A Story of Earth’s History
Introduction
The earth’s history is LONG! The best estimate we have says the earth is about 4.6
billion (with a b) years old. Earth’s history is divided most basically into the precambian
eon and the three eras after it. Here is the scoop
The Precambrian Eon
The name means: "before the Cambrian period." This old, but still common term was
originally used to refer to the whole period of Earth's history before the formation of the
oldest rocks with recognizable fossils in them. This eon covers almost 90% of the entire
history of Earth. It has been divided into three eras: the Hadean, the Archean and the
Proterozoic. Each era is very different.
Hadean ("Hades-like") Era
This era begins about 4.6 billion years ago with the formation of Earth from dust and
gas orbiting the Sun. During this era the surface of Earth is like popular visions of
Hades: oceans of liquid rock, boiling sulfur, and impact craters everywhere! Volcanoes
blast off all over the place, and the rain of rocks and asteroids from space never ends. It
is hard to take a step without falling into a pool of lava or getting hit by a meteor! The
air is hot, thick, steamy, and full of dust and crud. But you can't breathe it anyway: its
made of nothing but carbon dioxide and water vapor, with traces of nitrogen and smelly
sulfur compounds! Any rocks that do form from cooling lavas are quickly buried under
new lava flows or blasted to bits by yet another impact. No one has found any rocks on
Earth from this era. Only meteorites from space and moon rocks are this old. If any life
formed on Earth during this era, it was probably destroyed.
Archaean ("Ancient" or "Primitive") Era
This era begins about a billion years after the formation of Earth, and things have
changed a lot! Mostly everything has cooled down. Most water vapor in the air has
cooled and condensed to form a global ocean. Even most of the carbon dioxide is gone,
having been chemically changed into limestone and deposited at the bottom of the
ocean. The air is now mostly nitrogen, and the sky is filled with normal clouds and rain.
The lava also is mostly cooled to form the ocean floor. The interior of Earth is still quite
hot and active, as shown by the many erupting volcanoes. The volcanoes form lots of
small islands in long chains. The islands are the only land surface. The continents have
not formed yet. The islands are carried over the surface of Earth by the movement of
rock deep in Earth's interior. Occasionally the small islands collide with each other to
form larger islands. Eventually these larger islands will collide to form the cores of the
continents we know today.
What about life? If you look closely, you will see evidence of blue-green algae (actually
simple bacteria) floating in the ocean. That's all there is! Just single-celled bacteria in
the ocean. There is as yet no life on land. Life began in the ocean near the beginning of
this era. The oldest known fossils--the remains of different types of bacteria--are in
archean rocks about 3.5 billion years old.
The surface of Earth is still very active, but a few of the rocks that are forming now will
actually survive to the present in spite of wind, rain, and remelting. In fact, geologists
define the beginning of the Archean era as the age of the oldest rocks on Earth we can
still find today.
Proterozoic ("Early Life") Era
Well, here we are about 700 million years ago, near the end of the longest time period
in geologic history. It began about two billion years after the formation of Earth and
lasted about another two billion years! So what has happened in all that time?
Hmmmmm. There is a lot more land to be seen. In fact, there are two supercontinents,
one visible across the equator on this side of Earth and another one on the other side.
These huge masses of land formed by collisions of the many, many islands made by
volcanoes during the Archaean and most of the Proterozoic eras. Earth's interior has
cooled some more, and there are fewer volcanoes than in the Archean. Even though the
movements of Earth's surface we call Plate Tectonics are still very fast and continental
collisions are frequent (every few hundred million years or so!), the centers or cores of
the continents are now quite large and stable. In fact, geologists date the beginning of
the Proterozoic Era by the age of the oldest continental rocks that have not been
reheated or chemically altered.
Life has not changed much during the last two billion years, but the few changes are
significant. Life is still found only in the ocean, but sometime around 1.7 billion years
ago, single-celled creatures appeared that had a real nucleus. Another important change
is about to happen: true multi-celled life is about to appear, some 30 million years
before the end of the Proterozoic. These multi-celled creatures will have no hard parts
like shells or teeth in their bodies, so their fossils will be hard to find.
The atmosphere is about the same, mostly nitrogen, with a little water vapor and carbon
dioxide. But what's this? Free oxygen released by the algae floating in the oceans is
beginning to collect in the air. These single-celled plants have been producing oxygen
for about two billion years, but up until now the oxygen has been combining chemically
with iron and other elements to form great mineral deposits around the world.
Paradoxically, this oxygen, which we must have to live, is poisonous to most of the life
forms living on Earth during the Proterozoic, so another great change in the types of life
is about to occur.
Earth at this time is also very cold, with huge, bluish glacial ice sheets visible across the
supercontinent, even in the normally warm equatorial regions!
Paleozoic ("Ancient Life") Era
This is the first of three geologic eras squeezed into the last 10% of Earth's whole
geologic history. But things are really starting to happen! This last 10% of Earth's
history is called the "Phanerozoic Eon" or eon of "evident life." This means that rocks
from the Phanerozoic contain fossils, lots of fossils!
The Paleozoic period lasted about 325 million years, from about 570 million years ago to
about 245 million years ago. So much happened during the Paleozoic that it is divided
into seven geologic time periods, shown on the red steps of the "Staircase of Time."
Many different things happened during each period, but we can only give a summary of
them here. You can learn more by going to your library or searching the Internet for
words like "Paleozoic" or the names of each of the periods.
Here in the Paleozoic, Earth's interior has cooled down to something like modern levels,
so that volcanic activity is usually about as humanity experiences it: a few minor
eruptions like Mount St. Helens each year, and major ones like Krakatoa every century
or so. However, gigantic "hot-spot" type eruptions still occur every hundred million years
or so. Plate tectonics continues to push land masses across Earth's surface. At this
particular time--the middle of the Silurian Period--most of the land is still locked in two
supercontinents called Gondwanaland, which happens to be wandering over the South
Pole in our view, and Laurasia, which is on the other side of the globe. Huge glaciers
cover the interior of Gondwanaland, and Earth is experiencing one of its ice ages. Over
the next hundred million years, Gondwanaland will move north over the equator and
begin to break up, and the climate will warm up substantially
The composition of the atmosphere has continued to slowly change, mostly due to the
increase of oxygen produced by photosynthetic algae floating on the ocean. By the
Paleozoic, the composition of the air has reached something like what we breathe now:
about 4/5 nitrogen, 1/5 oxygen, and small amounts of carbon dioxide, water vapor, and
other gases. At long last the air is capable of supporting large animals, and almost in
response, life explodes into the Paleozoic!
At the beginning of the Paleozoic, life existed only in or near the ocean. Trilobites,
shellfish, corals, and sponges appeared, followed by the first fish. Land plants appeared
near the end of the Ordovician and for the first time we see the green of land plants in
our global view. Huge forests and swamplands formed during the warm climate of the
Mississippian and Pennsylvanian periods that later fossilized into the giant coal beds of
the eastern United States. Animal life also moved onto the land, first the arthropods
(spiders and insects to you), then the amphibians, and later the reptiles. The most
abundant animals on land and sea during the Paleozoic were those like shellfish and
insects that lacked backbones, so the Paleozoic is often called "The Age of
Invertebrates."
The Paleozoic was also marked by several mass extinctions: geologically short periods of
time during which large numbers of life forms died out. Mass extinctions occurred at the
end of the Ordovician, the Devonian, and the worst one of all at the end of the Permian,
when about 95% of all life on Earth died! These mass dyings were probably caused by
climate changes and periods of giant volcanic eruptions.
We can follow the development of life in detail during the Paleozoic, because at the
beginning of that Era, life forms developed hard parts like shells, teeth, bones, and
woody parts that were easily preserved as fossils. Earlier life forms were single-celled
and soft-bodied, so older rocks contain few fossils.
Mesozoic ("Middle Life") Era
This is the second of three geologic eras squeezed into the Phanerozoic ("Evident Life")
Eon that covers the last 10% of Earth's whole geologic history. This is the era we all
think of when we imagine the Ancient Earth! Rampaging dinosaurs! Dive-bombing
pterodactyls! Endless forests of giant ferns! Erupting Volcanoes! (Sorry, no cave men!
They didn't show up until the end of the Cenozoic.)
The Mesozoic Era lasted about 180 million years, from about 245 million years ago to
about 65 million years ago. The Mesozoic is divided into just three time periods: the
Triassic, the Jurassic, and the Cretaceous. Since lots of things were going on in each
time period, we can only summarize the events. You can learn more by going to your
library or searching the Internet for words like "Mesozoic" or the names of each of the
periods.
In the view above, we see Earth in the middle of the Jurassic Period, in mid-Mesozoic
times (sort of a middle-middle view). The supercontinents Gondwanaland and Laurasia
collided some time back to form a single super-super continent called Pangea ("AllEarth"). But plate tectonics continues its irresistible motions, and even as we look,
Pangea is beginning to break up into the continents we know now. At upper left, North
America is just breaking away from the northwest coast of Africa, and the Atlantic Ocean
and Gulf of Mexico are beginning to form. The Appalachian Mountains of the eastern
United States are a high, rugged mountain range, something like the Rocky Mountains
of today. Over the next fifty million years or so, South America, India, and Antarctica will
all break away from Africa and move toward their present positions.
Life is diversifying rapidly, and beginning to look familiar. The dominant animals on both
land and sea are reptiles, the most famous of which are the dinosaurs. Dinosaurs began
in the Triassic, spread during the Jurassic, and dominated Earth in the Cretaceous. They
are so prominent that the Mesozoic is also called "The Age of Reptiles." But dinosaurs
are not the only life form around: birds and mammals also appear during the Mesozoic,
as well as deciduous trees and flowering plants.
The climate during the Mesozoic is warm; so warm that there are no ice caps at all,
even at the poles! Plants grow like crazy in the warmth and moisture, so there is food
everywhere for your average hungry 50-ton Ultrasaurus! So what happened to this Dino
Paradise? More change! A mass extinction like those in the Paleozoic ended the idyllic
Mesozoic Era (if you can call dodging your friendly local T-Rex as idyllic). More than half
of all existing life forms disappeared, including virtually all of the dinosaurs. Why? There
are many hypotheses, including disease, volcanic eruptions, and giant impacts.
Cenozoic ("Recent Life") Era
This is the last of three geologic eras squeezed into the Phanerozoic ("Evident Life") Eon
that covers the last 10% of Earth's whole geologic history. This is the "Age of Mammals"
in which whales took over the oceans, saber-tooth cats shared the land with elephants
and giant sloths, and humans finally appeared.
The Cenozoic period began about 65 million years ago with the extinction of the
dinosaurs and continues through the present. The Cenozoic is divided into just two time
periods: the Tertiary, and the Quaternary. We can only summarize the many events of
these periods. You can learn more by going to your library or searching the Internet for
words like "Cenozoic" or the names of each of the periods.
The view of Earth above is that of today. During the last 65 million years, Pangea has
broken up into the continents, and they have moved into the positions which we see
them in now. The Atlantic Ocean has opened from a narrow valley to a vast ocean. India
has moved across the Indian Ocean and collided with Asia to make the Himalaya
Mountains. North and South America have moved westward over part of the Pacific
Ocean. The pressure has crumpled the western coasts of both continents to form the
Rocky and Andes Mountains. Part of the Pacific sea floor has been forced into the warm
interior under the American continents, causing melting and the formation of the
Cascade and Andes volcanoes on the surface.
The global climate has turned somewhat colder, and the last few million years have
seen the return of giant glaciers and ice caps to North America, Eurasia, and Antarctica.
After the disappearance of the dinosaurs, there were suddenly many empty places on
Earth where animals could live. Mammals, which were small, mouse-like animals at the
beginning of the Cenozoic, quickly spread out, diversified in kind, and grew in size. Soon
the plains and forests of Earth were occupied by giant rhinos and elephants, lions and
saber-tooth cats, horses and deer. The skies filled with bats and birds, and the seas
filled with whales and porpoises, as well as with fish and octopi. There have been mass
extinctions during the Cenozoic as there were during the Mesozoic and Paleozoic, but
not as many animals and plants have disappeared.
Finally, humanity appeared during the last two million years. In the last 10,000 years, a
blink of the eye in geologic time, humanity has spread across the lands and seas of
Earth, altering the face of Earth with cities and farms, destroying some plants and
animals and domesticating others. Humans have become the dominant terrestrial life
form: more numerous than any other large animal, and more fearsome than the most
terrible of dinosaurs.