Sea Sponges May Have Led Way to Ocean Oxygenation and Animal Evolution

The first animals to live on Earth may have oxygenated the planet's oceans, according to a group of scientists, who present a view contrary to the traditional belief that a rise in ocean oxygen is what led the way to the first animals.

Writing in the journal Nature Geoscience, the research team, led by scientists at the University of Exeter, expands upon earlier research that found that sea sponges, the first animals to evolve, only required small amounts of oxygen.

"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 lead study author Tim Lenton. "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."

Lenton and his collaborators considered ways the deep ocean could have been oxygenated during the Neoproterozoic Era (from 1 billion to 542 million years ago) without requiring an increase in atmospheric oxygen.

The supply and demand of oxygen in the deep ocean is influenced by the sinking of dead organic material, the more dead organic matter that sinks to the depths, the more oxygen is demanded. The researchers contend that the first animals were able to reduce the supply of organic matter both directly and indirectly.

Sponges are a key to this theory, as the creatures act as filters. They feed by pumping water through their bodies and collecting tiny particles of organic matter from the water. This feeding process helps oxygenate the areas where the sponges live, but it also reduces oxygen demand because the sponges, quick to soak up anything that comes their way, made quick work of the fast-sinking phytoplankton that hit their filters first. This natural selection process reduced oxygen demand, the researchers suggest.

"By oxygenating more of the bottom waters of shelf seas, the first filter-feeding animals inadvertently increased the removal of the essential nutrient phosphorus in the ocean," the researchers said in a statement. "This in turn reduced the productivity of the whole ocean ecosystem, suppressing oxygen demand and thus oxygenating the deep ocean."

An oxygen-rich ocean created an ideal living situation for animal evolution. Eventually the first predatory animals with guts evolved and began eating one another, setting the state for the marine food web.

"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. They created a world in which more complex animals could evolve, including our very distant ancestors," Lenton said.

"This study provides a plausible mechanism for ocean oxygenation without the requirement for a rise in atmospheric oxygen," said study co-author Simon Poulton of the University of Leeds. "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."