Introduction
Earth is
the only planet we know of that can support life. This is an amazing
fact considering that it is made out of the same matter as other planets
in our solar system, was formed at the same time and through the same
processes as every other planet, and gets its energy from the sun.
Earth has
an average size star of average brightness and is joined by eight other
planets, which support no known life forms, in its solar system.
Earth is
a planet is home to billions of plants and animals that share a common
evolutionary track. No one has been able to come close to knowing exactly
what led to the origins of life, and we may never know. After 4.5 billion
years of Earth's formation and evolution, the evidence may have been
lost. But scientists have made significant progress in understanding
what chemical processes that may have led to the origins of life.
There are
many theories, but most have the same general perspective of how things
came to be the way they are. Following is an account of life's beginnings
based on some of the leading research and theories related to the subject,
and of course, fossil records dating back as far as 3.5 billion years
ago.
Earth's
Beginnings
Earth began
to form over 4.5 billion years ago from the same cloud of gas (mostly
hydrogen and helium) and interstellar dust that formed our sun, the
rest of the solar system and even our galaxy. In fact, Earth is still
forming and cooling from the galactic implosion that created the other
stars and planetary systems in our galaxy, a process which began about
16 billion years ago as the Milky Way began to form.
As our solar
system began to come together some 6-7 billion years ago, the sun formed
within a cloud of dust and gas that continued to shrink upon itself
by its own gravitational forces. This caused it to undergo the fusion
process and give off light, heat and other radiation. During this process,
the remaining clouds of gas and dust that surrounded the sun began to
form into smaller lumps called planetesimals, which eventually formed
into the planets we know today.
The Earth
went through a period of catastrophic and intense formation during its
earliest beginnings about 4.5 - 4.6 billion years ago. By 3.8 to 4.1
billion years ago, Earth had become a planet with an atmosphere (not
like our atmosphere today) and an ocean. This period of time of Earth's
formation is referred to as the pre-Cambrian Period. The pre-Cambrian
is divided into three parts: the Hadean, Archean and Proterozoic Periods.
Pre-Cambrian
Period
The Earth
formed under so much heat and pressure that it formed as a molten planet.
For nearly the first billion years of its formation, called the Hadean
Period (or "hellish" period), Earth was bombarded continuously by the
remnants of the dust and debris, like asteroids, meteors and comets,
until it formed into a solid sphere, fell into an orbit around the sun,
and began to cool down.
As Earth
began to take solid form, it had no free oxygen in its atmosphere. It
was so hot that the water droplets in its atmosphere could not settle
to form surface water or ice. Its atmosphere was also so poisonous that
nothing would have been able to survive.
Earth's
atmosphere was formed mostly from the outgassing of such volatile compounds
as water vapour, carbon monoxide, methane, ammonia, nitrogen, carbon
dioxide, nitrogen, hydrochloric acid and sulphur produced by the constant
volcanic eruptions that besieged the Earth. It had no free oxygen.
About 4.1
billion years ago, the Earth's surface -- or crust -- began to cool
and stabilize, creating the solid surface with its rocky terrain. Clouds
formed as the Earth began to cool, producing enormous volumes of rain
water that formed the oceans. For the next 1.3 billion years (3.8 to
2.5 billion years ago), called the Archean Period, first life began
to appear (at least as far as our fossil records tell us... there may
have been life before this!) and the world's land masses began to form.
Earth's initial life forms were bacteria which could survive in the
highly toxic atmosphere that existed during this time. In fact, all
life was bacteria during the Archean Period.
Toward the
end of the of the Archean Period and at the beginning of the Proterozoic
Period, about 2.5 billion years ago, oxygen-forming photosynthesis began
to occur. The first fossils, in fact, were a type of blue-green algae
that could photosynthesize.
Some of
the most exciting events in Earth's history and life occurred during
this time which spanned about two billion years until about 550 million
years ago. The continents began to form and stabilize, creating the
super continent Rodinia about 1.1 billion years ago. (Rodinia is widely
accepted as the first super continent, but there were probably others
before it.) Although Rodinia is composed of some of the same land fragments
as the more popular super continent, Pangea, they are two different
super continents. Pangea formed some 225 million years ago and would
evolve into the seven continents we know today.
Free oxygen
began to build up around the middle of the Proterozoic Period -- around
1.8 billion years ago -- and made way for the emergence of life as we
know it today. This event, of course, created conditions that would
not allow most of the existing life to survive and thus made way for
the more oxygen dependent life forms.
By the end
of the Proterozoic Period, Earth was well along in its evolutionary
processes leading to our current period, the Holocene Period, also known
as the Age of Man. Thus, about 550 million years ago, the Cambrian Period
began. During this period, life "exploded," developing almost all of
the major groups of plants and animals in a relatively short time. It
ended with the massive extinction of most of the existing species about
500 million years ago, making room for the future appearance and evolution
of new plant and animal species.
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