A new analysis of some of the most ancient rocks on Earth suggests that life on land began more than 3 billion years ago - around 300 million years earlier than previous estimates.

Scientists have been studying samples from the Barberton greenstone belt in eastern South Africa - one of the oldest and most well-preserved pieces of continental crust on the planet. These rocks date from the Archaean aeon (2.5-4 billion years ago), when Earth's crust and layers had only just been formed.

This area, situated on the eastern edge of a crust mass called Kaapvaal Craton, doesn't just give us clues about early microbial life - it can also tell us how Earth was originally formed and knocked into the shape it is today.

Now scientists in Germany have found tiny grains of the iron sulphide mineral pyrite inside some of these rocks, and telltale signs of life inside that they say existed at least 3.22 billion years ago.

The team thinks the rocks' composition, the shape of the miniature crystals they discovered, and the layering seen on the research site is evidence of ancient living microbes in this area.

To date these particular rock samples, the researchers analysed the sulphur isotopes 34S and 32S present in the pyrite crystals, and found that the concentrations changed between the centre and the outer rim of the tiny grains.

The scientists think the changes in concentrations are due to the way microbes process sulphur, which suggests that these tiny lifeforms were already around some 3.22 billion years ago - and probably even earlier than that.

"It is interpreted that microbes living in the soil, at a level that was continually shifting between wet and dry conditions, subsequently produced the rim overgrowths on the pyrite crystals," the researchers explain.

As you might expect, some pretty heavy-duty equipment was required to reach these conclusions.

The researchers used secondary ion mass spectrometers (SIMS) installed at the GFZ German Research Centre for Geosciences in Germany to measure sample masses that were less than one-billionth of a gram.

These SIMS use finely focussed ion beams to detect the chemical make-up of materials, which is how the researchers were able to differentiate between the sulphur isotopes.

It's worth bearing in mind that looking for evidence of life so long ago is a complicated task even for the experts, and there isn't always consensus between scientists about when life on Earth began, what signs of early life should look like, or what counts as life in the first place.

In September, researchers working in Greenland announced the discovery of microbial evidence found in fossils dated to 3.7 billion years ago, which would pre-date the samples here.

And in March, another study found new signs of fungal life on land, which existed some 440 million years ago.  

And now this latest study, published in Geology, adds to the body of evidence showing just how ancient and widespread these basic forms of terrestrial life were on Earth.

We can't wait to see what scientists dig up next.