origin of life on earth

The Earth is one of eight major planets and a host of minor planets that circle a fairly average, middle-aged, main-sequence star. It formed from the accretion of residual material from the gravitational collapse of the solar nebula that produced the Sun. Based on their specific distances from the Sun, the planets formed into groups of similar chemical compositions, sizes, and densities. The four smaller inner planets are of higher density and are composed of a mixture of rock and metal. In contrast, the next four planets are massive gas giants with lower densities and larger sizes. The final grouping consists of small, low-density, icy worlds. Unlike the other inner planets, the Earth has remained a geologically active planet and has undergone continuous change since its formation. It is a planet dominated by the presence of liquid water. Throughout its long history, the Earth has evolved from a lifeless world into one that is populated by an uncountable number of species.
Based on radiometric dating analyses of moon rocks and meteorites, the Earth is believed to be about 4.5 billion years old. Many scientists believe that the Earth originated as a cold, undifferentiated body that internally heated up from the energy released through giant impacts, radioactive isotope decay, and the mass of the Earth itself. Once the appropriate melting temperatures were reached, heavy metallic elements sank to the planet's center of gravity, as lighter elements were displaced upward toward the surface. This process may have taken place in as little as 50 million years. It is fairly certain that the present core-mantle-crust structure was in place by 4 billion years ago.
Earth's core consists of both solid and liquid metal, presumably of nickel-iron composition. The outer, liquid metallic core revolves around the inner, solid metallic core and, in the process, generates an electric current. This electric current is responsible for the Earth's magnetic field. Sandwiched between