Our trip to the beginning of life starts from Wilpena Pound, in the Flinders Ranges (Australia). This is where the first signs of multi-cellular organisms, belonging to the so-called Ediacara fauna, were found. 

  The Sun rises over Wilpena Pound, in the Flinder Ranges

 
However, the Ediacara fauna appeared "only" 600 million years ago. Given that our planet is cosiderably older (about 4.5 billion years), what forms of life were predominant before the Ediacara fauna? These days, thanks to new advances in molecular biology, researchers are beginning to find traces of life in rocks as old as 3.7 billion years. These 'fossils', though, do not look like anything we would see in a museum: they are molecules that can be ascribed to the activity of organisms. 

In order to find something more palatable, we have to search in younger rocks (3.5 billion years old), where one can find stromatolites. Stromatolites are columnar features that are commonly thought to have been formed by the trapping, binding, and cementation of sedimentary grains by micro-organisms, especially blue-green algae. Living examples of these algae colonies can be observed only in a few places on Earth, like Shark Bay, in Western Australia. The sedimentary strata of the kind shown in the picture below are interpreted as being largely made of those colonies. For more information on stromatolites, have a look at http://www.fossilmall.com/Science/About_Stromatolite.htm                      

    Fossil stromatolites (Wilpena Pound, Flinder Ranges, Australia)

 
The first traces of multi-cellular organisms are found in rocks ranging in age from 600 to 545 million years ago. That's where the Ediacara fauna was discovered by Reginald Sprigg in 1946, in the Flinders Ranges. Traces of these organisms are limited to body-prints. Palaeontologists believe the Ediaca fauna was made of soft-bodied invertebrates that resembled the flatworms, soft corals and jellyfish we know today. For more information on the Ediacara fauna, click here                       

 
The Ediacara Fauna immediately pre-dated  the explosion of life-forms at the beginning of the Cambrian period, about 542 million of years ago. This explosion eventually led to the appearance of dinosaurs. The sedimentary strata shown below preserve traces of a stroll taken but a dinosaur in the Jurassic. It is probably worth spending a few words on the mechanisms of formation of such fossils. In this case, a dinosaur left its footprints while walking on a mud plain. The mud layer (and its footprints) were then rapidly buried underneath sandy sediments, which filled the voids left by the footprints. Later in the geological history the sedimentary layers were uplifted and tilted. The muddy layer was eroded away and we are now left with the sandy layer and the footprints. The process of formation of these fotprints is somewhat similar to the creation of a sculpture by putting molten iron in a cast and then, after the iron has cooled down, removing the cast.                      

  Fossil footprints of a dinosaur (Skyline drive, Canon City, Colorado, USA)

Silicization is another efficient mechanism of preservation of fossils. Silica (the element that makes up the glass in our windows), under particular circumstances, can replace the elements that originally constituted shell, bones and trunks, thus making them very resistant to erosion and weathering.