Many experts now believe that life evolved not long after the Earth formed some 4.5 billion years ago. However life remained in a very primitive state hardly changing at all for many hundreds of millions of years. Then, for some reason about 600 million years ago the very first bacteria evolved which eventually led to a sudden explosion in evolution with the evolution of many different plants and animals.
Bacteria was one of the earliest and most primitive forms of life. Some bacteria colonies are sticky and particles in the water gets stuck to them. The bacteria then grows around and on top of this material until, over thousands of years, mounds form on the sea bed. These mounds, which can still be found today in parts of Australia, sometimes even break though the surface of the water and continue to grow in the air. Bacteria is so small it cannot be seen with the naked eye, or sometimes just as a colouration on rocks, but these mounds, called stromatolites, were the first significantly visible sign that life was evolving on plant Earth.