Australian scientists have pinpointed the time when an Earth-changing form of life appeared - the bacteria which gave us the oxygen we breathe.

Writing in the journal Nature, a team from the Australian Geological Survey have identified chemical fossils of cyanobacteria in ancient rocks. It demonstrates that 2.5 billion years ago they were already changing the face of our planet.

Rod-shaped cyanobacteria found in Australia

It was the emergence of this type of bacteria, able to use sunlight to produce energy and also oxygen, that turned the sky blue. In a real sense, the Earth's blue sky has been coloured by life.

Before the cyanobacteria evolved the Earth was a very different place from the world we know today.

Several billion years ago the Earth's atmosphere was made of hydrogen, carbon monoxide and possibly methane, gasses that are highly toxic to most lifeforms today.

It is difficult for us to imagine what the Earth was like three billion years ago when no form of life was visible anywhere on the surface, before the continents formed, before it even rained water.

But even during these earliest times life was stirring - the microorganisms called cyanobacteria evolved.

As oxygen accumulated in the atmosphere the face of the planet was changed. Life took a new course as it adapted to use the abundance of oxygen.

Still around

We can still see the cyanobacteria today. They are often visible as surface blooms in lakes or oceans but they live on in an even more remarkable way.

Sometime in the past, perhaps a billion years ago, the cyanobacteria teamed up with other cells.

Some of them started to live inside other cells exchanging their freedom for an enclosed and stable environment inside another cell.

In turn, the host cells used the energy-producing ability of the cyanobacteria and the two organisms began a symbiotic relationship of mutual benefit.

So close has this relationship become that today we have forgotton that a widespread form of life on Earth is actually two forms of life working together.