Scientists believe that about 4.7 billion years ago, a swirling interstellar cloud of gas and dust began to fragment and form clusters that eventually became the Sun, Earth, and the other planets of our Solar System. On Earth, gravity, collisions with other bodies, and the radioactivity of some of the heavier elements caused the planet to begin melting. Lighter compounds floated outwards to form the Earth’s mantle and crust, while the heavier elements, mainly iron and nickel, sank in towards the centre to form the core. The resulting world was not quite a perfect sphere, and it remains slightly flattened today, thicker at the Equator than at the poles.
Once the planet was formed, volcanic eruptions caused light, volatile gases and vapours to escape from the mantle and crust. Some of these, primarily carbon dioxide and nitrogen, were captured by the Earth’s gravity and formed a primitive atmosphere, while water vapour condensed to form the world’s first oceans. Today water covers nearly 71 per cent of the world’s surface, and most of that forms the Pacific, Atlantic, and Indian oceans. The remaining 29 per cent of the Earth’s surface is covered by land, most of it concentrated on seven continents—Asia, Africa, North America, South America, Antarctica, Europe, and Australia.
BEGINNINGS OF LIFE
About 3.5 billion years ago, conditions developed in which it was possible for life to emerge. The world’s oceans and atmosphere, affected by the proliferation and evolution of early life-forms, underwent major transformations, many of which would later enable the evolution of higher life-forms. Since then, life has evolved from simple single-celled organisms into the micro-organisms, insects, plants, and animals we know today. The world’s atmosphere has evolved as well, both influencing and influenced by the life-forms living within it
Anthropologists estimate that the first modern members of our species, Homo sapiens, appeared about 100,000 years ago in southeastern Africa. They then migrated, spreading rapidly across Africa and northeast into Asia, using their intelligence and dexterity to adapt to different environments. About 70,000 years later humans had colonized Europe, Australia, and North America. Humanity occupied most of the world’s landmasses as early as 10,000 years ago, with the exception of several remote islands and the continent of Antarctica.
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