st, the mighty Roman empire rises and falls in just under four seconds.
在12月31号，强势的罗马帝国崛起又衰落，仅仅用了4秒钟。
And Columbus sets sail for what he thinks is India at three seconds to midnight.
在子夜的三秒钟之前，哥伦布扬帆起航，寻找他心目中的印度。
If you try to write the history of the Earth using just one page per year,
如果你想要把地球的历史写下来，一页纸便是一年，
your book would be 145 miles thick, more than half the distance to the international space station.
你这本书将会有145英里（约233公里）那么厚，比国际太空站一半的距离还长。
The story of the 3.2 million year-old Australopithecine fossil known as Lucy would be found on the 144th mile,
320万岁的南方古猿化石露西的故事将在第144英里处找到，
just over 500 feet from the end of the book.
仅仅还有500英尺便到了书的结尾。
The United States of America's Declaration of Independence would be signed in the last half-inch.
美国的独立宣言签署于最后的半英寸。

Or if we compared geologic time to a woman stretching her arms to a span of six feet,
或者如果我们将地质时间和女人伸直手臂到六英尺的长度相比较，
the simple act of filing her nails would wipe away all of recorded human history.
那么她整理指甲的简单动作便会抹去整个的人类历史。
Finally, let's imagine the history of the Earth as your life: from the moment you're born to your first day of high school.
最后，让我们把地球的历史想象成你的一生：从你出生的时刻，到你进入高中的第一天。
Your first word, first time sitting up, and first time walking
你说的第一个词，第一次坐起来，第一次行走，
would all take place while life on Earth was comprised of single-celled organisms.
统统发生于地球上的生物界是由单细胞生物构成的时候。
In fact, the first multicellular organism wouldn't evolve until you were 12 years old and starting 7th grade,
事实上，第一个多细胞生物直到你12岁的时候才出现，也就是你开始7年级的时候，
right around the time your science teacher is telling the class how fossils are formed.
这刚好是你的科学老师在课堂上告诉你化石是怎么形成的时候。
The dinosaurs don't appear until three months into 8th grade and are soon wiped out right around spring break.
恐龙直到你8年级上了三个月的时候才出现，但很快，在你春假的时候又消失了。
Three days before 9th grade begins, when you realize summer is over and you need new school supplies,
9年级开始的三天前，当你发现夏天已经结束你又需要新的学习用具的时候，
Lucy, the Australopithecine, is walking around Africa.
南方古猿露西正在非洲散步。
As you finish breakfast and head outside to catch your bus 44 minutes before school,
当你吃完早餐出门追赶公交车，还有44分钟就要上课的时候，
the Neanderthals are going extinct throughout Europe.
尼安德特人将从整个欧洲消失。
The most recent glacial period ends as your bus drops you off 16 minutes before class.
最近的一次冰河时期结束于你从公交车上下来，上课前16分钟的时候。
Columbus sets sail 50 seconds before class as you're still trying to find the right classroom.
当你还在试着找正确的教室时，哥伦布将于上课的50秒前起航。
The Declaration of Independence is signed 28 seconds later as you look for an empty seat.
《独立宣言》签署于28秒钟之后，这时你正在寻找空座位。
And you were born 1.3 seconds before the bell rings.
然后你出生于上课铃响1.3秒之前。
So, you see, the Earth is extremely, unbelievably old compared to us humans
所以，你看，跟我们人类比起来，地球真的是极其不可思议的高龄，
with a fossil record hiding incredible stories to tell us about the past and possibly the future as well.
化石记录隐藏了太多关于过去的惊人故事，还可能包括将来的。
But in the short time we've been here, we've learned so much
但在我们存在的这短短的时间内，我们已经了解了如此之多，
and will surely learn more over the next decades and centuries, near moments in geological time.
在未来的几十年、几百年，仅仅在这地质年代相近的时刻，我们一定会了解更多。