Dust and ice around the Sun’s atmosphere were drawn together by gravitational force to form a cold mass which we now call Earth. The heat produced by meteorite activity, compaction and radioactivity caused the earth to melt, thereby reorganizing elements of varying density into three layers called core, mantle and crust. Gases such as hydrogen, nitrogen, water vapor, nitrogen, methane and carbon dioxide escaped from earth’s gravity.
These gases formed a layer around the earth called atmosphere. All these gases are made of basic units called elements, which can be further broken down to atoms. Carbon, Hydrogen, Nitrogen, Oxygen and another 21 naturally-occurring elements are critical for the existence of life on earth. As the temperature of the earth lowered, the water vapor in the atmosphere condensed and came down as rain. The hollow surfaces on earth got filled with rain water and thus formed oceans.
Life on earth was sparked by a random combination of physical and chemical reactions, approximately four billion years ago. Cells are the building blocks for any life form that exists and the cell’s DNA determines the nature of the cell. All chemical reactions inside a cell are enabled by proteins called enzymes. The duplication and mutation of enzymes over several thousand years causes diverse life to evolve into existence. All living beings have an underlying structure which in spite of being complex follows some orderly pattern.
Living organisms also share certain common features such as the ability to reproduce, grow, evolve and respond to the external stimuli. The interaction of one life form with another or the environment forms the basis for an ecosystem. Energy is a critical constituent in any ecosystem that makes life on earth possible. No form of energy can be destroyed, since energy which is used up in one form is converted to another. Hence, Reference: Campbell, N. A. , Reece, J. B. , & Simon, E. J. (2006). Essential Biology with Physiology. Benjamin Cummings.