The Origin of Life

Prokaryotes were the first organisms to evolve.  This would have occurred somewhere between 4.1 Ga, after the Earth's crust began to solidify and 3.5 Ga, when the planet was inhabited by bacteria that was advanced enough to build stromatolites.  There are various hypothesis's on the origin of life, taking into account the few factors that we know about ancient earth (little oxygen, volcanic activity, asteroids, ultraviolet radiation...)  One hypothesis states that the first organisms were the products of a chemical evolution that took place in four stages;

  1. the abiotic synthesis and accumulation of small organic molecules or monomers, such as amino acids and nucleotides
  2. the joining of these monomers into polymers, including proteins and muclies acids
  3. the aggregation of abiotically produced molecules into droplets called protobionts, that had chemical characteristics different from their surroundings;
  4. the origin of heredity
Ancient Fossil Bacteria : Pictured above are two kinds cyanobacteria from the Bitter Springs chert of
 central Australia, a site dating to the Late Proterozoic, about 850 million years old. On the left is a
 colonial chroococcalean form, and on the right is the filamentous Palaeolyngbya. ( 

more about prokaryotes.....