A BRIEF STORY OF OUR PLANET

Our Unique Home in Space Earth — our home planet — is but a ‘tiny’ sphere in the vastness of space. The Earth, along with its companion planets, orbits the Sun, the fiery yellow star which lights up our daytime skies. At the centre of our Solar System, the Sun governs the movements of all the 9 planets that circle around it at increasingly greater distances. Closer to the Sun lie the four inner planets: Mercury, Venus, Earth and Mars. Then

comes a band of rocks and dust called the ‘asteroid belt’, beyond which at much greater distances lie the four gas giants: Jupiter, Saturn, Uranus and Neptune. And even further, towards the outer reaches of space, is the dwarf planet, Pluto. Each planet has its own special nature. Moving outwards from the Sun we would come upon the scorching cratered surface of Mercury and then the densely clouded, superheated world of Venus — the two planets nearest to the Sun. Going out much further we would encounter the extremely cold, giant gaseous balls of Jupiter and ringed Saturn, and much beyond these the remote, icy world of Pluto. The Earth, however, is unique. Lying between the pathways of Venus and Mars, not too far away from the Sun, the blue-green Earth is the only living planet in the Solar System. Origins of the Solar System and the early Earth The whole of our Solar system probably came into being around 4,600 million years ago. Scientists think that it all began with a vast swirling cloud of dust and gases, leftovers from a Super Nova — the huge explosion of a dying giant star. The revolving cloud

1 slowly pulled in most of the material towards its own centre, giving shape to the beginnings of a new star — our Sun. The remaining particles also gathered and joined together into shapes that collected more material as they swept through the swirling cloud. These shapes revolving around the central ‘proto-sun’ were the ‘proto-planets’. Among them was the infant Earth. The proto-Earth swept up as much material as there was available around its pathway, growing into a lumpy, rocky object moving in space. The impact of collisions with other space rocks heated up the Earth and ‘nuclear changes’ of particles inside the Earth began to add to this heat. A profound change took place. As the heat melted the new-born planet, it changed into a spinning mass of semi- liquid material: a semi-molten ball that spun itself into a nearly spherical shape. Its heavier material began to sink down towards the centre, while lighter material floated up to the outside. The Earth became a spherical planet with a layered structure. As the hot pasty rocks of the surface slowly cooled in the darkness of space, a thin crust of solid rock gradually hardened on the outside. Hot, liquid rock from the inside, however, kept breaking through the crust in lots of places. It was a time of planet-wide volcanic activity. This was more than 4 billion years ago.

The Birth of the Atmosphere, Oceans and …Life It was the Earth’s early volcanoes that gave birth to its oceans and atmosphere. They erupted and spewed out all the lighter material that had been mixed into the Earth when it first formed. This included a great deal of water vapour and many other gases. The water vapour formed enormous clouds of steam that collected all around the planet. As the Earth’s surface cooled, around 3,800 million years ago these clouds started condensing. There were violent storms, torrential rain poured down for a long time and all the depressions were flooded to form the first oceans. Oceans now covered almost the whole Earth. The other volcanic gases — carbon dioxide, hydrogen and sulphur dioxide — formed the early atmosphere. Liquid water in the Earth’s oceans! No other planet has this. The blue waters became the site for dramatic new developments. The first living cells — a kind of algae or bacteria — appeared in the waters 3,500 million years ago, probably near volcanic ridges and vents in the deep oceans. How this happened, no one truly knows. But by 2,500 million years ago, various simple forms of plants had begun to grow in the seas. Plant life was especially rich in the shallow seas near the edges of

2 newly formed continents that had built up above the oceans. Plants have a very special property: as they grow they absorb carbon dioxide and produce oxygen. Plant life thus started changing the composition of our atmosphere, by removing carbon dioxide and releasing vast amounts of oxygen. Oxygen and the Evolution of Life Forms Earth thus became the only planet to have oxygen in its atmosphere. This highly reactive gas easily combines with the rocks of the crust and it can even burn up simple living things. The presence of oxygen in the air called for the development of more complex life forms and the earliest animal cells appeared in the oceans of our planet. These creatures used up oxygen in a controlled manner for their life functions. The proportion of oxygen was thus kept in a fine balance — neither too much, nor too less. The varieties of animal and plant life now began to evolve side by side, rapidly producing newer and bigger forms of life.

The once barren lands on the continents were first ‘colonized’ by plants and completely new landscapes must have appeared on the planet. Animals followed the plants onto land (around 500 million years ago), crawling out of the seas and developing new features and body parts in order to adapt to life on solid ground. Animals had learnt to feed on plants and on other species of animals, and out of this came the many patterns of ‘ecological relationships’ among living things. The Earth became home to larger and more complex creatures. There were already many types of creatures without backbones and those which had developed backbones evolved from fish to amphibians to reptiles to birds, and eventually the early mammals.

3 Dinosaurs, Mammals … and Humans By around 350 million years ago many kinds of plants were a well-established feature of the warmer landscapes on Earth. Enormous ferns, grasses, shrubs, and later trees and flowering plants: they began to produce landscapes with a brilliant green cover. About 160 million years ago a variety of huge reptiles — the Dinosaurs — came to dominate the Earth.

They flourished for several millions of years and then quite mysteriously became extinct around 60 million years ago. We know of them only through the evidence of their bones and skeletons that can still be found today. The passing away of the Dinosaurs made it possible for the first mammals — a kind of rodent — to develop rapidly into many new forms. The Earth now began to support early forms of the elephant, horse and bison and later there appeared the cat and the dog family. Meanwhile there had already appeared on Earth the primates, a branch of the ‘evolutionary tree’ from which monkeys, apes and eventually human beings came into existence. The earliest human-like creatures probably lived around a million years ago on the Savanna grasslands of Africa, living as animals among animals. But this creature, walking on two legs and keeping his hands free for other uses, was already at the beginning of an evolutionary path that set him apart from all other living creatures. Most of the Earth’s story had unfolded without any humans. But even in the extremely short duration that humans have lived on Earth, they have come to increasingly influence the planet, its landscapes and all the other creatures who share the planet with them. This impact is far-reaching and, at present, highly destructive. Even though we may be among the most recent arrivals on this planet, the Earth’s future and therefore the future of all species that live on Earth appears to lie in the hands of us human beings.

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Draw a ‘PICTURE STORY’ OF THE PLANET EARTH in your notebook

Our unique home in space Birth of the solar system

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Formation of the first Oceans Continents are colonized by plants

5 Dinosours appear Humans appear On earth on the on the savannas

Questions Q1. The Earth is a unique planet because it supports life on it. What reasons can you find for the unique development of life on this planet?

Q2. Complete this ‘time-chart’ that starts from the ‘Formation of the Earth’ and mentions the major changes that have taken place on Earth up to the coming of the human species. Continue this chart: 4,600 million years ago Formation of the Earth 4,000 million years ago Planet-wide volcanic activity 3,800 million years ago

Q3. In what ways do you think have human beings proved to be destructive towards other living species or the planet as a whole?

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