New findings suggest that an explosion of animals 540 million years ago may have stemmed from huge rises in oxygen levels.

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SYDNEY: The sudden appearance of large animals more than 500 million years ago may be due to a huge increase of oxygen in the world's oceans, according to Canadian researchers.

The mystifying, abrupt appearance of large animals, known as 'Darwin's Dilemma', is a problem that has perplexed scientists right back to Charles Darwin himself, said the study's lead author, Guy Narbonne, a palaeontologist at Queen's University in Ontario.

Geochemical studies have determined the oxygen levels in the world's oceans at the time ancient sediments at Avalon, on the coast of Newfoundland, were laid down.

"Our studies show that the oldest sediments on the Avalon Peninsula, which completely lack animal fossils, were deposited during a time when there was little or no free oxygen in the world's oceans," says Narbonne. "Immediately after this ice age there is evidence for a huge increase in atmospheric oxygen to at least 15 per cent of modern levels, and these sediments also contain evidence of the oldest large animal fossils."

The research analysed sediments found in 2002 by Narbonne and his research team that contained fossilised 575 million-year-old complex life forms sandwiched between layers of sandstone. Their find pushed back the age of Earth's earliest known complex life to soon after the melting of the "snowball" glaciers of the massive Gaskiers Glaciation 580 million years ago.

In a paper published online in Science Express, Narbonne's team argued that the huge increase in oxygen following the Gaskiers Glaciation corresponds with the first appearance of large animal fossils on the Avalon Peninsula in Newfoundland.

The findings shed light on why, after three billion years of mostly single-celled evolution, these large animals suddenly appeared in the fossil record.

Geochemical studies by the team included measurements of iron and sulphur to determine the oxygen levels in the world's oceans at the time these sediments accumulated in Avalon.

The close connection between the first appearance of oxygenated conditions in the world's oceans and the first appearance of large animal fossils confirms the importance of oxygen as a trigger for the early evolution of animals, the researchers said.

They suggested that melting glaciers increased the amount of nutrients in the ocean and led to a proliferation of single-celled organisms that liberated oxygen through photosynthesis. This began an evolutionary radiation that led to complex communities of filter-feeding animals, then complex mobile animals, and ultimately to the Cambrian 'explosion' of skeletal animals 542 million years ago.