You don’t need oxygen when you’re a Bread-crumb Sponge (Image: D P Wilson/FLPA)

The most primitive animals may have thrived in water that contained almost no oxygen. The finding suggests that the rise of animals could have created our modern, oxygen-rich oceans, rather than oxygen-rich oceans triggering the rise of animals.

Sponges similar to the world’s first animals can survive in water containing extremely low levels of oxygen. The finding challenges the standard view that the evolution of animals was delayed by a lack of sufficient oxygen for them to breathe – and fits with a theory that the first animals may have helped raise oxygen levels.

Daniel Mills of the University of Southern Denmark in Odense, and his colleagues, collected breadcrumb sponges (Halichondria panicea) from oxygenated waters in a Danish fjord. They kept the sponges in an aquarium and gradually removed the oxygen. 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. If modern sponges can live with little oxygen, early animals probably could too.


“There are still many researchers who contend that animals could not have arisen until oxygen levels became relatively high,” says Mills. “Our results challenge that.”

Sponges vs jellies

Mills says his results support a radical alternative reading of events. One reason the early oceans were poor in oxygen may have been because they were full of dead microbial matter, which consumes oxygen as it rots (Geobiology, doi.org/cn5g8s). Some geologists now think early animals like sponges fed on this dead matter, helping to clear the water of it. This allowed oxygen levels to rise and triggered the evolution of more complex animals that need more oxygen.

The results also fit with the genetic evidence, says William Martin at the University of Düsseldorf, Germany. Although animal fossils appear only about 600 million years ago, around when the oceans became fully oxygenated, animal DNA today is so genetically diverse that the first animals must have evolved at least 100 million years earlier, when there was a lot less oxygen.

What’s more, the mitochondria in cells that generate energy can, in many simple animals, function without oxygen for substantial periods of time. Studying how the sponges’ mitochondria behave under low-oxygen conditions could reveal how they survive,” says Martin.

The results are elegant, says Antonis Rokas at Vanderbilt University in Nashville, Tennessee. But it is not clear that sponges were the earliest animals. A study late last year suggested a group called the comb jellies appeared before sponges (Science, doi.org/qgp). “The relationships are devilishly difficult to decipher,” says Rokas. He wants to know if comb jellies can also survive in low-oxygen conditions.

It is perfectly possible that sponges came before, and helped bring about, fully oxygenated oceans, says Timothy Lyons at the University of California, Riverside, who studies the variation in oxygen levels on early Earth. But this doesn’t mean that oceans needed animals in order to get oxygen. “We know, for example, that oxygen was likely very high a bit more than 2 billion years ago – but there were no animals,” he says. “Evolution wasn’t ready.”

Journal reference: PNAS, DOI: 10.1073/pnas.1400547111