Earth’s First Animals May Have Lived in a Dead Zone

Breadcrumb sponges show how Earth’s first animals may have got by with barely any oxygen

Breadcrump sponges, Halichondria panicea, can survive with minimal oxygen. Ken-ichi Ueda

Dead zones, where oxygen is so scarce life struggles to survive, and their weaker cousin, hypoxic zones, can occur all over the world, often in the wake of massive algae blooms or in connection with agricultural runoff. They're a growing plague for marine life, but in a sense, says new research, they're also a sort of homecoming. According to a new study, the first animal life on Earth, likely either some form of comb jelly or sponge, arose in a world hopelessly deprived of oxygen.

The presiding scientific thought is that the development of animal life (rather than, say, microbes) was delayed because there just wasn't enough oxygen to go around. But in their research, this group of scientists found that modern sponges can survive in conditions with just 0.5 percent of the current oxygen concentration.

“Even with 200 times less oxygen than is currently found in the atmosphere, the sponges survived until the end of the study, 10 days after oxygen levels finished dropping,” says New Scientist. “If modern sponges can live with little oxygen, early animals probably could too.”

The rise of the sponges may have even assisted the evolution of more complex life, says New Scientist. One reason the Earth may have been so oxygen-deprived is because, just like in modern dead zones, an abundance of decaying organic matter saps oxygen from the water. If these early sponges gobbled up the waste, it could have boosted the oxygen availability to levels needed to support other types of animals. 

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