Discovery Challenges Theory Behind Reason for Cambrian Explosion

Ample oxygen saturated Earth's atmosphere prior to the sudden eruption in the development of life during the Cambrian explosion some 542 million years ago, according to a new study published in the Proceedings of the National Academy of Sciences. 

Led by an international team of researchers, the report reveals that the oxygen content was roughly the same 2.1 billion years ago as 500 million years ago. The finding challenges a long-held theory that the Cambrian explosion was in part triggered by a sudden uptick in the element required by all higher organisms.

"We have examined rocks that are 2.15 billion - 2.08 billion years old," co-author Emma Hammarlund, a researcher from the Nordic Center for Earth Evolution at the University of Southern Denmark, said in a statement. "They show us that there was oxygen in deep water and thus also in the atmosphere at that time. We cannot say exactly how much, but there was probably ample oxygen and also ample time to permit advanced life to evolve."

Previously, the team discovered a series of unique fossils from the same site they suggest represents evidence of a specimen that attempted to evolve into a multicellular life form.

"It was not a life form that in any way is comparable to large life as we know it today," she said. "It was rather microbes that experimented with a way to evolve into some form of multicellular existence. It had enough oxygen for the experiment, but its destiny is unknown."

One possible reason for the apparent eruption of life 540 million years ago, the scientists suggest, is simply an absence of fossils from previous life forms. An organism that lacked bones or a shell would be unlikely to leave any kind of trace behind 2 billion years later, they point out.

Not only does the discovery potentially rewrite the history of evolution, it also offers a new narrative regarding Earth's development. Atmospheric oxygen, it shows, has fluctuated several times throughout history, rising to 25 percent between 250-300 million years ago, up from 21 percent today and more than double the estimated 10 percent of the Cambrian explosion.

Still, Hammarlund doesn't worry about the world hitting another super low point.

"Not even if we let all organic material rot at the same time would the decay process use all the atmospheric oxygen," she said. "Most of it would still remain. Perhaps some large external disaster could remove all the oxygen from Earth's atmosphere, but I cannot see what that could be."