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.