- Scientists have analysed microscopic remnants from the earliest years of Earth
- The study found that the fossil likely stemmed from organic compounds
- It would have had to endure temperatures of around 300°C (572°F)
- Oxygen had not formed on Earth by this time and the prokaryotic organism likely survived by using minerals as fuel
Tiny fossilised bacteria dating back 3.4 billion years could be the first evidence of life on Earth, say scientists.
The microscopic remnants provide experts the opportunity to see at what the world looked like when it was barely one billion yeas old.
During these formative years of planet Earth the planet was vastly different to how it is today, with temperatures reaching up to a sweltering 300°C (572°F).
As well as weathering the extreme heat, the primitive lifeforms would have been fuelled by minerals as oxygen and ozone had yet to form in this tumultuous period in Earth's history.
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Conditions for life were far from ideal and plants had yet to develop the ability to photosynthesise.
The single celled organisms are believed to be the most primitive type of bacteria called prokaryotes.
This means they don't even have a nucleus or other structures such as mitochondria in which the biochemical processes of respiration and energy production occur.
They were dug out of a rock formation known as Strelley Pool in Pilbara, Western Australia, five years ago.
Now scientists have declared they are possibly the world's oldest microfossils after a chemical analysis identified characteristics similar to modern bacteria.
This all but confirms their 'biological origin', the international team told the Goldschmidt geochemistry conference in Boston.
Dr Julien Alleon, of the Massachusetts Institute of Technology, said: 'There are a couple of important points which come out of this work.
'Firstly, we demonstrate that the elemental and molecular characteristics of these 3.4 billion year old microfossils are consistent with biological remains, slightly degraded by fossilisation processes.
'This effectively supports the biological origin of the Strelley Pool microfossils.
'Secondly, it is remarkable that these echoes of past life have survived the extreme conditions they have experienced over the last 3.4 billion years.
'We know from the molecular structure of the microfossils that they have been exposed to temperatures of up to 300 °C for long periods. And yet we are still able to see signs of their original chemistry.
'This is a step forward to confirming that these are indeed the oldest fossils yet discovered.'
The study published in Geochemical Perspectives Letters shows the chemical residuals match those of younger bacterial fossils - and so are likely to have been laid down by early life forms.
There is some evidence life may have developed even earlier, with rocks that date back 4.1 billion years potentially harbouring signs of life.
This theory has yet to be confirmed however, as it has not been subjected to the same rigorous examination as the Strelley Pool specimen.
Dr Alleon said: 'There are competing claims over which microfossils are actually the world's oldest. This analytical strategy needs to be applied to other ancient samples to help settle the controversy.'
His team analysed the Strelley Pool samples using a scanning technique called synchrotron-based X-ray absorption spectroscopy.
They then compared the analytical results with 1.9 billion year old fossils unearthed on the shores of Lake Superior, Ontario and modern-day bacteria.
All of the samples showed similar absorption features, indicating the residual chemicals were made from the same building blocks.
Earth scientist Professor Vickie Bennett, of the Australian National University, Canberra, who was not involved in the study, described it as exciting and said it was 'compelling evidence' the Strelley microfossils were evidence of life on ancient Earth.
She said: 'This is in line with other observations for early life from the Strelley Pool rocks, including stromatolites interpreted as microbial mats, and further confirming that the minimum age for life on Earth is 3.4 billion years.'
But Professor Bennett added: 'The techniques used here are not applicable to the older rocks that host the claims for the oldest terrestrial life, as these rocks were exposed to much higher temperatures.
'These samples include the 3.7 billion year old stromatolites from Isua, Greenland, and the 4.1 billion year old Canadian microfossils.
'However, this work shows how quickly the field is developing and that new capabilities for testing and confirming earlier evidence of life are in reach.'
Earth is believed to have formed about 4.6 billion years ago, with life only emerging after the planet had cooled down from its original molten state and developed a solid crust with oceans created from atmospheric water vapour.
Many scientists think these primordial seas gave rise to life with hot, mineral-rich volcanic vents acting as catalysts for chemical reactions across the surface of tiny water bubbles, which led to the first cell membranes such as prokaryotes.
Other bubbles are thought to have formed self-replicating substances by attracting chemicals from around them.
Over time the two combined to produce energy-using, living cells.
As the numbers of these microscopic bacteria multiplied and supplies of their chemical fuel were eaten up, they sought out an alternative energy source.
New varieties began to harness the power of the sun through photosynthesis - a move that would ultimately lead to simple plants and which opened the planet up to animal life.
WHAT ARE THE OLDEST FUNGI EVER DISCOVERED?
For many years, fungi were grouped with, or mistaken for plants.
Not until 1969 were they officially granted their own 'kingdom', alongside animals and plants, though their distinct characteristics had been recognised long before that.
Yeast, mildew and molds are all fungi, as are many forms of large, mushroom-looking organisms that grow in moist forest environments and absorb nutrients from dead or living organic matter.
Unlike plants, fungi do not photosynthesise, and their cell walls are devoid of cellulose.
Geologists studying lava samples taken from a drill site in South Africa discovered fossilised gas bubbles 800 metres (2,600 feet) underground.
In April 2017, they revealed that they are believed to contain the oldest fungi ever found.
Researchers were examining samples taken from drill-holes of rocks buried deep underground, when they found the 2.4 billion-year-old microscopic creatures.
They are believed to be the oldest fungi ever found by around 1.2 billion years.
Earth itself is about 4.6 billion years old.
They could be the earliest evidence of eukaryotes - the 'superkingdom' of life that includes plants, animals and fungi, but not bacteria.
The previous earliest examples of eukaryotes - the 'superkingdom' of life that includes plants, animals and fungi, but not bacteria - dates to 1.9 billion years ago. That makes this sample 500 million years older.
It was believed that fungi first emerged on land, but the newly-found organisms lived and thrived under an ancient ocean seabed.
And the dating of the find suggests that not only did these fungus-like creatures live in a dark and cavernous world devoid of light, but they also lacked oxygen.