Earth’s earliest animals triggered a global warming event

A new study led by the University of Exeter has found that the first animals on Earth caused a global warming event by making small changes to the seafloor.

Animal life evolved in the ocean around 520 million years ago. At this time, the animals began breaking down organic material on the ocean floor, releasing carbon dioxide and diminishing levels of oxygen.

The conditions on the planet became much harsher over the next hundred million years as ocean oxygen levels continued to drop and carbon dioxide triggered global warming.

“Like worms in a garden, tiny creatures on the seabed disturb, mix and recycle dead organic material – a process known as bioturbation,” said study co-author Professor Tim Lenton.

“Because the effect of animals burrowing is so big, you would expect to see big changes in the environment when the whole ocean floor changes from an undisturbed state to a bioturbated state.”

Professor Filip Meysman from the University of Antwerp said: “We did indeed see a decrease in oxygen levels in the ocean around 520 million years ago. But evidence from the rock record showed sediment was only a little disturbed.”

According to Professor Simon Poulton, this meant that the animals living in the seafloor at that time were not very active, and did not move very deep into the seabed.

“At first sight, these two observations did not seem to add up,” he said.

Study lead author Dr. Sebastiaan van de Velde explained, “The critical factor was to realize that the biggest changes happen at the lowest levels of animal activity. This meant that the first bioturbators had a massive impact.”

The researchers said that this realization was the “missing piece of the puzzle” which enabled them to construct a mathematical model of Earth at the time of the earliest life forms and examine the changes that they made to the planet.

“When we ran our model, we were surprised by what we saw,” said Dr. Benjamin Mills. “The evolution of these small animals did indeed decrease the oxygen in the ocean and atmosphere, but also increased atmospheric carbon dioxide levels to such an extent that it caused a global warming event.”

“We knew that warming occurred at this point in Earth history, but did not realize it could be driven by animals.”

Global warming made conditions far worse for the animals, and possibly contributed to several mass extinction events that occurred during the first 100 million years of animal evolution.

“There is an interesting parallel between the earliest animals changing their world in a way that was bad for them, and what we human animals are doing to the planet now,” said Professor Lenton.

“We are creating a hotter world with expanding ocean anoxia which is bad for us and a lot of other creatures we share the planet with.”

The study is published in the journal Nature Communications.

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By Chrissy Sexton, Earth.com Staff Writer