Early animal life burrowing into the seafloor were the cause of a global warming event half a billion years ago, new research published Monday demonstrates.
It took about 100 million years for the little animals burrowing in the sandy seabed to breathe up the oxygen in the water and poison their own environment with carbon dioxide, report the scientists in Nature Communications. But they got there.
Life began to evolve around 4 billion years ago, based on albeit controversial evidence of fossil bacteria in the few crustal rocks surviving from that primordial time. (The Earth itself is thought to have been created 4.6 billion years ago.)
The first life would have been simple cells without nuclei, that resembled today's bacteria. Then complex cells with nuclei, the eukaryotes, would have evolved, and then multi-cellular life, plant and animal.
When all these things happened is fiercely debated: fossils suspected to be multi-cellular fungi found in South Africa date to 2.4 billion years.
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We can say however that the true explosion of animal speciation happened in the Cambrian, around 550 million years ago. That evolutionary burst resulted in an extraordinary diversity of animals, arthropods with simple nervous systems but sporting eyes, legs and claws.
Scientists had long wondered what triggered this explosive Cambrian burst of life forms. Before it, the ocean had been oxygen-poor and what animals it had were likely small, sluggish and probably confined to eating bacterial mats on the seafloor.
In 2016, a paper in Nature proposed that the development of complex animals was enabled by oxygenation of the oceans. Primitive algae absorbed carbon dioxide and generated oxygen. Primordial animals living in the seabed slime could breathe easier, literally, and had the metabolic luxury of evolving advanced features like legs and predatory habits – which in turn would have sparked an evolutionary arms race.
Some of the animals of the Cambrian were not small: The fearsome Anomalocaris canadensis for instance could be a fearsome 60 centimeters long. And some early animals burrowed.
Tunnel to hell
But to the dismay of the Cambrian animals ravening at each other, that luxurious oxygenation was not to last, report Sebastiaan van de Velde of Brussels' Vrije Universiteit and colleagues from the Universities of Exeter, Leeds and Antwerp in their new paper. They suspect it was the burrowers' fault.
As the animals frisked and flayed each other in the oceans, and ate of the bacterial mats and algae, they breathed the oxygen in and breathed the carbon dioxide out, just as we do. And the carbon dioxide concentration in the air rose and rose, just like our emissions are doing.
Over about 100 million years, write the scientists, these primordial animals gradually ruined their own environment. "Ocean oxygen levels fell and carbon dioxide caused global warming," they sum up.
Why would they think the burrowers dunnit? "Like worms in a garden, tiny creatures on the seabed disturb, mix and recycle dead organic material - a process known as bioturbation," explained Prof. Tim Lenton, from the University of Exeter. If they "bioturbate" the whole ocean floor, you get critical mass.
Apparently, there was a critical mass of burrowing animals back then. Prof Filip Meysman of Antwerp University confirms that a decrease in ocean oxygen levels occurred around 520 million years ago; and geologic evidence points at rising CO2 levels.
"We knew that warming occurred at this point in Earth history, but did not realize it could be driven by animals," said Dr Benjamin Mills of Leeds University.
Carbon dioxide-driven temperatures increases on Earth may have underlain the multiple mass extinctions that followed the Cambrian explosion, and the parallel is obvious. Though what it took worms 100 million years, humans have achieved in 100, and we have done it more spectacularly. Scientists note with horror that never before have carbon dioxide concentrations in the atmosphere (and water) risen as fast as in the last 200 years. But just like our predecessors on the planet, we are poisoning it for ourselves and everyone else, and like them, we may come to an end because of it.