Rising levels of oxygen in the oceans were a key causative factor in the emergence of skeletal animals 550 million years ago, according to a new study. This answers why there was a sudden surge in complex organisms on Earth, beyond algae, after a lull of 3 billion years.

At the beginning of the Neoproterozoic era, ranging 1,000 to 541 million years ago, anoxic conditions in the ocean prevailed. And this why it took so long for complex animal life to appear on Earth, said lead author Rosalie Tostevin of the Department of Earth Sciences at the Oxford University.

The study, published in the journal Nature Communications, tender geochemical evidence that a rise in oxygen levels in the oceans coincided with the appearance of complex animals.

The researchers included geochemists, palaeoecologists and geologists from University College London and Cambridge, Leeds, Edinburgh Universities and Namibia's Geological Survey office.

According to the lead researcher, a causal link on that matter has not been established. Yet the study is a trailblazer in terms of making a distinction between waters with high and low levels of oxygen in supporting the rise of complex organisms.

The study has succeeded in demonstrating that the presence of skeletal animals was more defined in well-oxygenated waters as a proof of oxygenated waters having supported life.

The study also inferred that there must have been a scarcity of well-oxygenated environments. That led to earliest animals facing problems from skewed oxygenated habitats.

The team analyzed the elemental composition at Namibia's ancient seafloor in the Nama Group. It was a well-preserved rock with abundant fossils of Namacalathus, Namapoikia and Cloudina animals.

They noted that cerium and iron volumes present in the rocks hinted at low-oxygen conditions that existed between highly oxygenated surface waters and deep waters where oxygen was in complete short supply.

"We honed in on the last 10 million years of the Proterozoic Eon as the interval of Earth's history when today's major animal groups first grew shells and churned up the sediment," said Graham Shields-Zhou, one of the co-authors.

Shields-Zhou added that oxygen levels were crucial in the relationship between environmental conditions and early development of animals.

The new thesis partially debunks the argument that the long gap on the emergence of complex organisms was due to the slow evolution process. Rather, the rising levels of oxygen in the oceans became the trigger for complex life-forms to attain mobility and features of modern animals.

Threat Of Falling Oxygen Levels

Meanwhile, researchers at the National Center for Atmospheric Research (NCAR) warned that oceans are likely to face a decline in their oxygen concentrations by 2030 as global warming intensifies.

In the NCAR study, Matthew Long and colleagues examined how climate change will impact oxygen levels in the Earth's oceans.

They analyzed that oxygen levels in the oceans have been diminishing, though they are not sure how much global warming has contributed to it.

The team developed a timeframe on the basis of computer simulations and said global warming could erode much of ocean's oxygen in the next couple of decades, which could pose breathing problems for most marine organisms.

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