Global warming caused by evolution of Earth's first animals over 500m years ago, scientists claim

Breaking down of organic material on sea floor led to more carbon dioxide and less oxygen in atmosphere

Four of Europe's leading universities have joined forces to uncover the source of global warming and found that it was first caused by the evolution of Earth's first animals more than 500 million years ago.

Published in the science journal Nature Communications, the research comes from the Universities of Exeter, Leeds, Antwerp, and the Vrije Universiteit Brussel.

It suggests that between 520 and 540 million years ago, animal life evolved in the ocean and began breaking down organic material on the seafloor, leading to more carbon dioxide and less oxygen in the atmosphere.

So, in the 100 million years that followed, conditions for these earliest animals became much more harsh, as ocean oxygen levels fell and carbon dioxide caused 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 Professor Tim Lenton, from the University of Exeter.

"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."

The scientists said that while they saw a decrease in oxygen levels in the ocean around 520 million years ago, the evidence from the rock record showed sediment was only a little disturbed.

Professor Simon Poulton, from the University of Leeds, explained: "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."

They then found "the missing piece of the puzzle" when they realised that the biggest changes happened at the lowest levels of animal activity, meaning the first bioturbators had a "massive impact".

Finding this out meant they could construct a mathematical model of Earth around that time to look to the changes caused by these early life forms.

"When we ran our model, we were surprised by what we saw," said Dr Benjamin Mills, also from the University of Leeds, who led this part of the research.

"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 realise it could be driven by animals."

They came to the conclusion that it was this process that made conditions worse for these animals, possibly contributing to a number of mass extinction events 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," added Professor Lenton, director of Exeter's new Global Systems Institute.

"We are creating a hotter world with expanding ocean anoxia (oxygen deficiency) which is bad for us and a lot of other creatures we share the planet with."