Between 5,000 and 4,000 million years ago the earth was formed.  The atmosphere of the earth consisted of methane (a simple hydrocarbon), ammonia (a nitrogen – hydrogen combination), carbon dioxide (the gas we breathe out) and other gases in a lesser quantity.  In laboratories today, these conditions have been simulated.  With the addition of water and high voltage electrical energy (the same as natural lightning), complex near-organic substances are known to be able to form spontaneously. These include Amino Acids (the things that form proteins including Enzymes) and Purines and Pyridines (the things that form RNA and DNA). 

From this ‘primeval soup’ simple bacteria-like creatures and algae are believed to have formed.  These creatures evolved and more complex creatures began to appear at the beginning of the Cambrian period, 570 million years ago.  It was at the stage where fossilised evidence of bodied animals begins to appear.  Obviously, hard bodied animals dominate fossil records (as they create fossils more easily).  Between 400 and 300 million years ago, vertebrate animals (animals with a spine or backbone) appeared more often.  Seas covered the earth and around this time fish-like creatures were spread over the earth. 

250 million years ago, lunged animals began to appear and amphibians and reptiles began to move to land.  From these early amphibious and reptilian species evolved more advanced reptiles such as dinosaurs, as well as mammals and birds.  A sudden catastrophic event at around 65 million years ago caused the abrupt extinction of dinosaurs.  Mammals filled the gaps that were left and the animals we see today evolved.  At around 1.5 million years ago the basic human evolved from the primates (so your great-great-great-great ..... grandfather was actually an ape according to this theory). 

As you can see around you today, there is a large diversity of life surrounding us, and evolutionary theory has provided us with an explanation of how it, and we, got here.