In the very beginning of earth's history,
this planet was a giant, red hot, roiling, boiling sea of molten rock
- a magma ocean. The heat had been generated by the repeated high speed
collisions of much smaller bodies of space rocks that continually clumped
together as they collided to form this planet. As the collisions tapered
off the earth began to cool, forming a thin crust on its surface. As
the cooling continued, water vapor began to escape and condense in the
earth's early atmosphere. Clouds formed and storms raged, raining more
and more water down on the primitive earth, cooling the surface further
until it was flooded with water, forming the seas.
It
is theorized that the true age of the earth is about 4.6 billion years
old, formed at about the same time as the rest of our solar
system. The oldest rocks geologists have been able to find are 3.9
billion years old. Using radiometric
dating methods to determine the age of rocks means scientists
have to rely on when the rock was initially formed (as in - when its
internal minerals first cooled). In the infancy of our home planet the
entire earth was molten (melted) rock - a magma ocean.
Since we can only measure as far back in time as
we had solid rock on this planet, we are limited in how we can measure
the real age of the earth. Due to the forces of plate
tectonics, our planet is also a very dynamic one; new
mountains forming, old ones wearing down, volcanoes melting and reshaping
new crust. The continual changing and reshaping of the earth's surface
that involves the melting down and reconstructing of old rock has
pretty much eliminated most of the original rocks that came with earth
when it was newly formed. So the age is a theoretical age.
When Did Life on Earth Begin?
Scientists are still trying to unravel
one of the greatest mysteries of earth: When did "life" first appear
and how did it happen? It is estimated that the first life forms on
earth were primitive, one-celled creatures that appeared about 3 billion
years ago. That's pretty much all there was for about the next two
billion years. Then suddenly those single celled organisms began to
evolve into multicellular organisms. Then an unprecedented profusion
of life in incredibly complex forms began to fill the oceans. Some
crawled from the seas and took residence on land, perhaps to escape
predators in the ocean. A cascading chain of new and increasingly
differentiated forms of life appeared all over the planet, only to
be virtually annihilated by an unexplained mass extinction. It would
be the first of several mass extinctions in Earth's history.
Scientists have been looking increasingly
to space to explain these mass extinctions that have been happening
almost like clockwork since the beginning of "living" time. Perhaps
we've been getting periodically belted by more space rocks (ie. asteroids),
or the collision of neutron stars happening too close for comfort?
Each time a mass extinction occurred, life found a way to come back
from the brink. Life has tenaciously clung to this small blue planet
for the last three billion years. Scientists are finding new cues
as to how life first began on earth in some really interesting places
- the deep ocean.
Checking the Fossil Record
Scientists have studied rocks using
radiometric dating methods to
determine the age of earth. Another really cool thing they've found
in rocks that tells us more about the story of earth's past are the
remains of living creatures that have been embedded in the rocks for
all time. We call these fossils.
It has been the careful study of earth's fossil record that has revealed
the exciting picture about the kinds of creatures that once roamed
this planet. Fossilized skeletons of enormous creatures with huge
claws and teeth, ancient ancestors of modern day species (such as
sharks) that have remained virtually unchanged for millions of years,
and prehistoric jungles lush with plant life, all point to a profusion
of life and a variety of species that continues to populate the earth,
even in the face of periodic mass extinctions.
By studying the fossil record scientists
have determined that the earth has experienced very different climates
in the past. In fact, general climactic conditions, as well as existing
species, are used to define distinct geologic time periods in earth's
history. For example, periodic warming of the earth - during the Jurassic
and Cretaceous periods - created
a profusion of plant and animal life that left behind generous organic
materials from their decay. These layers of organic material built
up over millions of years undisturbed. They were eventually covered
by younger, overlying sediment and compressed, giving us fossil fuels
such as coal, petroleum and natural gas.
Alternately, the earth's climate
has also experienced periods of extremely cold weather for such prolonged
periods that much of the surface was covered in thick sheets of ice.
These periods of geologic time are called ice
ages and the earth has had several in its history. Entire
species of warmer-climate species died out during these time periods,
giving rise to entirely new species of living things which could tolerate
and survive in the extremely cold climate. Believe it or not, humans
were around during the last ice age - the Holocene (about 11,500 years
ago) - and we managed to survive. Creatures like the Woolly Mammoth
- a distant relative of modern-day elephants - did not.
Read about a really exciting recent
find of a perfectly-preserved, frozen
Woolly Mammoth! This was a particularly exciting find because
it wasn't a fossil that scientists found, but actual tissue, which
still has its DNA record intact.
Also, read more about the Ice
Man - another frozen tissue sample of a human being who was frozen
into the high mountains of France. He was just recently discovered
as thousands of years of ice pack have finally melted from around
his body.
