The First Animals Caused Global Warming Too

Trilobites were among the animals to emerge in the Cambrian era. These animals mixed the upper layers of the seafloor sufficiently to cause a runaway greenhouse effect and deplete the ocean of oxygen. Abrilla/Shutterstock

When future generations ask how the decision makers of today could let the planet's temperature run out of control, perhaps they'll be told “tradition”. Messing with the Earth's thermostat is apparently something animals have been doing for more than 500 million years. When the first animals sent carbon dioxide levels skyrocketing and induced an intense greenhouse effect, it took 100 million years for the world to recover.

"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 of the University of Exeter in a statement. The first creatures capable of doing this appeared around 520-540 million years ago.

Lenton was part of a team that looked for signs of this bioturbation in rocks of the appropriate age. He and his colleagues were surprised at how little the sediment of the day, as preserved in rocks, was disturbed. Only the top 1-3 centimeters showed signs of mixing. In contrast, modern sea floors are marked by the mixing of minerals and the exchange of soluble molecules with the water above as a result of animal actions to depths of tens of centimeters.

Yet around 520 million years ago, oxygen levels in the ocean plunged, as would be expected if organic material on the seabed were being recycled and changing the planet's chemistry in the process. Professor Simon Poulton of the University of Leeds summed up the team's thinking: “At first sight, these two observations did not seem to add up."

In Nature Communications, Lenton and Poulton explain the paradox by arguing that even these shallow burrowers induced reactions in enough organic material to soak up most of the oxygen in the oceans, releasing enormous amounts of carbon dioxide instead. The effect lasted 100 million years until land plants started adding to the oxygen produced in the oceans.

The extra carbon dioxide had the predictable effect of dramatically increasing global temperatures, which in turn raised sea levels. Paleoclimatologists had previously identified these higher sea heights, without tying it to animal activity.

Given the long timescales involved, the animals that triggered these changes to the Earth's chemistry probably had time to adapt to much warmer temperatures, and the rising oceans would have been no issue. The loss of oceanic oxygen was a different matter. The shortage of oxygen limited the energy the animals could expend, and therefore how deep into the mud they could burrow. They also probably triggered several mass extinction events through this period.

"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 Lenton. The nearly brainless worms, however, couldn't have seen it coming.

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