Evolution of the earth

At first the earth was barren, rocky and hot, with a thin atmosphere of hydrogen and helium. Over about 4,600 million years it became a planet with water, a life-friendly atmosphere and living things. It now has a layered structure: the atmosphere is least dense, and density rises towards the centre.

Evolution of the lithosphere:

• In its early stage the earth was mostly volatile. As density increased, the temperature inside rose and materials separated by density: heavy ones (like iron) sank towards the centre, light ones rose to the surface. This is differentiation.
• It cooled, solidified and shrank, forming a crust. The giant impact that formed the moon heated the earth again.
• Result: crust, mantle, outer core, inner core, with density increasing from crust to core.

Evolution of the atmosphere and hydrosphere (three stages):

• Loss of the primordial atmosphere: the early hydrogen and helium were stripped away by solar winds, as on all the terrestrial planets.
• Degassing: as the earth cooled, gases and water vapour poured out of the interior, helped by constant volcanic eruptions. This atmosphere had water vapour, nitrogen, carbon dioxide, methane and ammonia, and very little free oxygen.
• Photosynthesis by living things changed its composition. Today the atmosphere is mainly nitrogen and oxygen.

• As the earth cooled, water vapour condensed; rain dissolved carbon dioxide, cooling things further and bringing more rain. Water collected in depressions to form oceans within 500 million years of the earth's formation, so the oceans are about 4,000 million years old.
• Life began to evolve about 3,800 million years ago; photosynthesis evolved about 2,500-3,000 million years ago. Life stayed in the oceans for a long time. The oceans filled with oxygen, and about 2,000 million years ago oxygen began to flood the atmosphere.

Origin of life. Scientists see it as a kind of chemical reaction that produced complex organic molecules able to copy themselves, turning non-living matter into living. Life's record is in fossils: microscopic structures like today's blue algae are found in rocks older than about 3,000 million years.

Example (new): shake a jar holding sand, water and a little oil, then leave it: the sand sinks to the bottom and the oil floats on top. Differentiation sorted the young earth's material by density in the same way, with heavy iron at the centre and light rock at the surface.