Around 540 million years ago, the first burrowing animals, worms, evolved and started to dig into the floor of the oceans. In so doing they stirred up the sediments in a process known as bioturbation affecting the ocean's phosphorus cycle and altering the amount of oxygen in the Earth's atmosphere. That's the conclusion of researchers at the University of Southern Denmark. “Our research is an attempt to place the spread of animal life in the context of wider biogeochemical cycles, and we conclude that animal activity had a decreasing impact on the global oxygen reservoir and introduced a stabilizing effect on the connection between the oxygen and phosphorus cycles," explains team member Richard Boyle. The team linked data from the fossil record to well-established connections between the phosphorus and oxygen cycles in a computer model to draw their conclusion.