[Global] The ‘explosion’ of animal life on Earth may have taken more than 100 million years to get going, according to a new study.
Scientists believe the first life on Earth was anaerobic, able to survive without the need for oxygen which was almost totally absent from the atmosphere between 3.5 billion and 600 million years ago.
However, around 600 million years ago there appears to have been an evolutionary burst of animal life known as the Cambrian explosion, and this was kickstarted by increased levels of oxygen.
Rock samples from across the US, Canada and China, made from sediment laid down at the time, have now revealed just how long it took for this process to get going.
Researchers said it took 100 million years for the oxygen levels to rise from 1 per cent to 10 per cent of the levels seen in the atmosphere today.
This, according to the scientists, suggests early animal evolution was probably kick-started by increasing oxygen levels rather than changes in animal behaviour causing oxygen levels to rise.
Dr Philip Pogge von Strandmann, a biogeochemist at University College London, said: ‘We wanted to find out how the evolution of life links to the evolution of our climate.
‘The question on how strongly life has actively modified Earth’s climate, and why the Earth has been habitable for so long is extremely important for understanding both the climate system, and why life is on Earth in the first place.
‘We were surprised to see how long it took Earth to produce oxygen and our findings dispel theories that it was a quick process caused by a change in animal behaviour.’
It is widely believed oxygen began trickling into the atmosphere around 2.5 billion years ago as a photosynthetic microbe called cyanobacteria began releasing it as a waste product.
However, much of this oxygen was absorbed by rocks and the oceans at first, until around 600 million years ago when there was a sudden spike in oxygen levels.
Exactly what caused this spike, however, has puzzled scientists, with some claiming it may have been the appearance of animals on the planet which altered the oxygen in the atmosphere.
In the latest research, which is published in the journal Nature Communications, the scientists measured the selenium isotopes in rock samples that were laid down under the sea between 770 million and 520 million years ago and used that to calculate oxygen levels at the time.
Dr Pogge von Strandmann said: ‘We took a new approach by using selenium isotope tracers to analyse marine shales which gave us more information about the gradual changes in oxygen levels than is possible using the more conventional techniques used previously.
‘There are two big oxygen rises in Earth history – one was 2.5 billion years ago, and took oxygen from nothing to under 1 per cent of the present level.
HOW ALGAE TOOK OVER THE LAND
Plants are commonly thought to have first emerged on land as a result of algae creeping out of the water.
But recent research suggests not all terrestrial plans are descended from aquatic algae.
Instead it seems a branch of algae may have evolved already capable of living on the land hundreds of millions of years ago.
But now, a group of scientists are challenging this theory, saying not all plants are descendants of aquatic algae.
Instead it appears some algae may have grown in the sandy soil along shorelines, developing the ability to venture further inland.
Analysing some of the earliest known plants and green algae, the researchers found their shared algal ancestors already possessed the sets of genes needed to interact with a fungi called arbuscular mycorrhiza, which often forms a symbiotic relationship with the roots of plants to help them draw nutrients from the soil.
Photo: The explosion in animal life that occurred at the start of the Cambrian period may have started with more of a ‘wimper’ as oxygen levels slowly increased over 100 million years, according to a new study. A fossil of a Marrellomorpha is pictured above, one of the animal species that became abundant during the Cambria