In the search for life's origins, it's no longer a question whether the chicken or the egg came first, but whether it was oxygen or sponges.

Research led by the University of Exeter in the United Kingdom challenges the long-held scientific belief that oxygenation of the atmosphere preceded most if not all complex life.

In fact, say researchers, the planet's initial animals, sponges, oxygenated the oceans first -- which eventually led to the evolution of more sophisticated creatures.

The new findings, published in the journal Nature Geoscience, are anchored on the previous work of scientists in Denmark who found sponges -- identified as the first animals to evolve -- require only minimal amounts of oxygen to thrive.

"There had been enough oxygen in ocean surface waters for over 1.5 billion years before the first animals evolved, but the dark depths of the ocean remained devoid of oxygen," said study lead Tim Lenton, a professor at Exeter. "We argue that the evolution of the first animals could have played a key role in the widespread oxygenation of the deep oceans. This in turn may have facilitated the evolution of more complex, mobile animals."

The scientific team set out to determine what processes led to the oxygenation of the deep oceans during the Neoproterozoic Era, from about 1 billion to 542 million years ago, without any great uptick of oxygen in the atmosphere. They already understood oxygen levels in the deep ocean are determined through a balance of oxygen supply and demand and a lot of oxygen is used by dead organic material sinking to the ocean bottom.

The study argues that the earth's first animals fed on organic matter in the water, thereby reducing the demand for oxygen, which in turn promoted the development of larger types of life.

A more oxygen-rich ocean, the researchers said, became ideal for the evolution of more mobile animals, which require higher amounts of oxygen to flourish. And that ultimately led to the rise of predatory animals, with digestive systems, which started to eat one another.

"The effects we predict suggest that the first animals, far from being a passive response to rising atmospheric oxygen, were the active agents that oxygenated the ocean around 600 million years ago," Lenton continued in the release. "They created a world in which more complex animals could evolve, including our very distant ancestors."

Said Professor Simon Poulton of the University of Leeds, who co-authored of the research paper: ″This study provides a plausible mechanism for ocean oxygenation without the requirement for a rise in atmospheric oxygen. It therefore questions whether the long-standing belief that there was a major rise in atmospheric oxygen at this time is correct. We simply don't know the answer to this at present, which is ultimately key to understanding how our planet evolved to its current habitable state. Geochemists need to come up with new ways to decipher oxygen levels on the early Earth.″