-
HOW LIFE BEGAN ON THE EARTH
地球上生命的起源
No
one knows exactly how the earth began, as it
happened so long ago. However, according
to
a
widely
accepted
theory,
the
universe
began
with
a
Bang
that
threw
matter
in
all
that, atoms began to
form and combine to create stars and other bodies.
没有人确切知道地球是怎么样开始形成的，
因为在很早很在以前
地球就形成了。
然而，
一种
普遍为人们
所接受的理论是：宇宙起源于一次大爆炸，这次大爆炸将物质投射到四面八方。
然后，原
子开始形成并结合成恒星和其他天体。
For
several billion years after the
was to
become was uncertain until between 4.5 and 3.8
billion years ago when the dust settled into
a solid globe. The earth became so
violent that it was not clear whether the shape
would last or not.
It exploded loudly
with fire and rock. They were in time to produce
carbon, nitrogen, water vapour
and
other gases, which were to make the earths
atmosphere. What is even more important is that
as the earth cooled down, water began
to appear on its surface.
大爆炸后好几十亿年过去了，但
地球还只是一团尘埃。随后它会变成什么没人能知道，直到
38
-45
亿年前，这团尘埃才慢
慢地形成一个团体的球状物。<
/p>
地球开始变得激烈动荡，
不知道这个固体形状是否会继续存下
去。它猛烈地爆炸喷出了烈火与岩石，最终产生了碳，氮气，水蒸气和其他多种气体，从而<
/p>
形成了地球的大气层。更为重要的是，地球冷却了下来，地球的表面就开始出现了水。
Water had also
appeared on other planets like Mars but, unlike
the earth, it had disappeared
later. It
was not immediately obvious that water was to be
fundamental to the development of life.
What many scientists believe is that
the continued presence of water allowed the earth
to dissolve
harmful gases and acids
into the oceans and seas. This produced a chain
reaction, which made it
possible for
life to develop.
在火星这些其他行星上都出现了水，
但和地球不同的是，
这些水后
来都消失了。水对于生命
的发展会起关键作用，这一点在当时并不明显。很多科学家相信，
由于地球上长期有水存
在，
使得地球得以把有害气体和酸性物质溶解在海洋里。
这就产
生了
一系列的反应，使得生命就有可能开始发展了。
Many millions of years later, the first
extremely small plants began to appear on the
surface
of the water. They multiplied
and filled the oceans and seas with oxygen, which
encouraged the
later development of
early shellfish and all sorts of fish. Next, green
plants began to grow on land.
They were
followed in time by land animals. Some were
insects. Others, called amphibians, were
able
to
live
on
land
as
well
as
in
the
water.
Later
when
the
plants
grew
into
forests,
reptiles
appeared for the first time. They
produced young enerally by laying eggs. After
that, some huge
animals, called
dinosaurs, developed. They laid eggs too and
existed on the earth for more than
140
million
years.
However,65
million
years
ago
the
age
of
the
dinosaurs
ended.
Why
they
suddenly
disappeared
still
remains
a
mystery.
This
disappearance
made
possible
the
rise
of
mammals on the earth.
These animals were different from all life forms
in the past, because they
gave birth to
young baby animals and produced milk to feed them.
好几百万年后，
水的表面开始出现了极小的植物。
他们繁殖起来就使得早期的海洋充满了氧
气，
这为早
期的贝类及其他各种鱼类的发育进一步创造了条件。
接着，
绿色
植物开始出现在
陆地上。随后，陆地上出现了动物，其中有一些是昆虫，另外一些叫两栖
动物，他们既能在
陆地上生活，也能在海里生存。后来，当植物长大成了森林的时候，爬
行动物出现了。他们
一般是通过孵蛋而繁衍后代的。
再后来一些
叫做恐龙的巨型动物出现了。
他们也生蛋，
在地
球上生存了一亿四千多万年。然而，
六千五百万年前，恐龙时代结束了。他们为
什么会突然