Precambrian Era: Hadean Period
4,500 Million To 3,800 Million Years Ago
Four billion, five hundred million years ago, the Earth was born. New and
small, it came into being within a large cloud of gas and dust around the sun. The
sun itself formed within such a cloud of gas and dust. Our sun was one of
thousands of billions of stars formed as the universe began, perhaps from the
explosion of an old, massive star. The sun’s gravity caused it to press in on itself,
creating more and more pressure and heat until it created nuclear fusion and began
to give off light and heat.
Hot gaseous particles rose from the earth’s surface, then cooled and sank back
to earth, slowly cooling the earth’s surface. Some gases stayed in the atmosphere.
After about 250 million years, the surface was cool enough to accept rain and
running water and the earth’s first oceans formed, thick and soupy with dissolved
mineral and chemical compounds. Some liquids cooled to form solid earth. The
heaviest solids sank to the center of the earth. Igneous rocks date from the time of
cooling - about 3.8 billion years ago.
As a thin crust formed upon the cooling earth, hot elements underneath pushed
up and broke through in violent explosions. Volcanic activity surrounded the earth
with smoke and steam. Covered in hot lava, with a poisonous atmosphere, the
newborn earth was uninhabitable. After a long, long time the volcanoes slowed
down. Steam turned to rain, the smoke cleared, and the earth cooled, wrinkled up
and shrank. Rain flowed into the low spots forming our rivers and oceans. The air
cleared and the sun shone on the beautiful earth.
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Precambrian Era: Archean and Proterozoic Periods
3,800 Million to 544 Million Years Ago
Three billion, eight hundred million years ago, the atmosphere was composed of poisonous
gases - methane, ammonia, and other gases that would be toxic to most life on our planet today.
The gasses of the early atmosphere dissolved in the oceans, creating a warm chemical soup.
Ultraviolet radiation from the sun bombarded this soup. Beneath its cooling surface, Earth was
still hot as a furnace, yet even as lava continued to pour out from underground, glaciers appeared
in places and spread over the earth. There were violent thunder storms and torrential rains. The
oceans were like a gigantic chemical laboratory, forming one complex chemical after another.
Quietly and slowly, microscopic life began to develop in the murky seas during the Archaen
Period, 3.8 to 2.5 billion years ago. Our oldest fossils are from cyanobacteria, also called blue
green algae. These bacteria microfossils date back roughly 3.5 billion years. It may seem
surprising that such small organisms can leave fossils at all, but cyanobacteria are larger than
most bacteria, secrete a thick cell wall and form large layered structures in aquatic environments.
When sectioned very thinly, fossils of these layers may be found to contain exquisitely preserved
fossil cyanobacteria and algae. By the time of the Proterozoic Era, 2500 million years ago,
cyanobacteria were common and widespread.
Cyanobacteria have been tremendously important in shaping the course of earth's ecological
history. During the Proterozoic Era, minerals washing from exposed areas of the earth’s crust
built up in the seas and oceans, clogging and poisoning the waters. Colonies of cyanobacteria
were able to trap these minerals within a sticky layer of mucilage. These sediment layers, called
stromatolites were widespread during the Proterozoic Era, and were ecologically important as the
first reefs. Many Proterozoic oil deposits are attributed to the activity of cyanobacteria, and their
photosynthetic activity was primarily responsible for the rise in atmospheric oxygen.
Many of the most exciting events in the history of Earth occurred during the Proterozoic
Period. One celled organisms learned to convert sunlight and carbon dioxide gas into simple
sugars in a process called photosynthesis. Stable continents first appeared. The first "pollution
crisis" hit the Earth about 2.2 billion years ago as oxygen levels began rising rapidly. Oxygen is
a powerful degrader of organic compounds, and many bacteria and protists are killed by oxygen.
This global catastrophe spelled doom for many bacterial groups, but made possible the
explosion of multicellular algae, and toward the end of the Proterozoic, the first animals,
organisms capable of using oxygen for respiration. The first animals, tiny wormlike sea
dwellers, evolved just before the end of the Precambrian Era.
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Paleozoic Era: 544 to 245 million years ago
544,000,000 years ago, much of the world was
covered by oceans and shallow seas. The land
was completely barren and rocky and the
atmosphere was still very bad. The climate was
warm to hot over most of the world, with no
seasons. During this part of the Paleozoic Era,
called the Cambrian Period, multicelled animals
underwent a dramatic "explosion" in diversity.
Soft bodied creatures such as jellyfish and
sponges made their homes in the seas. Corals
built great reefs. The most dominant and
striking form of life in the Cambrian Period were the trilobites. These small crab-like creatures
ruled the seas. The shelled animals used minerals from the water to create protection for them,
and at the same time cleaned and purified the polluted waters.
505,000,000 years ago, the earth experienced an ice age, followed by a long spell of mild and
pleasant climate. During the Ordovician and Silurian Periods, trilobites began to die out. The
seas were populated with clams, starfishes, and sea urchins. Giant sea scorpions three meters
long appeared along with the mighty nautili, five meters long. Coral reefs expanded. Seaweed
and crinoids, looking like giant flowers, made the bottom of the seas into huge gardens. Toward
the end of the Ordovician Period, the first jawless armored fish appeared, and were soon
followed by more advanced fish with jaws.
During the Silurian Period seas receded. Some
algae, left stranded, adapted to life on dry land.
These were the first land plants, prickly, with no
leaves. A second ice age 410,000,000 years ago
ended the Silurian period.
The next time period, the Devonian Period, is
often called the Age of Fish. The first slow
bottom-dwelling fish were covered with plates of heavy armor. They were followed by fish with
scales, then true fish with backbones. Some grew to large sizes and were fearsome predators.
During the Devonian Period long rainy periods brought rising seas, which covered forests,
burying them in mud. When dry weather returned, new forests grew on top of the old. Seas
appeared, then disappeared, then appeared again. This process was repeated again and again,
and layers of sediment left by dead plants compressed over millions of years, first into peat, then
into coal. Each time the seas receded they left behind muddy swamps, and stranded many
animals and plants. Plants adapted by developing roots and leaves.
By the end of the Devonian Period, much of the land was covered by the world’s first forests.
Lobe-finned fish, stranded by receding seas, learned to use their swim bladders for lungs. The
first amphibians evolved about 370,000,000 years ago. They had to stay near water because their
skin tended to dry out, and eggs had to be laid in water. The first insects and earliest arachnids
colonized the land. Toward the end of the Paleozoic Period, 300,000,000 years ago, reptiles
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appeared, with two advantages over amphibians. Their thicker skin kept body moisture in, and
leathery egg coverings protected eggs from losing moisture, so they could lay eggs in sand or
mud and the sun could warm them. This gave reptiles more freedom from water.
Formation of the Continents
The earth we walk on is not as solid as it seems. It is made up of about a dozen large 'plates’
floating on a dense, semi-liquid mantle. These plates are always in motion, moving about two
cm a year. Early in the Paleozoic Era, there were six major continental land masses, each
consisting of parts of the modern continents. Today's western coast of North America ran eastwest along the equator. Africa was at the South Pole. Throughout the Paleozoic Era, the shallow
seas grew deeper, and Paleozoic continents experienced tremendous mountain building along
their margins. Our Appalachian mountains originated at this time.
The idea that our continents are drifting on a semi liquid mantle was first developed by a
German astronomer and meteorologist named Alfred Wegener. Noticing that Africa and South
America appear to fit together along their continental shelves, Wegener found that the mountains
running from east to west across South Africa seemed to link with the range in Argentina.
Distinctive rock layers in South Africa, were identical to rock layers in Brazil. This remarkable
fit between the Atlantic coasts of Africa and South America led Wegener to suggest that Earth's
continents had at one time been joined. He also found fossil evidence for this theory. One plant,
Glossopteris, left behind leaf fossils in both Africa and South America.
Wegener hypothesized that all of the southern continents, together with India, were once
joined into a supercontinent which he named Pangaea. Scientists have now accumulated
extensive evidence that about 225 million years ago, as the Permian Period and the Paleozoic Era
ended, the earth’s continents all came together to form this supercontinent. This may have caused
global climate changes that triggered the largest mass extinction in history. Pangaea apparently
covered about half the Earth and was completely surrounded by a world ocean called
Panthalassa. Then, 245 million years ago, Pangaea began to split apart at one of the plate
boundaries. A rift developed beneath the continent, and Pangaea separated into Northern and
Southern components, Laurasia and Gondwana.
Laurasia included North America, Europe, and Asia (except peninsular India). Africa,
Antarctica, Australia, India, and South America were part of Gondwana. Laurasia fragmented
into the present continents when westward drift of the Americas opened the Atlantic Ocean about
208 million years ago. As the Atlantic Ocean spread between South America and Africa,
Gondwana also broke up. The Indian block drifted across the Equator towards Asia. Tethys, an
equatorial ocean that is believed to have separated Laurasia from Gondwana during much of the
Mesozoic Era, closed 66.4 million years ago when the Indian peninsula collided with the rest of
Eurasia and pushed up the modern Alpine-Himalayan ranges. The eastern part of the
Mediterranean Sea is a remnant of the Tethys Sea. Antarctica became isolated and of the
Antarctic polar cap developed. The African-Arabian plate joined to Asia, closing the seaway
which had previously separated Africa from Asia.
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Mesozoic Era: 244 - 65 Million Year Ago
Mesozoic means “middle animals.” The Mesozoic Era is
framed by mass extinctions. At the end of the Paleozoic Era the
largest mass extinction in history wiped out nearly 90% of all
marine animal species. The Mesozoic Era was characterized by
the rise of the dinosaurs, who dominated the land for over 160
million years. At the end of the Mesozoic Era dinosaurs,
pterosaurs, the large sea reptiles, and many other animals suddenly
died out. This extinction may have been triggered by the collision
of a massive asteroid with the earth 65 million years ago. It has
been suggested that this collision caused an explosive dust cloud
which produced a worldwide winter, unsurvivable by nearly all large animals.
During the early Mesozoic Era, the Triassic Period, the seas were dominated by mollusks
with horn shaped shells called ammonites. These ancestors of today’s octopus developed an
astounding variety of forms, and were abundant throughout the Mesozoic Seas. Modern conifers
first appeared in their current recognizable forms in the early Triassic Period. The first dinosaurs
evolved, small and fast. They stood on hind legs and could use their hands for grasping.
Dinosaurs soon overshadowed other land-based life forms, because they could resist the heat of
the sun with their armor-like skin covering. Some reptiles took to the air, first with wings
stretched tightly over long ribs, then later with webbed structures extending from their legs and
arms.
By the time of the Jurassic Period, great plant-eating dinosaurs roamed the earth, feeding on
lush growths of ferns. Diplodocus, Brachiosaurus and Apatosaurus were the largest land animals
of all times. Smaller but vicious carnivores stalked the great herbivores. Some reptiles, like
Ichthyosaurus and plesiosaur took to the water with a shape like fish. Pterodactyl developed
wings that enabled it to fly. Archaeopteryx was about the size of a crow, with feather covered
wings. It is believed to be the dinosaur ancestor of modern birds.
Little reptiles called theraspids evolved into the first mammals. The ability to maintain a
constant body temperature, a covering of fur, and the capability of producing milk to nourish
their young made these animals adaptable to a variety of climates and gave them an advantage
over the less adaptable reptiles.
During the last part of the Mesozoic Era, deciduous trees evolved from conifers and many
mountains were formed. Towards the end of this era the earliest flowering plants appeared and
began to diversify, largely taking over from other plant groups. Dinosaurs dominated the land
more totally than any other species up to that time. The Cretaceous Period gave rise to
Triceratops and the gargantuan Tyrannosaurus Rex. Over 13 meters long and six meters high, it
weighed up to 8 tons. Stegosaurus, a giant herbivore, was as large as an elephant, but had a brain
the size of a walnut. Pteranadon took to the air, with a wingspan of seven meters. Then, for some
reason, over a relatively short time, they all disappeared. Many scientists now think dinosaurs
live on today as the birds. Based on features of the skeleton, most people studying dinosaurs
consider birds to be dinosaurs. This shocking realization would make even the smallest
hummingbird a legitimate dinosaur!
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Cenozoic Era: Tertiary Period
65 Million to 1.8 Million Years Ago
The Cenozoic Era is the most recent of the three major subdivisions of animal history. "Cen"
means recent. "Zoic" comes from the root "zoo,” which means animal. This Era is sometimes
called the Age of Mammals, because the largest land animals have been mammals during the last
65 million year. It could have been called the "Age of Flowering Plants,” the "Age of Insects" or
the "Age of Birds" just as accurately. Conifers, mosses, flowering plants, insects, mollusks,
birds, mammals, and others from this time period survive to modern times. The Cenozoic Era is
divided into two main sub-divisions: the Tertiary Period (from 65 million years ago to 1.8
million years ago), and the Quaternary Period (the last 1.8 million years.)
For the first 27 million years of the Cenozoic Era climates were warm
and mild. Sea level dropped and the continents expanded. Vegetation was
lush and tropical and there were great areas of swampy forests. South
America separated from Gondwanaland. All the main land masses moved
near to their present positions. The oceans were rich in new species and
on land plant-eating mammals evolved into many new forms. Ants and
bees appeared, as did penguins, the first horse (only 12 inches tall), and the
first small elephant, looking like a big pig with a flexible snout. The time
period also saw the first cats, dogs, bears, bats and poisonous snakes. One
terrifying animal, Diatryma, was a large, flightless bird nearly seven feet tall with a parrotlike
beak and enormous claws.
During the time period between 38 million and five million years ago, climates first cooled,
then warmed again. First the ice cap formed over South Pole. This formation of the ice cap used
up great quantities of sea water, exposing more dry land. As global climate first warmed and
then cooled, deserts, tundra, and grasslands expanded, while tropical forests diminished. With
the expanding grasslands came many new species of grazing animals. Horses grew to 23 inches
tall, then to pony size. Elephants as tall as 13 feet appeared, with long snouts, and wide spade
like teeth. Kelp forests emerged for the first time in the seas. Cathedral-like kelp groves
provided homes for a multitude of marine species within their broad canopies. India and Africa
moved closer and closer to Asia and Europe, finally crashing together. Australia separated from
Asia.
Between five million and two million years ago, giant ice caps developed at both poles.
Climates cooled, forests disappeared, and grasslands spread. A narrow land bridge between
South America and North America formed. Herbivores acquired many new forms. Modern
looking elephants emerged, as well as huge mastodons. Glyptodon was an armadillo bigger than
a car. Megatherium was a monstrous 20 foot long sloth that walked on its knuckles and the
sides of its feet. Many herbivores gained protection from predators by living in herds. Some,
like horses, developed one toed hooves and other adaptations that gave them speed in running.
Predators also developed speed and power. Some carnivores began to hunt in packs. By
teamwork they could attack and kill large herbivores. Team hunting led to other social behaviors
based on ability, discipline, good communication and loyalty to the pack leader.
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Cenozoic Era: Quaternary Period
1.8 Million Years Ago to Today
It was during the early part of the Quartenary Period that the most recent
ice ages took place. Much of the world's temperate zones were
alternately covered by glaciers during cool periods and uncovered during
the warmer interglacial periods when the glaciers retreated. The large
mammals of the Pleistocene weathered several climate shifts. This time
period also saw the evolution and expansion of our own species, Homo
Sapiens. By the close of the Pleistocene, humans had spread through
most of the world. Tropical ecosystems retreated, and northern
coniferous forests expanded. Seasonality increased. This led to a
rediversification of temperate ecosystems and many anatomical changes in animals.
Mammals and birds in particular developed distinctive new forms,
whether as fast-running herbivores, large predatory mammals and birds,
or small quick birds and rodents. Mammoths and mastodons,
longhorned bison, sabre-toothed cats, giant ground sloths, and many
other large mammals characterized Pleistocene habitats in North
America, Asia, and Europe. Native horses and camels galloped across
the plains of North America. Great teratorn birds with 25-foot
wingspans stalked prey. Both plants and animals were close to modern
forms. Many species of conifers, mosses, flowering plants, insects,
mollusks, birds, mammals, and others survive to this day from the early Quartenary Period.
Around the two million years ago mark, many species became extinct, perhaps due to an ice
age, perhaps because of the spread of human hunters. It has also been hypothesized that some
disease wiped out species after species in the Pleistocene. The issue remains unsolved; perhaps
the real cause of the extinction was a combination of these factors.
The last 11,000 years of the Earth's history, since the end of the last major ice age, has been a
relatively warm period in between ice ages. Humans of our own subspecies, Homo sapiens,
evolved and dispersed all over the world well before the start of this time. All of humanity's
recorded history and the rise and fall of all its civilizations have taken place during the last
11,000 years. Few species have ever changed the globe as much, or as fast, as our species is
doing. Scientists agree that human activity is responsible for "global warming," an observed
increase in mean global temperatures that is still going on.
Habitat destruction, pollution, and other factors are causing an ongoing mass extinction of
plant and animal species. According to some projections, 20% of all plant and animal species on
Earth will be extinct within the next 25 years. This time period, however, has also seen the
development of human knowledge and technology, which can and is being used to understand
the changes that we see, to predict their effects, and to stop or ameliorate the damage they may
do to the Earth and to us. Paleontologists are part of this effort to understand global change.
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