Take a Breath and Thank a Sponge

If Tim Lenton is right, we all owe sponges a deep debt of gratitude. It may be hard to give much credit to these simple animals, which spend their uneventful lives on the sea floor trapping floating bits of food. But Dr. Lenton, an earth systems scientist at the University of Exeter, suspects that sponges played a crucial role in the rise of the animal kingdom.

Some 700 million years ago, he and his colleagues argue, sponges re-engineered the planet. The sponges unleashed a flood of oxygen into the ocean, which before then had scarcely any oxygen at all. Without that transformation, we might not be on earth today.

“This story is about the first animals bootstrapping the environment into one where more complex animals could evolve,” said Dr. Lenton. “This is essentially the birth of the modern world.”

Dr. Lenton and his colleagues describe their hypothesis in the journal Nature Geoscience.

The researchers developed their hypothesis after growing dissatisfied with earlier explanations for the rise of animals. The most influential of these came from the Canadian biologist John Ralph Nursall in 1959. At the time, scientists had found animal fossils reaching back just over half a billion years. But they knew of microbial fossils that were billions of years older. Dr. Nursall proposed that the evolution of animals had been blocked because earth first had to accumulate enough oxygen to support them.

Dr. Nursall based his argument on the fact that living animals, generally speaking, consume a lot of oxygen to run their bodies. “If you want to be like us and have a brain, that’s quite oxygen-demanding,” said Dr. Lenton. “Good eyesight and carnivory also require a lot of oxygen.”

Today, animals can enjoy an abundance of oxygen, which makes up 21 percent of the atmosphere. But when the earth formed, the ocean and atmosphere were nearly oxygen-free.

That started to change when bacteria evolved the ability to harness sunlight to grow. As they carried out photosynthesis, they absorbed carbon dioxide and released oxygen.

But, for reasons that remain unclear,  oxygen remained very low for billions of years afterward. With so little oxygen on earth, animals could not evolve, Dr. Nursall argued. When the oxygen level finally rose high enough, that obstacle was removed.

In recent years, a number of scientists have fleshed out Dr. Nursall’s hypothesis. They have searched for the geological forces that might have raised the oxygen in the atmosphere and ocean. Some have argued that shifting continents altered the planet’s chemistry. Others have investigated the planet-wide glaciers that grew from time to time.

But recent studies have challenged the idea that animals couldn’t have evolved on a low-oxygen earth. Among the first animals to appear in the fossil record are sponges. While they lack the brains and other organs of more complex animals, they are animals nonetheless. Recently Donald E. Canfield of the University of Southern Denmark and his colleagues ran an experiment to see how little oxygen sponges needed to survive.

“We just kept reducing the oxygen levels,” said Dr. Canfield. Eventually they got to the lowest level their equipment could handle — less than 4 percent of today’s levels.

“The sponges did O.K.,” said Dr. Canfield, who reported the experiment with his colleagues last month.

Experiments like these suggest that, contrary to what Dr. Nursall proposed, early animals such as sponges could have evolved long before oxygen levels rose to more comfortable levels. Paleontologists are now finding evidence that jibes nicely with those experiments. They’re unearthing fossils and chemical traces of sponges dating back as far as 700 million years ago — long before the oceans gained much of their oxygen.

Dr. Lenton and his colleagues suspect that not only did animals, like sponges, exist at low levels of oxygen. They propose that those first animals themselves were responsible for raising the oxygen in the oceans that led to more complex forms of life.

They argue that before a billion years ago, the surface of the ocean was dominated by photosynthetic bacteria. When the bacteria died, they floated in the surface waters where predatory microbes fed on them.

The photosynthetic bacteria produced oxygen, but it was quickly consumed at the surface by the predatory microbes feeding on them. As a result, the deep ocean remained oxygen-free.

That began to change, Dr. Lenton and his colleagues argue, when algae and other larger single-celled organisms evolved. Thanks to their size, they sank when they died and escaped the oxygen-consuming scavengers. As a result, oxygen had a chance to seep down from the top of the ocean into its depths.

When sponges came on the scene, the scientists suspect they had a huge impact by acting as seawater filters. As they trapped organic matter, they lowered vital nutrients in the water, among them phosphorus. Without phosphorus, photosynthetic bacteria couldn’t grow. In the absence of those bacteria competing for sunlight, the algae thrived, which means more of the oxygen they produced made it to the deep ocean.

Sponges also made life worse for photosynthetic bacteria in another way: by eating them. Sponges have tiny pores that are good for trapping bacteria but leave larger algae unharmed.

Sponges thus created a new environment — an oxygen-rich ocean — where new kinds of animals could evolve. Animals now had enough oxygen to swim or crawl. And these new animals, in turn, increased the supply of oxygen, Dr. Lenton argued. In life, they released feces that fell quickly into the deep ocean, and in death, their bodies sank there as well.

“I love this idea, I really do,” Dr. Canfield said of Dr. Lenton’s hypothesis. He found the new paper a plausible way to explain the evidence scientists have gathered so far about the history of animals and the ocean.

Andrew H. Knoll of Harvard said it was “a very interesting approach,” although he cautioned that other researchers are developing different models that are coming to different conclusions. “There are a lot of moving parts, and many reasonable people can turn them in different directions,” he said.

Dr. Knoll expected that more data from ancient rocks and sophisticated computer simulations would help scientists determine which model does the best job at explaining the intertwined history of oxygen and animals.

The possibility that sponges cleared our evolutionary path won’t be the last surprise, he predicted. “Why else would anyone continue to be interested in the problem?” he asked.