EVOLUTION OF THE EARTH
The
planet earth initially was a barren, rocky and hot object with a thin
atmosphere of hydrogen and helium. This is far from the present day picture of
the earth. Hence, there must have been some events– processes, which may have
caused this change from rocky, barren and hot earth to a beautiful planet with
ample amount of water and conducive atmosphere favouring the existence of life
The
earth has a layered structure.From the outermost end of the atmosphere to the
to the centre of the earth, the material that existis not uniform. The
atmospheric matter has the least density. From the surface to deeper depths,
the earth’s interior has different zones and each of these contains materials
with different characteristics.
Geologic Time, time scale that covers Earth’s
entire geologic history from its origin to the present. Before the growth of a
geologic time scale in the 19th century natural historians recognized that
Earth has a lengthy history, but the scale used today developed over the last
200 years and continues to evolve. A geologic time scale helps scientists think
about the history of the planet in manageable sections of time.
The
present geologic time scale is based on radiometric dating and the record of
ancient life preserved in layers of rock. Most boundaries in recent geologic
time coincide with periodic extinctions and appearances of new species.
Divisions in the older part of the record are based on dates provided by
radiometric dating methods.
Division of Time:
The
geologic time scale adopted by the International Union of Geological Sciences
in 2004 breaks Earth’s history into distinct intervals of varying lengths
measured in calendar years. The longest intervals are eons. Each eon is
subdivided into eras. Each era is made up of periods, which are further divided
into epochs.
There are
three eons: the Archean, the Proterozoic, and the Phanerozoic. The Archean, the
earliest eon, is defined as about 3.8 billion to 2.5 billion years before
present. The time before the Archean Eon, simply called pre-Archean, is marked
by the formation of the planet. The Proterozoic Eon lasted from about 2.5
billion to 542 million years before present. The Archean and Proterozoic eons
are also collectively called Precambrian time. An explosion of invertebrate
life marks the end of the Proterozoic and the beginning of the Phanerozoic.
The
Phanerozoic Eon started about 542 million years ago and continues into the
present. It is divided into three eras: the Paleozoic (542 million to 251
million years before present), Mesozoic (251 million to 65 million years before
present), and Cenozoic (65 million years before present to present).
The
Paleozoic Era is divided into six periods. From oldest to youngest they are the
Cambrian (542 million to 488 million years before present), Ordovician (488
million to 444 million years before present), Silurian (444 million to 416
million years before present), Devonian (416 million to 359 million years
before present), Carboniferous (359 million to 299 million years before
present), and Permian (299 million to 251 million years before present). The
Paleozoic began with the appearance of many different life-forms, which are
preserved as abundant fossils in rock sequences all over the world. It ended
with the extinction of over 90 percent of all living organisms at the end of
the Permian Period. The cause of this event is currently unknown.
The
Mesozoic Era is made up of the Triassic (251 million to 200 million years
before present), Jurassic (200 million to 145 million years before present), and
Cretaceous (145 million to 65 million years before present) periods. The
Mesozoic began with the appearance of many new kinds of animals, including the
dinosaurs and the ammonites, or extinct relatives of modern squid. The Mesozoic
ended with another major extinction in which about 80 percent of all living
organisms died. This extinction may have been the result of a large asteroid
that crashed into Earth on the present-day northern Yucatán Peninsula of
Mexico.
The
Cenozoic Era has two geologic periods, the Paleogene (65 million to 23 million
years before present) and the Neogene (23 million years before present to the
present). The Paleogene Period is made up of three epochs: the Paleocene (65
million to 56 million years before present), Eocene (56 million to 34 million
years before present), and Oligocene (34 million to 23 million years before
present). The Neogene Period is divided into four epochs: the Miocene (23
million to 5.3 million years before present), Pliocene (5.3 million to 1.8
million years before present), Pleistocene (1.8 million to 11,500 years before
present), and Holocene (11,500 years to the present) epochs. The Holocene is
marked by the rapid retreat of the last continental ice sheets in Europe and
North America, an accelerated rise in sea level, climatic moderation, and the
expansion of human societies in every part of the world.
Geologic
Time
The
Precambrian is a time span that includes the Archean and Proterozoic eons and
began about 4 billion years ago. The Precambrian marks the first formation of
continents, the oceans, the atmosphere, and life. The Precambrian represents
the oldest chapter in Earth’s history that can still be studied. Very little
remains of Earth from the period of 4.6 billion to about 4 billion years ago
due to the melting of rock caused by the early period of meteorite bombardment.
Rocks dating from the Precambrian, however, have been found in Africa,
Antarctica, Australia, Brazil, Canada, and Scandinavia. Some zircon mineral
grains deposited in Australian rock layers have been dated to 4.2 billion
years.
The Precambrian is also the longest chapter in
Earth’s history, spanning a period of about 3.5 billion years. During this
timeframe, the atmosphere and the oceans formed from gases that escaped from
the hot interior of the planet as a result of widespread volcanic eruptions.
The early atmosphere consisted primarily of nitrogen, carbon dioxide, and water
vapor. As Earth continued to cool, the water vapor condensed out and fell as
precipitation to form the oceans. Some scientists believe that much of Earth’s
water vapor originally came from comets containing frozen water that struck
Earth during the period of meteorite bombardment.
By studying 2-billion-year-old rocks found in
northwestern Canada, as well as 2.5-billion-year-old rocks in China, scientists
have found evidence that plate tectonics began shaping Earth’s surface as early
as the middle Precambrian. About a billion years ago, the Earth’s plates were
centered around the South Pole and formed a supercontinent called Rodinia.
Slowly, pieces of this supercontinent broke away from the central continent and
traveled north, forming smaller continents.
Life originated during the Precambrian. The
earliest fossil evidence of life consists of prokaryotes, one-celled organisms
that lacked a nucleus and reproduced by dividing, a process known as asexual
reproduction. Asexual division meant that a prokaryote’s hereditary material
was copied unchanged. The first prokaryotes were bacteria known as
archaebacteria. Scientists believe they came into existence perhaps as early as
3.8 billion years ago, but certainly by about 3.5 billion years ago, and were
anaerobic—that is, they did not require oxygen to produce energy. Free oxygen
barely existed in the atmosphere of the early Earth.
Archaebacteria were followed about 3.46 billion
years ago by another type of prokaryote known as cyanobacteria or blue-green
algae. These cyanobacteria gradually introduced oxygen in the atmosphere as a
result of photosynthesis. In shallow tropical waters, cyanobacteria formed mats
that grew into humps called stromatolites. Fossilized stromatolites have been
found in rocks in the Pilbara region of western Australia that are more than
3.4 billion years old and in rocks of the Gunflint Chert region of northwest
Lake Superior that are about 2.1 billion years old.
For billions of years, life existed only in the
simple form of prokaryotes. Prokaryotes were followed by the relatively more
advanced eukaryotes, organisms that have a nucleus in their cells and that
reproduce by combining or sharing their heredity makeup rather than by simply
dividing. Sexual reproduction marked a milestone in life on Earth because it
created the possibility of hereditary variation and enabled organisms to adapt
more easily to a changing environment. The very latest part of Precambrian time
some 560 million to 545 million years ago saw the appearance of an intriguing
group of fossil organisms known as the Ediacaran fauna. First discovered in the
northern Flinders Range region of Australia in the mid-1940s and subsequently
found in many locations throughout the world, these strange fossils appear to
be the precursors of many of the fossil groups that were to explode in Earth's
oceans in the Paleozoic Era.
Paleozoic Era.
At the
start of the Paleozoic Era about 543 million years ago, an enormous expansion
in the diversity and complexity of life occurred. This event took place in the
Cambrian Period and is called the Cambrian explosion. Nothing like it has
happened since. Almost all of the major groups of animals we know today made
their first appearance during the Cambrian explosion. Almost all of the
different “body plans” found in animals today—that is, the way an animal’s body
is designed, with heads, legs, rear ends, claws, tentacles, or antennae—also
originated during this period.
Fishes first appeared during the Paleozoic Era,
and multicellular plants began growing on the land. Other land animals, such as
scorpions, insects, and amphibians, also originated during this time. Just as
new forms of life were being created, however, other forms of life were going
out of existence. Natural selection meant that some species were able to
flourish, while others failed. In fact, mass extinctions of animal and plant
species were commonplace.
Most of the early complex life forms of the
Cambrian explosion lived in the sea. The creation of warm, shallow seas, along
with the buildup of oxygen in the atmosphere, may have aided this explosion of
life forms. The shallow seas were created by the breakup of the supercontinent
Rodinia. During the Ordovician, Silurian, and Devonian periods, which followed
the Cambrian Period and lasted from 490 million to 354 million years ago, some
of the continental pieces that had broken off Rodinia collided. These
collisions resulted in larger continental masses in equatorial regions and in
the Northern Hemisphere. The collisions built a number of mountain ranges,
including parts of the Appalachian Mountains in North America and the
Caledonian Mountains of northern Europe.
Toward the close of the Paleozoic Era, two large
continental masses, Gondwanaland to the south and Laurasia to the north, faced
each other across the equator. Their slow but eventful collision during the
Permian Period of the Paleozoic Era, which lasted from 290 million to 248
million years ago, assembled the supercontinent Pangaea and resulted in some of
the grandest mountains in the history of Earth. These mountains included other
parts of the Appalachians and the Ural Mountains of Asia. At the close of the
Paleozoic Era, Pangaea represented over 90 percent of all the continental
landmasses. Pangaea straddled the equator with a huge mouthlike opening that
faced east. This opening was the Tethys Ocean, which closed as India moved
northward creating the Himalayas. The last remnants of the Tethys Ocean can be
seen in today’s Mediterranean Sea.
The Paleozoic came to an end with a major
extinction event, when perhaps as many as 90 percent of all plant and animal
species died out. The reason is not known for sure, but many scientists believe
that huge volcanic outpourings of lavas in central Siberia, coupled with an
asteroid impact, were joint contributing factors.
Mesozoic
Era
The Mesozoic Era, beginning 248 million years ago,
is often characterized as the Age of Reptiles because reptiles were the
dominant life forms during this era. Reptiles dominated not only on land, as
dinosaurs, but also in the sea, in the form of the plesiosaurs and
ichthyosaurs, and in the air, as pterosaurs, which were flying reptiles. See
also Dinosaur; Plesiosaur; Ichthyosaur; Pterosaur.
The Mesozoic Era is divided into three geological
periods: the Triassic, which lasted from 248 million to 206 million years ago;
the Jurassic, from 206 million to 144 million years ago; and the Cretaceous,
from 144 million to 65 million years ago. The dinosaurs emerged during the
Triassic Period and were one of the most successful animals in Earth’s history,
lasting for about 180 million years before going extinct at the end of the
Cretaceous Period. The first birds and mammals and the first flowering plants
also appeared during the Mesozoic Era. Before flowering plants emerged, plants
with seed-bearing cones known as conifers were the dominant form of plants.
Flowering plants soon replaced conifers as the dominant form of vegetation
during the Mesozoic Era.
The Mesozoic was an eventful era geologically with
many changes to Earth’s surface. Pangaea continued to exist for another 50
million years during the early Mesozoic Era. By the early Jurassic Period,
Pangaea began to break up. What is now South America began splitting from what
is now Africa, and in the process the South Atlantic Ocean formed. As the
landmass that became North America drifted away from Pangaea and moved
westward, a long subduction zone extended along North America’s western margin.
This subduction zone and the accompanying arc of volcanoes extended from what
is now Alaska to the southern tip of South America. Much of this feature,
called the American Cordillera, exists today as the eastern margin of the
Pacific Ring of Fire.
During the Cretaceous Period, heat continued to be
released from the margins of the drifting continents, and as they slowly sank, vast
inland seas formed in much of the continental interiors. The fossilized remains
of fishes and marine mollusks called ammonites can be found today in the middle
of the North American continent because these areas were once underwater. Large
continental masses broke off the northern part of southern Gondwanaland during
this period and began to narrow the Tethys Ocean. The largest of these
continental masses, present-day India, moved northward toward its collision
with southern Asia. As both the North Atlantic Ocean and South Atlantic Ocean
continued to open, North and South America became isolated continents for the
first time in 450 million years. Their westward journey resulted in mountains
along their western margins, including the Andes of South America.
Cenozoic Era
The Cenozoic Era, beginning about 65 million years
ago, is the period when mammals became the dominant form of life on land. Human
beings first appeared in the later stages of the Cenozoic Era. In short, the
modern world as we know it, with its characteristic geographical features and
its animals and plants, came into being. All of the continents that we know
today took shape during this era.
A single catastrophic event may have been
responsible for this relatively abrupt change from the Age of Reptiles to the
Age of Mammals. Most scientists now believe that a huge asteroid or comet
struck the Earth at the end of the Mesozoic and the beginning of the Cenozoic
eras, causing the extinction of many forms of life, including the dinosaurs. Evidence
of this collision came with the discovery of a large impact crater off the
coast of Mexico’s Yucatán Peninsula and the worldwide finding of iridium, a
metallic element rare on Earth but abundant in meteorites, in rock layers dated
from the end of the Cretaceous Period. The extinction of the dinosaurs opened
the way for mammals to become the dominant land animals.
The Cenozoic Era is divided into the Tertiary and
the Quaternary periods. The Tertiary Period lasted from about 65 million to
about 1.8 million years ago. The Quaternary Period began about 1.8 million
years ago and continues to the present day. These periods are further
subdivided into epochs, such as the Pleistocene, from 1.8 million to 10,000
years ago, and the Holocene, from 10,000 years ago to the present.
Early in the Tertiary Period, Pangaea was
completely disassembled, and the modern continents were all clearly outlined.
India and other continental masses began colliding with southern Asia to form
the Himalayas. Africa and a series of smaller microcontinents began colliding
with southern Europe to form the Alps. The Tethys Ocean was nearly closed and
began to resemble today’s Mediterranean Sea. As the Tethys continued to narrow,
the Atlantic continued to open, becoming an ever-wider ocean. Iceland appeared
as a new island in later Tertiary time, and its active volcanism today
indicates that seafloor spreading is still causing the country to grow.
Late in the Tertiary Period, about 6 million years
ago, humans began to evolve in Africa. These early humans began to migrate to
other parts of the world between 2 million and 1.7 million years ago.
The Quaternary Period marks the onset of the great
ice ages. Many times, perhaps at least once every 100,000 years on average,
vast glaciers 3 km (2 mi) thick invaded much of North America, Europe, and
parts of Asia. The glaciers eroded considerable amounts of material that stood
in their paths, gouging out U-shaped valleys. Anatomically modern human beings,
known as Homo sapiens, became the dominant form of life in the
Quaternary Period. Most anthropologists (scientists who study human life
and culture) believe that anatomically modern humans originated only recently
in Earth’s 4.6-billion-year history, within the past 200,000 years.
