As the newborn Earth consolidated, its internal heat became so fierce that it caused the solids to melt. The heavier elements in the planet's make-up were drawn down into the center. Lighter elements, including gases, moved to the surface. This surface began to form a crust. Above it, an atmosphere began to form from the gases ejected through the cooling crust by the pressures within. They included carbon dioxide, nitrogen and water vapor. The Earth's gravitational pull held onto the atmosphere being created from the volcanic emissions. Volcanic steam condensed to form thick clouds that blanketed the planet. The surface continued to cool. Eventually, the clouds began to pour down rain. It was a rain that lasted for millions of years.
The rains created the first oceans. By the time the Earth's surface had cooled enough to interrupt the almost endless cycle of evaporation and condensation, the planet had begun to demonstrate its unique characteristic. It is the only body in the Solar System with a surface covered largely by water. This cover of water had a significant affect on the atmosphere. It dissolved some of the gaseous elements in the air, making the atmosphere thinner. Composed mainly of carbon dioxide and nitrogen, it also included ammonia, carbon monoxide, hydrogen and methane. There was no significant amount of oxygen yet.
Scientists who study the origins of the Earth have conducted many experiments. Gases found in the early atmosphere have been mixed with water and subjected to certain natural traumas such as ultraviolet radiation and the high voltage discharges typical of electrical storms and lightning bolts. Complex substances have been found as a result. These materials include carbon chemicals and amino acids. Both are fundamental to living matter. The warm, shallow lakes and ocean margins were ideal for the beginning of life. The water stewed, waiting for the right exposure to ultraviolet rays from the Sun, or lightning strikes from the rumbling clouds. Once the new, biologically active, molecular chains were set in motion, they could sink into the protective depths of their watery environment. There, protected from prolonged exposure to harmful ultraviolet radiation, they could begin to evolve.
Blue-green bacteria were Earth's predominant life form long before humans ever existed on Earth. Over hundreds of millions of years, the bacteria colonized the planet. Through a process similar to photosynthesis, these primitive cells released oxygen, which they extracted from oxygen compounds into the atmosphere. This process radically altered the atmosphere of the Earth. The way was cleared for oxygen-consuming organisms to develop.
Oxygen also built a protective barrier of ozone. This is a special form of oxygen that is found only in the upper levels of the atmosphere. It shields living things from the Sun's ultraviolet rays. Life forms that could leave the protection of the water and exist on dry land were now able to develop.
Volcanoes are one of many signs that the Earth's interior is hot. On the beds of oceans, there are ridges where plates move apart. The gap is constantly replenished with molten rock from below. The motion sometimes sticks along the slip lines. When it jolts onward again, an earthquake may accompany the sudden movement.
The Earth's atmosphere is as unique as its water cover and shifting crust. The air breathed at ground level is 78.08% nitrogen, 20.94% oxygen, 0.93% argon, and varying amounts of water vapor and other gases. In the upper atmosphere, solar ultraviolet radiation changes some of the oxygen into ozone. The ozone absorbs ultraviolet radiation. This helps to protect life from unfiltered ultraviolet radiation, which would destroy it. The "greenhouse" gases in our atmosphere help to seal in the weather below. The air around the Earth also acts as a friction shield. It burns up the billions of meteors that bombard the planet every day, most the size of specks of dust.