Friday, November 16, 2007

History of the Earth

The age of the earth is estimated at 4.6 billion years. During the course of this immense period of time, the earth evolved from a cloud of cosmic dust, undergoing constant changes into what we know today.

The earliest era, called Precambrian, encompasses the time period from the very first beginnings of the earth to the time when the first rocks appeared. These rocks contained petrified residues from which it was possible to determine the age of one layer of rocks. No petrified remnants dating back to the Precambrian proper were found. It is not possible to provide any precise information concerning the formation of the rocks of that time because, since that time, the earth has been undergoing constant changes. It is possible to classify only certain processes of the origins of the mountains (most importantly in Northern America, Scotland, and Greenland - Laurentian Plateau formation).

The earth originated probably as a result of a formation of clusters of minuscule particles of cosmic dust and gases, where these were constantly bombarded and joined by new mass particles until finally the whole mass began circling around the sun as the third of a total of nine planets. In the beginning, the earth was hot and it dotted with countless volcanoes. It lacked oxygen and atmosphere. During that stage, life on Earth was impossible.

The earth was slowly cooling down, allowing water vapour and gases to escape and create an atmosphere. It started to rain and the oceans formed. It is probable that the first single-cell animals originated already 3.6 billion years ago, most likely by means of the spontaneous joining of molecules. Later on algae and bacteria evolved and, finally, the first multi-cell animals, such as articulates and cnidaria (coelenterata), which are made of simple cavities surrounded by soft tissue.

Approximately 4 billion years ago, the earth had a solid lithosphere. Later, about 3.9 billion years ago, water began to form on its surface. Immense earth masses originated some 3 billion years ago. In the past, their form and distribution had undergone changes. From a single giant continent encompassing the entire surface of the Earth there emerged the first two continents, which drifted away from each other and divided further.

The Precambrian era is followed by the Paleozoic era. Our knowledge concerning this period rests mainly on the evolution of animal life (fauna). Fossils were found in the rocks of the layers of the earth and, by determining their age, it was possible to estimate the age of the individual layers. The most important fossils of this kind are the trilobites and graptolites. However, within this immense time period, there already existed numerous mollusks, crustaceans, first vertebrates, etc., which evolved in ever greater diversity. By the end of the Paleozoic era there existed the first vertebrates - reptiles, which deposited their eggs on the ground. They evolved from the amphibians. In addition, during the Paleozoic era, the mountain range of Ural originated, uniting Europe and Asia into one continental block. The Appalachians also originated near the end of this era.

During the following era, the Mesozoic, which ended 66 million years ago, emerged the American Andes and Rocky Mountains. There occurred a mass extinction of approximately 90 percent of all animal species. For a long time, dinosaurs dominated life on Earth. They included inhabitants of the oceans, flying dinosaurs as well as land animals. However, they were not the only ones who were evolving. First birds and even small mammals began to appear. During the Jurassic period, i.e., the middle part of the Mesozoic era, a major portion of the European continental mass was flooded, but during the period that followed, the Cretaceous, this continent returned to its original form. These floods explain the fact that marine fossils were found in mountainous areas. When the dinosaurs and numerous reptiles became extinct and, with the dawn of the Tertiary period of the newest era of the Earth 65 million years ago, the Cenozoic, a great diversity of mammals began to appear. Some 55 million years ago horses, proboscideans, and artiodactyls (even-toed ungulates or hoofed mammals) first appeared, followed by (38 million years ago) the anthropoids (apes), hogs, deer, and other animals. The mountain ranges of Alps, Pyrenees, Caucasus, and Atlas emerged during this period.

During the Quaternary period of the Cenozoic period (following the Tertiary period), the global climate change occurred. Cold periods alternated with warm periods, giant glaciers and enormous inland masses of ice formed and, in the warm periods that followed, there were great floods as a consequence of the melting of these ice masses. Present-day human emerged at the end of this period.

The Earliest Time Period of the Earth (Precambrian)

Geologically, Precambrian Time is the earliest era. It encompasses the time from the formation of the earth's crust more than four billion years ago to the beginning of the Cambrian Era, approximately 590 million years ago, and represents more than 80 percent of the entire history of the earth.

The earliest part of the Precambrian, called Archaean, encompasses the first 1.5 billion years from the origin of the earth. We know very little about this time period. Also, it is not absolutely clear how the earth originated. A part of the sun may have broken away, resulting in the planetary system, which includes our earth, or it may been the result of clusters of cosmic particles.

The prevailing hypothesis is that, when the sun originated through tremendous heat and cooling (contraction), it began to cool down. Matter particles swirling around the sun were transformed by condensation into gases, ice, and radioactive matter, forming new celestial bodies (planetoids). The planetoids, due to their gravity, attracted more and more particles until they became planets circling the sun.

It took at least 10 billion years for the Sun and protoplanets to emerge. The process of formation of planets, together with their moons, possibly ended approximately 5 billion years ago. We call this period "the star era." This is when the Archaean period begins. Following its formation, the earth began to cool down. This process proceeded relatively quickly because the temperature difference between the universe and the earth's crust was considerable.

The atmosphere, which most likely consisted mainly of water vapour, carbon dioxide, methane, and nitrogen, also cooled down. Water began to form, evaporating constantly, until it turned into "primeval rain" and flooded the earth's surface, which still continued to cool down. The existing rocks were therefore scattered and formed the first rock layers. Oxygen, indispensable for animals and plants, did not yet exist, for which reason life could not originate on earth at that time.

The Archaean period is followed by the Proterozoic period. In recent years, rocks were discovered in different regions of the world which, according to detailed studies and calculations, date back 3.8 billion years. The earliest ones come from sedimentary rocks from South Africa and Greenland. Very few fossils exist from that time and the majority of them are fossilised plants. In the earliest rocks we find mainly filament and spheroidal microorganisms - single-cell algae, while in younger rocks there are already branching filamentous algae and primitive fungi (lichens). These were found in coarse-grained limestone in Ontario and in the rocks from Minnesota and Great Britain.

The Precambrian animal fossils are documented very seldomly. Probably only at the end of this era there existed mainly cnidarians, articulates, hydrozoans, and medusoids. For these mollusks the fossilisation was rather difficult and therefore findings are very rare. During the Cambrian period, however, live organisms having solid body parts began to occur on a larger scale, for which reason their fossils are may be more commonly found. By all indications, in the Precambrian period, the continental regions were empty and desolate.

There were probably inhospitable masses of bare rocks since the slowly emerging life was developing in shallow seas. While the first prokaryota (meaning lacking nucleus) consumed "primeval soup" and during the process of acquiring energy liberated only a small amount of oxygen, with the occurrence of organic matter interacting with solar rays, the production of oxygen increased considerably.

In this manner the content of oxygen in the atmosphere increased, a consequence of which was the formation of a protective ozone layer which deflects damaging ultraviolet sunrays, where only under this protective layer could life on Earth originate.

It is assumed that the temperature was between 0 and 50 degrees Celsius because only on the basis of this assumption could life originate. It is possible that, at that time, ice covered extensive regions. However, it may be assumed that the Earth was not completely covered by masses of ice. As documented by numerous fossils, a number of primitive marine species evolved during the early Cambrian.

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The Paleozoic Period of the Earth

The Paleozoic period of the Earth is the time period between the Precambrian and Mesozoic periods. This era lasted approximately 340 million years and is divided into several periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian.

Cambrian is the earliest part of the Paleozoic. Its name is taken from the Roman name for northern Wales, "Cambria," where rocks from this period were found. At that time, approximately 590 million years ago, the continents formed one giant supercontinent called "Gondwana." It included the continents of Africa, Antarctica, South America, Western Australia, as well as parts of India, Florida, Mexico, southern Europe and, possibly, China. These continental masses were situated in the southern hemisphere and extended almost to the south pole. They were separated from the north by a wide band of ocean and then towards the north pole there followed other more articulated continental masses separated by seas.

First the climate was cool to moderate. Later on global warming took place, as documented by scientific studies of red sandstones and saline deposits, such as rock salt, and gypsum.

An extensive development of flora and fauna begins during the Cambrian period. In the seas there developed a diverse number of animals such as fungi (mould), trilobites, and crayfish-like creatures, which had a hard chitin armour. There were also prehistoric mussels. They all lived on the seafloor and medusae existed in the waters. Flora consisted mainly of a vast variety of algae and seaweed. Land plants and animals were not yet in existence but evolved later.

The Cambrian period is followed by the Ordovician period. This period takes its name from a Celtic tribe of Ordovics who inhabited northern Wales and covers the time period between 505 and 439 million years ago. The geographic situation was similar to the one that existed during the Cambrian period but the continents continued to come closer and their coasts were became more flooded. There was an ice age during the Ordovician period. In South America, it is possible to see the moraines deposited by glaciers. The fauna of the invertebrates evolved mainly in the seas during warm climate regions.

Deposits in dark slates indicate that, in the deep waters, lived multi-cell (metazoan) graptolites, an extinct type of flagelliform with a hard exterior chitin skeleton, who formed colonies. A great variety of calcarea evolved in shallow seas. In grainy calcareous layers, we find generally brachiopods and bryozoa, relatives of today's mussels, as well as a great diversity of trilobites. The body of these articulates was covered by a chitin carapace consisting of three interconnected parts. In addition, there were corals and crinoids. The first vertebrates also evolved and possibly the first primitive fish which, however, did not have yet fins or jaws.

There were no complex plants yet, but blue and green algae were spreading from salty seawater to fresher coastal waters. Traces of tracks of reptilian animals similar to centipedes were found in petrified sandbanks dating to the Ordovician. If any animal species existed on land, there would have had to be plants to sustain these animals. However, according to extensive research and calculations, plants did not exist until the following period, called the Silurian period.

The Silurian period is the third period of the earth's Paleozoic stage and its name is derived from the pre-Celtic tribe Silurs, who lived in Wales. It encompasses a period between 438 and 408 million years ago.

From a geographical point of view, there existed two completely interlocked continents; the northern part called the Laurasia and the southern part called the Gondwana. Both parts were alternatively more or less under water. The north pole at that time was probably in the northern Pacific Ocean and it is assumed that the south pole was situated in southwestern Africa. The equator crossed from southeastern Europe, over northern Australia and Greenland to the centre of America.

The climate was mostly humid and warm, turning a little drier at the end of the Silurian. This is indicated by limestone, gypsum, and rock salt dating from that time. This rock salt forms today's salt deposits.

The most common marine animals were invertebrates such as graptolites, trilobites (articulates), and simple corals (cnidarians). Crinoids, as common at that time as algae, consisted of calycinal bodies anchored in the bottom of the sea by means of their stalk. Their bodies were surrounded by tentacles. Corals, which built enormous cliffs that still fascinate scientists today, became extinct approximately 230 million years ago. They were replaced by today's corals, now found in warm seawaters. Marine scorpions and articulates grew to gigantic proportions of up to two metres (Pterygotus).

First jawless fish, which did not have bones but rather a cartilaginoid skeleton, appeared in the seas. We are aware of their existence because they had small bony plates on their head and body, where these plates became fossilised. Two of these species survive today: the eel-like fresh-water lampreys and the slimy, worm-like blennies (blenniidae). At the end of the Silurian period, the first primeval ferns and club moss (lycopodium) appeared on land.

The Devonian period, the fourth period of the earth's Paleozoic stage, which is named after the English county of Devonshire, comprises the period between 490 and 360 million years ago.

The land masses of the continents were constantly moving also during this period. The climate of the northern hemisphere was warm while, in the area of the south pole, there were several ice belts. In linkage to the first beginning in the preceding Silurian period, the intensive evolution of higher types of terran plants continued. These plants were generally horsetails and ferns which, in the late Devonian period, would grow as tall as trees. Thus fauna was injected with new evolutionary impulses.

Numerous new kinds of animals began to evolve, mainly fish. The Devonian period is therefore also called the period of fish. The jawless fish of the preceding Silurian period evolved into shield-headed fish which, as the first vertebrates, also had jaws. Originally the skeleton of the first vertebrates was not formed by bones but by cartilaginoid matter.

During the Devonian period, there were also thorny-finned/lobe-finned fish (ichthyostega). They had thick fins and rigid skeleton and they were able to remain on land. By means of these fins, they could move on land and even abandon a dried-out lake or river and find another water habitat. The ichthyostega are considered as a transitional form toward land animals.

The sea was the habitat mainly of corals, ammonites, snails, conchoidal crayfish and echiderms, as well as trilobites and similar kinds. Scorpions, arachnids, terran reptiles, myriapeds and early, wingless insects were evolving on land.

The Carboniferous period (from the Latin word for coal), is the fifth geological segment of the Earth's Paleozoic. This period, which began 360 million years ago and which ended 286 million years ago, is also called the period of anthracite because, during that time, the largest deposits of this coal in the history of the Earth were formed, a result of a conjunction of favourable climatic, biological, and geological factors.

The climate in central Europe, which at that time was situated near the equator, was tropical and humid, similar to the climate that is found today in the rainforest of the Amazon. During the course of the Carboniferous period, there were powerful movements of the lithospheric plates which resulted in the formation of mountain ranges and valleys between them. Many regions were alternately just above the sea level and slightly below the sea level and dense forests grew in the coastal regions. These were periodically covered by water; when the sea level decreased, these forests underwent vigorous growth again.

The forests consisted of club moss growing up to 30 metres, ferns and horsetails and resembled today's tropical swampy forests. As a consequence of the fluctuation of the water levels, these forests were constantly flooded by water, which formed sediments. When oxygen could not penetrate, peat began to form and later on coal.

The first land vertebrates began to evolve in this environment. The transitional form between fish and land vertebrates (ichthyostega), numerous kinds of amphibians (amphibia), as well as amphibians with rigid skeleton began to evolve.

At the end of the Carboniferous period, many animals using only lungs for breathing and which were are not forced to live only in water, begin to appear. These reptiles did not have to return to the water to lay their eggs in that their eggs generally had leathery shells providing protection against predators and the weather. The group (anapsida), predecessor of today's turtles, was among these reptiles.

During the Carboniferous period, there also lived gigantic myriapods reaching lengths of up to one metre, arachnids, scorpions, prehistoric dragonflies (meganeura), cockroaches, and insects. They lived in higher situated forests consisting already of conifers. Analogous to the Devonian, numerous kinds of algae, plant and animal single-cell organisms, corals, ammonites, and articulates inhabited the oceans. By now, the trilobites had disappeared, although a great variety of fish continued to exist.

The southern hemisphere of the globe was completely different. There was still Gondwana, the continuous continent consisting of a part of North America, India, Africa, Australia, and Antarctica. A major portion of this continental mass was situated near the equator of that time and, during the transitional period toward the Permian period, was covered by gigantic ice mass (the Permian-Carboniferous ice age). When, at the end of the Carboniferous period, Gondwana moved over the south pole toward the north, it collided with Laurasia to form a contiguous continental mass called Pangea.

The last geological period of the earth's Paleozoic stage was the Permian period. It represents the period between 286 and 248 million years ago and was named after a former Russian province of Perm situated west of the Ural Mountains.

At that time, Gondwana connected with the northern continents to create a gigantic continental mass called Pangea, resulting in an extensive rising of mountains. This supercontinent was surrounded by a great ocean (Panthalassa). Here, corals, ammonites, and large single-cell animals continued to exist. This was the only period when all the continents formed one continental mass.

Many of the kinds of animals indicated earlier became extinct during the Permian. Causes of this phenomenon are not clear. It is possible that it was linked to the gradual receding of the oceans.

A number of the amphibian animals remained in the vicinity of the water. Another part of this group gradually withdrew. During the Permian period the amphibians of the earth's Paleozoic stage reached their greatest expansion, although this period brought changes for the amphibians. In some regions simultaneously appear early forms of reptiles similar to mammals. They were forcing the amphibians out of their environment or at least competed against them for the space.

Following the unification of Laurasia and Gondwana, the prehistoric amphibians and reptiles penetrated into Gondwana, where these kinds of animals did not exist yet. During the Permian period, there begin to appear cammsaurs - real reptiles. Dimetrodon a Edaphosaurus are examples of this group.

The flora of this period is characterised by the penetration of conifers and the appearance of ginkgophyta. Club moss, horsetails, and ferns, which depended on humid and warm climate, began to recede.

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