Earth almost starved of oxygen one billion years ago

Iron in Earth’s oceans one billion years ago almost starved the world of oxygen and destroyed any chance of complex life evolving, study finds

Phytoplankton that lived in the ocean’s a billion years ago produced oxygen
But the presence of iron inhibited this process and limited photosynthesis
Oxygen dropped to around one per cent of today’s oxygen levels

Published: 18:26 GMT, 2 March 2020 | Updated: 18:26 GMT, 2 March 2020

Evolution of life on Earth was nearly nipped in the bud one billion years ago as key chemical reactions in the world’s oceans starved the planet of oxygen.

During this period of time, known as the Neoproterozoic, oxygen was produced via photosynthesis from phytoplankton – the ‘plants’ of the ocean.

But the oceans became rich in iron around 800 million years ago which restricted the oxygen-producing ability of the phytoplankton and stifled oxygen production.

Oxygen dropped to around one per cent of modern levels and was barely sufficient for life to survive.

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During the Neoproterozoic, oxygen was produced via photosynthesis from phytoplankton – the ‘plants’ of the ocean (pictured). Oeans became rich in iron around 800 million years ago which restricted the oxygen-producing ability of the phytoplankton and stifled oxygen production

An international team of researchers led by the University of Exeter assessed the role the ocean’s played in helping and hindering life on Earth to blossom.

‘Ocean chemistry in this period changed to become ‘ferruginous’ (rich in iron),’ said Dr Romain Guilbaud, of CNRS in Toulouse.

‘We know ocean chemistry affects the cycle of phosphorus, but the impact on phosphorus availability at this time hadn’t been investigated until now.

‘By analysing ocean sediments, we found that iron minerals were very effective at removing phosphorus from the water.’

Oxygen dropped to around one per cent of modern levels and was barely sufficient for life to survive. Had it dropped any further it would have potentially delayed or destroyed the evolution of life on Earth

The study found the amount of phosphorus available remained ‘just sufficient’ to support the oxidising atmosphere, around one per cent of current levels.

‘Our observations suggest significant potential variability in atmospheric oxygen concentrations across Earth’s “middle age”,’ said Professor Tim Lenton, Director of the Global Systems Institute at the University of Exeter.

He added: ‘One question about the emergence of complex life is why it didn’t happen sooner.

‘Lack of oxygen and lack of nutrients are two possible reasons, and our study suggests both of these may have been the case in the early Neoproterozoic era.

‘In fact, if phosphorus levels in the water had dropped any lower, it could have tipped the world back into a ‘reducing’ atmosphere suitable for bacteria but not for complex life.’

A return to a ‘reducing’ atmosphere would have reversed the Great Oxidation Event, which occurred about 2.5 billion years ago, during which photosynthesis by cyanobacteria in the oceans introduced free oxygen to the atmosphere.

HOW DO SCIENTISTS THINK LIFE ON EARTH BEGAN?

Life on Earth may have started thanks to a modified version of modern-day DNA’s sister molecule, scientists believe.

DNA is the backbone of life and almost all of our planet depends on it but, on primordial Earth, a primitive version of its lesser-known sister – RNA – was the focal point for evolution, experts say.

RNA is structurally similar to DNA, except one of the four fundamental pieces, thymine, is substituted for uracil.

This changes the shape and structure of the molecule and researchers have long believed this chemical was vital to the development of Earth’s first lifeforms.

An accidental discovery by Harvard academics published in December 2018 found that a slightly different version of RNA may have been the key ingredient allowing life on Earth to blossom.

Scientists claim that a chemical called inosine may have been present in place of guanine, allowing for life to develop.

This slight change to the bases, known as a nucleotides, may provide the first known proof of the ‘RNA World Hypothesis’ – a theory which claims RNA was integral to primitive lifeforms – they say.

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