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The air on Earth was a toxic soup of chemicals for almost two billion years as life was evolving on the planet, according to new research.

Scientists have found the oldest evidence for what our planet's atmosphere was like 4 billion years ago, just 600 million years after the Earth first took shape.

By analysing the different levels of isotopes trapped inside some of the oldest rocks in the world, they were able to build up a picture of what the air was like at the time.

Researchers studied chemicals from the early atmosphere trapped in rocks in Hudson Bay, Quebec (above)

They found that the Earth's early atmosphere contained oxygen levels that were far lower than they were today and were even lower than scientists had previously believed.

The study also suggests that the atmosphere may have contained considerably more sulphur dioxide, more methane gas and carbon dioxide than it does today.

The composition is almost identical the chemical fingerprints found in rocks that formed around 2-3 billion years ago.

The results suggest that the atmosphere of our planet remained almost unchanged as the first bacteria began to appear and these may have controlled the atmosphere.

It could provide new clues as to what the first life was like on Earth and how it evolved.

Dr Boswell Wing, a geologist at McGill University who oversaw the research, said: 'Those younger rocks contain clear signs of microbial life.

'One interpretation is that biology controlled the composition of the atmosphere on early Earth, with similar microbial biospheres producing the same atmospheric gases from Earth's infancy to adolescence.

'We can't rule out, however, the possibility that the biosphere was decoupled from the atmosphere.

'In this case geology could have been the major player in setting the composition of ancient air, with massive volcanic eruptions producing gases that recurrently swamped out weak biological gas production.'

The researchers, whose work is published in the journal Proceedings of the National Academy of Sciences, analysed rocks from an area known as the Nuvvuagituq Greenstone Belt in Porpoise Bay, Hudson Bay, northern Quebec, Canada.

Using mass spectrometery they were able to measure the levels of sulphur isotopes trapped inside the rocks when they were created.

The Nuvvuagituq Greenstone Belt is thought to be between 3.7 billion and 4.2 billion years old. 

The Earth is thought to have formed around 4.6 billion years ago and the first signs of life is thought to have appeared around 4 billion years ago.

Sulphur isotopes in the crystals of the rock in the Nuvvuagittuq Greenstone Belt (above) allowed the scientists to analyse what the Earth's atmosphere was like when the rock was forming around 4 billion years ago

Porpoise Bay (above), which sits within Hudson Bay, Quebec, is home to some of the oldest rocks in the world

The result suggested that oxygen levels were just 20 per cent of the levels today, making up just 0.0002 per cent of the atmosphere. This is far lower than previous estimates, which have said said oxygen made up around 0.01 per cent of the atmosphere.

Oxygen currently makes up about 21 per cent of the gases in the atmosphere.

Previous work by the researchers also looked at the composition of rocks that were between 2 to 3 billion years old, showed they had similar levels of sulphur.

 Many palaeobiologists believe that the first life to appear on Earth were bacteria capable of living in the harsh environments that existed around 4 billion years ago.

They would have been similar to the 'extremophile' bacteria that are found living around volcanic vents and acidic hot springs today.

Ancient archaebacteria similar to this Sulfolobus acidicaldarious, which can grow in temperatures of 70 degrees Celsuis and acid with pH2, would have been able to survive the harsh conditions in the early Earth

It is thought that as these bacteria evolved, they became able to photosynthesis and began producing oxygen around 2.6 billion years ago.

This then led to a fundamental change in the atmosphere, allowing the life that needs oxygen to live to develop.

Dr Emilie Thomassot, a geologist led the research and now works at the Centre de Recherches Pétrographiques et Géochimiques (CRPG) in Nancy France, said: 'We found that the isotopic fingerprint of this atmospheric cycling looks just like similar fingerprints from rocks that are a billion to 2 billion years younger.

'This unusual atmospheric fingerprint has been present since the end of the Hadean, the earliest eon of Earth history.'