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Scientists have long been baffled by why animals took so long to thrive on Earth.

Prehistoric creatures first began to prosper at the end of the Proterozoic period, around 800 million years ago, but for a billion years before that, only very basic forms of life developed.

Geologists have put this 'boring' billion years down to poor air quality by discovering that oxygen levels were insufficient to support animals.

Prehistoric creatures first began to prosper at the end of the Proterozoic period, about 800 million years ago because before that, oxygen levels were too low - 0.1 per cent of what they are today -  to support the rise of animals such as the comb jelly (pictured) a primitive jellyfish among the first complex creatures

International researchers led by a team at Yale University in New Haven, found that oxygen levels during the 'boring' period were only 0.1 per cent of what they are today.

Noah Planavsky, Assistant Professor at Yale, explained that Earth's atmosphere couldn't have supported a diversity of creatures, no matter what genetic advancements were poised to occur.

'There is no question that genetic and ecological innovation must ultimately be behind the rise of animals, but it is equally unavoidable that animals need a certain level of oxygen,' he said.

Professor Noah Planavsky of Yale University (pictured right) and Professor Chris Reinhard of Georgia Tech (left) were among experts who studied chromium isotopes in ancient sediments to study ancient oxygen levels

'We're providing the first evidence that oxygen levels were low enough during this period to potentially prevent the rise of animals.'

Together with Professor Chris Reinhard, he analysed chromium isotopes in ancient sediments from China, Australia, Canada, and the United States to examine ancient oxygen levels.

Chromium is found in the Earth's continental crust, and chromium oxidation is directly linked to the presence of free oxygen in the atmosphere, according to the study, published in the journal Science.

They studied samples deposited in shallow, iron-rich ocean areas, near the shore and compared their data with other samples taken from younger locations known to have higher levels of oxygen.

Scientists have long been baffled about oxygen's role in controlling the first appearance of animals and Professor Planavsky believes that chromium has helped to solve the mystery.

Previous estimates put oxygen levels at 40 per cent of today's conditions during pre-animal times, suggesting that oxygen was already plentiful enough to support animal life.

In the new study, the researchers acknowledged that oxygen levels were 'highly dynamic' in the early atmosphere, with the potential for occasional spikes.

However, they say, 'It seems clear that there is a first-order difference in the nature of Earth surface chromium cycling' before and after the rise of animals.

Co-author of the study, Tim Lyons of the University of California, Riverside said: 'If we are right, our results will really change how people view the origins of animals and other complex life, and their relationships to the co-evolving environment. This could be a game-changer.'