The first cells, chemoautotrophs and photoautotrophs

The Urey Miller experiment caused a great deal of excitement and was partially responsible for launching a whole new branch of biology called exobiology.

Variations of the Urey-Miller experiment were soon trialed.  It was found that sugars could be produced, lipids that form bilayers could be formed, proteins could be assembled, and molecules that self replicate could be made.  However, how these were able to be aggregated into something resembling a cell is still a matter of conjecture.  However, it is agreed that first cells appeared on Earth between 3.5 to 3.8 billion years ago.  The first fossil evidence is dated to 3.45 billion years ago.

Bacterial specimens found in rocks from the Barbeton Mountains, South Africa.  The rocks are dated to over 3.4 billion years old.

  
It is thought that the first cells were likely to be heterotrophic. That is, they consumed the complex energy yielding chemicals that were in the environment.  This would have been limited and would have run out fairly quickly.  Varieties of bacteria that were able to have produced their own food (autotrophs) would have survived while most of the heterotrophs would have perished.  These bacteria are likely to be the ancestors of what we now call the Archaebacteria.  These are found in extreme environments such as submarine vents in the deep oceans, hot springs, salt lakes etc.

These bacteria utilise fairly simple chemical compounds to obtain their own energy and complex substances needed for growth and division.  Most of these Archaebacteria are anaerobic, supporting the idea that they had their origins in an oxygen free environment.

From this group of bacteria, a new type arose. Rather than being powered from the chemicals produced by volcanoes, they utilised the sun to power the processes need to grow and divide.  These were the cyanobacteria, and their activities would change the Earth and the path life would take.



The first fossil evidence of photosynthesis were the appearance of fossilised stromatolites in WA dated to 3.4 billion years old. The largest colony of stromatolites in the world is currently in Shark Bay, also in Western Australia.
These are colonies of photosynthetic bacteria which form columns as they grow.  A by-product of their metabolism is oxygen.  These organisms were largely responsible for the removal of iron from the oceans, and a for adding large portion of the atmospheric oxygen and ozone layer.  For over 2 billion years, these bacteria were the dominant fossil found in the fossil record.