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Big Bang
About 13.7 billion years ago a tremendous
explosion started
the expansion of the
universe. This explosion
is known as the Big
Bang. At the point
of this event all
of the matter and
energy of space was
contained at one point.
What existed prior
to this event is completely
unknown and is a matter
of pure speculation.
The Big Bang produced
a fireball of titanic
proportions with a
temperature measuring
in the billions of
degrees. In the first
milliseconds immediately
following the blast,
matter and energy
burst outward in all
directions, literally
stretching space.
As space expanded,
the matter and energy
released by the fireball
cooled. As matter
cooled, highly energetic
subatomic particles
slowed enough to interact
with each other, filling
the universe with
the simplest elements
of Hydrogen and Helium.
Driven by gravity
and by their own momentum,
these gases began
to accumulate, first
into small pockets
and then into larger
clumps that gradually
grew hotter and brighter
and evolved into stars.
Stars were formed
in this manner throughout
the universe, and
large groups of stars
-- some numbering
in the hundreds of
billions -- formed
the largest structures
in the universe, galaxies.
The elusive dark matter
and dark energy were
also created during
the Big Bang. It is
estimated by scientific
studies that that
the dark matter may
form over 90% of all
the matter in the
Cosmos.
Galaxies
Galaxies tend to occur in groups known
as clusters. One such cluster, called
the local group, contains our own
galaxy, the Milky Way, and our neighbor
the Andromeda galaxy. The Milky Way
is a spiral galaxy; that is, if viewed
from afar it would resemble a spiral.
Other major galaxy types are globular
clusters, elliptical galaxies, and
barred spirals, which have bars of
dust, gas, and stars running across
the galaxy's center like the spokes
of a wheel. Still other galaxies are
irregular, with no definite form.
Some clusters of galaxies contain
thousands of galaxies, and there are
roughly 100 billion galaxies in the
universe.
Within the galaxies, stars continued
to form, burn, explode, die, and re-form.
In a remote corner of the Milky Way
galaxy about 5 billion years ago,
a single gaseous cloud began to contract
under the force of gravity. In the
process it became less amorphous in
shape. Its interior temperature began
to rise. At the center of the cloud,
a hot, dense sphere of Hydrogen gas
held together by gravitation began
to radiate. A star had formed. This
star was the Sun. Our local star,
Sun, is third generation star i.e.
it contains heavier elements like
carbon, Iron and Uranium that were
created by earlier stars which exploded
in a Nova explosion when they ran
out of their Hydrogen fuel. The heavier
elements then seeded the new forming
stars and planets. All life contains
atoms that were created in the stars
and we all are made of 'star dust'.
Solar System
Stars like the Sun form when giant clouds
of gas and dust condense into a ball,
igniting thermonuclear fusion, which
is the conversion of primarily hydrogen
and helium into heavier elements.
Theory holds that the cloud ought
to light up as the density increases
during its collapse, which can take
hundreds of millions of years. Theorists
figure a pre-solar nebula began to
collapse about 5 billion years ago,
possibly triggered by a shock wave
from a nearby exploding star. The
Sun likely formed in a crowded environment
before wandering to its present lonely
location, many astronomers believe.
As the cloud collapsed under its own
weight and rotated faster, some material
was spun into a doughnut-shaped disk.
Clumps of dust became rocks. Some
were destined to be planets. Others
became asteroids, comets and smaller
meteoroids. Smaller condensations
of matter began to form within the
gaseous cloud, slowly revolving around
the Sun. These were the protoplanets,
whirling accumulations of gas that
would, in time, give rise to the planets.
The protoplanets grew in size and,
through the force of their own gravitation,
absorbed other matter in the cloud,
including other protoplanets. In this
way the number of protoplanets lessened.
Finally, the Solar System took its
modern shape: spherical planets with
their moons locked into orbit around
a central sun, located some 30,000
light years away from the center of
the Milky Way. Our Solar System was
formed 4.6 billion years ago.
Earth
Scientists theorize that Earth began
as a waterless mass of rock
surrounded by a cloud of gas
about 4.6 billions years ago.
Radioactive materials in the
rock and increasing pressure
in Earth's interior produced
enough heat to melt the interior
of Earth. The heavy materials,
such as Iron, sank. The light
silicate rocks rose to Earth's
surface and formed the earliest
crust. The heat of the interior
caused other chemicals inside
Earth to rise to the surface.
Some of these chemicals formed
water, and others became the
gases of the atmosphere.
The leading theory for the Moon's formation
has a Mars-sized object slamming
into Earth about 4 billion years
ago, shortly after our planet
formed. The evidence is partly
in the Moon's composition, which
is similar to the upper portions
of Earth. The shock of the impact
strips material from the outer
layers of Earth and the impacting
object. The mostly iron cores
of both bodies meld into Earth's
core. Moon was predominantly
produced from material of the
mantle of the impactor and also
includes nearly 20% of Earth's
crust blasted by the impact.
The blasted material is captured
by Earth's gravity and it forms
a ring around the Earth. This
ring of material would have
coalesced to form the moon.
In addition, the moon is moving
away from the earth (currently
at 2 inches per year), as it
has been since its creation.
The known history of Earth is divided
into four long stretches of time called
eons. Starting with the earliest,
the eons are Hadean, Achaean, Proterozoic,
and Phanerozoic. The first three eons,
which together lasted nearly 4 billion
years, are grouped into a unit called
the Precambrian. The Phanerozoic Eon,
when life became abundant, is divided
into three eras. They are, from the
oldest to the youngest, the Paleozoic,
Mesozoic, and Cenozoic eras. Eras
are divided into periods, and periods
are divided into epochs. These divisions
and subdivisions are named for places
where rocks of each period were studied.
Periods are mostly separated by important
changes in the types of fossils found
in the rocks. As a result, the lengths
of eras, periods, and epochs are not
equal.
Life
It was early in the Archaean Era, 3.8
to 2.5 billion years ago, that life
first appeared on Earth. Our oldest
fossils date to roughly 3.5 billion
years ago, and consist of bacteria
microfossils. In fact, all life during
the more than one billion years of
the Archaean was bacterial. Life began
to flourish, and the autotrophic organisms
had tapped a new niche allowing the
biomass of Earth at the time to dramatically
increase. Carbon Dioxide was constantly
being absorbed by these organisms,
and after the biological reactions
responsible for creating energy in
them, Oxygen would be released as
a by-product. This meant that Oxygen
began to accumulate in the oceans
where life existed. This new material
would in turn be taken advantage of
by the adapting organisms leading
to the creation of aerobic organisms,
which used Oxygen as a component of
their energy creation. The mitochondria
are present in both animal and plant
cells in today's world, suggesting
that the arrival of the mitochondria
in the evolutionary chain was slightly
before recognizable taxonomical differences
between animals and plants. The beginning
of the Cambrian era saw a widespread
arrival of multi-cellular organisms,
particularly in the form of sponges.
One major event in time is the development
of sexual reproduction. Previous species
method of reproduction was simply
mitosis, repeated cell division which
produced new organisms, and exact
copy of their ancestors. Of course,
mutations and other factors over time
changed their genome causing them
to evolve. The first animals in the
fossil record appeared between 620
and 550 million years ago in the Vendian
Period.
The Cambrian Period, about 544 million
years ago, marks an important point
in the history of life on earth; it
is the time when most of the major
groups of animals first appear in
the fossil record. This event is sometimes
called the "Cambrian Explosion",
because of the relatively short time
over which this diversity of forms
appears. Some point to the increase
in Oxygen that began around 2 billion
years ago supporting a higher metabolic
rate and allowing the evolution of
larger organisms and more complex
body structures. Internal genetic
factors were also crucial in the Cambrian
Explosion. Recent research suggests
that the period prior to the Cambrian
explosion saw the gradual evolution
of a "genetic tool kit"
of genes that govern developmental
processes. Once assembled, this genetic
tool kit enabled an unprecedented
period of evolutionary experimentation
-- and competition. Once the body
plans that proved most successful
came to dominate the biosphere, evolution
never had such a free hand again,
and evolutionary change was limited
to relatively minor tinkering with
the body plans that already existed.
During the Ordovician, 505 million years
ago, the first plants appeared. But
it was not until the late Silurian
before they resembled modern plants.
The Silurean period, 440 millions
years ago, was the time when some
plants and animals left the water
and colonized the land for the first
time. Arthropods were the first animals
to adapt to the land by about 420
million years ago. Fossil footprints
of arthropods found in Western Australia
that were made in the sandy flats
surrounding temporary lakes, indicate
that these animals may have accompanied
the landward march of plants. In most
ways they were pre-adapted to life
on land. During the Devonian, 410
million years ago, two major animal
groups dominated the land. The first
Tetrapods, or land-living vertebrates,
appeared during the Devonian, as did
the first terrestrial arthropods,
including wingless insects and the
earliest arachnids which had already
ventured onto land during the Silurian.
In the oceans, brachiopods flourished.
In the Carboniferous Period, 360 million
years ago, the greatest evolutionary
innovations were the amniotic egg
which allowed early reptiles to move
away from waterside habitats and colonize
dry regions. The amniotic egg allowed
the ancestors of birds, mammals, and
reptiles to reproduce on land by preventing
the embryo inside from drying out,
so eggs could be laid away from the
water. It also meant that in contrast
to the amphibians the reptiles lay
fewer eggs, they had no larval stage
and fertilization was internal. In
the Permian Period, 286 million years
ago, the most striking transitions
in the evolution of life occurred
when mammals evolved from one lineage
of reptiles.
Roughly 248 million years ago, the Permian-Triassic
extinction occurred. This is the largest
extinction known. About 95% of all
species and about 60% of the genera
died out, including many marine animals
(like the trilobite). The cause of
the Permian extinction might have
been global cooling, volcanic eruptions,
or a decrease in the continental shelf
area during the formation of Pangaea.
During the late Triassic, 220 million
years ago, the first true mammals
appeared. These primitive mammals
were tiny and are thought to have
been nocturnal. During the Jurassic
Period, 213 million years ago, dinosaurs
dominated the near-tropical Earth.
The gigantic Sauropod dinosaurs, like
the Diplodocus and Apatosaurus, diversified.
Carnivorous Theropod, like Allosaurus
and Compsognathus, were abundant.
Bird-like dinosaurs also flourished.
Cretaceous Period, 145 million years
ago, was the age of the dinosaurs.
Huge carnivores like Tyrannosaurus
rex and Giganotosaurus appeared, as
did Triceratops and many, many others.
There was a tremendous diversity in
dinosaur species. Mammals were flourishing,
and flowering plants developed and
radically changed the landscape. The
Cretaceous period ended about 65 million
years ago, a mass extinction due to
impact of an asteroid at Chicxulub
in Mexico wiped out the dinosaurs,
except for the birds.
Mammals
The Paleocene epoch, 63 millions ago,
is a crucial time in the history of
mammals, it was a world without dinosaurs.
Mammals appeared first in the late
Triassic, at about the same time as
dinosaurs. Throughout the Mesozoic,
most mammals were small, fed on insects
and lead a nocturnal life, whereas
dinosaurs were the dominant forms
of life on land. After the abrupt
changes about 65 million years ago,
when dinosaurs disappeared with the
exception of their descendants, the
birds, the world was practically without
larger sized terrestrial animals.
This unique situation was the starting
point for the great evolutionary success
of the mammals. Only ten million years
later, at the end of the Paleocene,
they had occupied a large part of
the vacant ecological niches, often
competing with giant carnivorous birds,
especially in South America. By this
time, the landscape was teeming with
small insectivorous and rodent-like
mammals, medium sized mammals were
searching the forests for any kind
of food they could cope with, the
first large (but not yet gigantic)
mammals were browsing on the abundant
vegetation, and carnivorous mammals
were stalking their prey.
Primates
Where and when the first primates - the
group to which we belong - appeared
remains uncertain, but the oldest
confirmed primate fossils date to
about 60 million years ago. It is
widely agreed that primates emerged
from archaic terrestrial and nocturnal
insectivores (shrew-like animals)
with early primates resembling lemurs
or tarsiers and probably lived in
trees in tropical or subtropical areas.
Many of their characteristic features
are well suited for this habitat:
hands specialized for grasping, with
five digits and, in most primates,
opposable thumbs, rotating shoulder
joints and stereoscopic (three dimensional)
vision. Other traits include a large
brain cavity and nails instead of
claws. Modern primates range from
Prosimians such as the pygmy mouse
lemur, through the monkeys, to anthropoid
apes such as the gorilla- and humans.
In the Pliocene epoch, 5.3 million years
ago, the primates
continued to diversify.
Humans and Chimpanzees
shared their last
common ancestor around
7 million years ago,
and have since followed
separate evolutionary
paths. We share about
98.8% of our DNA with
Chimpanzees, which
are thus our closest
relatives amongst
the primates. The
first known hominids
or humanlike primates
evolve in eastern
Africa about 5.2 million
years ago. Hominids
feature prominent
jaws and most species
have large brains
relative to those
of apes. Most hominids
probably lived in
groups either in or
near forests and some
later species made
and used tools. The
oldest fossils, a
jawbone teeth and
a toe bone found in
Ethiopia, date to
5.3 million years.
A younger near complete
hominid skeleton named
'Lucy' by its discoverers
and a set of remarkably
preserved footprints
in Hadar, Tanzania
revealed more about
their appearance and
one of their most
distinctive traits:
even the earliest
hominids could walk
upright on two legs.
This adaptation afforded
certain advantages
such as the ability
to see over the top
of high vegetation
and to easily carry
food or tools and
weapons while traveling.
The Pleistocene, 10
million to 10,000
years ago, also saw
the evolution and
expansion of Homo
sapiens, and by the
close of the Pleistocene,
humans had spread
through most of the
world. A fossil jaw
found in Mauer, Germany,
of Homo Heidelbergensis
dates these early
humans to approximately
500,000 years ago.
They display physical
characteristics of
modern humans, with
an increased brain
capacity, smaller
teeth and a face that
slopes less than that
of other hominid ancestors.
About 130,000 years
ago modern humans,
Homo Sapiens, disperse
throughout Africa,
the Middle East and
Europe. They were
characterized by a
more gracile skeleton,
and higher, domed
skull than their European
contemporaries, the
Neanderthals. The
Toba volcano explosion,
Sumatra Island in
Indonesia, about 73,000
years ago created
world wide catastrophy
as the volcanic fine
ash accumulated in
the upper atmosphere
blocking the sunlight
. The temperature
dropped and many plant
and animal life died.
Nearly all of the
humans died of starvation
and only few survived
this disaster. All
the humans in the
world today are the
descendents of these
few survivors. Cave
paintings suggest
that by 40,000 years
ago, Paleolithic Period,
Cro-Magnon had developed
a sophisticated culture;
some authors equate
this to the appearance
of complex spoken
language.
The Age of Humans
The final epoch of the Quaternary period,
spanning the time from the end of
the Pleistocene, 10,000 years ago
to the present is the Holocene epoch,
also called the Anthropogene, the
'Age of Humans'. The DNA studies suggest
that all humans descended from a single
African ancestor who lived some 60,000
years ago. The modern humans migrated
from Africa to colonize the rest of
the world. During the Ice Age, water
levels dropped and land bridges connected
Africa with Asia, Asia with America,
and Indonesian peninsula was very
close to Australia. One human migration
was from Africa to Australia passing
through Arabia, Iran, South Asia,
Southeast Asia, and Indonesia. The
other migration path took Humans to
Central Asia. If Africa was the birth
place of humanity then the Central
Asia was its nursery. The humans migrated
over the Bering Bridge connecting
Asia to America about 10,000 years
ago. The glaciers started to melt
as Earth warmed up about 10,000 years
ago, the sea level rose and land bridges
disappeared under water, and many
coastal areas were flooded and coastal
highlands turned into Islands.
Mesolithic Era is marked by the transition
from roaming and hunting
to an agricultural
society and begins
around 11,000 years
ago. The first evidence
for domestication
of plants and animals
come from temporary
campsites and are
first seen from 11,000
years ago onwards.
Village communities
started appearing
around 8,000 years
ago and were common
by 6,000 years ago.
In the Neolithic era,
cities began to be
built around 8,000
to 6,000 years ago.
Beginning around 8000
years ago, many human
cultures became increasingly
dependent on cultivated
crops and domesticated
animals to secure
their supply of food.
The oldest city in
the world was excavated
at Mehrgarh
in
Pakistan and it
was settled about
9,000 BCE. By 7000
years ago sedentary
agriculture was able
to support towns such
as Jericho in Palestine
and Çatal Höyük
in Turkey with populations
of more than 1,000.
By 3500 years ago
the first civilizations
appeared in the Mesopotamia
Tigris- Euphrates
valley in Iraq, Nile
valley in Egypt, Indus
valley in Pakistan,
and Yellow river valley
in China.
Page last updated:
Tuesday, February 14, 2006 12:53:43 -0500 |
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