Why did it take so long for animals to evolve on Earth? Scientists say this was due to oxygen deficiency.

The advent of complex animal life on the Earth started to flourish over 800 million years ago during the end of Proterozoic period. For about a billion years before this period, life existed but only with very simple organisms.

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The gap has long triggered a scientific debate among scientists.

In a study published in the journal, Science, Noah Planavsky of Yale University and his team found that oxygen levels in the mid-Proterozoic period were only about 0.1 percent compared with the amount of oxygen present today.

Animals must need a certain level of oxygen, along with genetic and ecological innovation, to survive and diversify. The study claimed to provide the first evidence that oxygen levels in the earliest period were incredibly low enough to prevent the rise of animals.

Evidence of the findings were analyzed using the reactivity of oxygen with ancient sediments collected across China, Australia, Canada and the U.S.

When oxygen first entered the atmosphere in considerable amounts, it chemically reacted with the oldest rocks. These rocks often contain chromium, which likely to remain stable after oxidation and subsequent burial. This feature makes chromium a reliable factor in measuring the amount of oxygen present in the atmosphere when the minerals were deposited.

The results were at odds with previous estimates, which showed oxygen was at least 40 percent of today's conditions.

The more generous estimates were established on a basis of slow but steady oxygen increases over time while the latest method indicated a more rapid rise of oxygen.

"If correct, the very low levels of oxygen found in this study would have provided a major restriction on early animal evolution, thus suggesting that it was indeed a rise in oxygen that ultimately stimulated the evolution of our earliest animal ancestors," said geochemist Simon Poulton. 

Tagsevolution, Complex life, diversity, Animals, Proterozoic Period, Low levels of oxygen