Scientists have now discovered microbes living and reproducing as deep as 5km (3.1 miles) below ground and studies have shown that they are likely to have survived in complete isolation from the surface biosphere for millions and perhaps even billions of years.

One of the latest studies into the deep biosphere has found that these microbes form a distinct subsurface community of genetically similar individuals despite living on opposite sides of the world. This global similarity of such an isolated life-form suggests that they may have evolved directly from a common ancestor that lived as long ago at the period when life on earth originated, some 3.5 billion years ago.

An increasing number of researchers believe that life could have first got going in the tiny cracks of underground rocks, fuelled not by the energy of sunlight but by chemical fuel in the form of hydrogen and methane which can be produced in certain types of rock under high temperatures and pressures.