Oxygen didn't trigger multicellular organisms, study claims

Researchers have made the extraordinary claim that an increase in oxygen did not catalyse the swift blossoming of Earth’s first multicellular organisms, defying a 70-year-old assumption about what caused an explosion of oceanic fauna hundreds of millions of years ago.

Between 685 and 800 million years ago, multicellular organisms began to appear in all of Earth’s oceans during what is known as the Avalon explosion, a forerunner era of the more famed Cambrian explosion. During this era, sea sponges and other bizarre multicellular organisms replaced small single-celled amoeba, algae and bacteria, which had had run of the planet for more than two billion years.

Up until now, it was believed that increased oxygen levels triggered the evolutionary arrival of more advanced marine organisms. This has now been disproved in a new study published in the journal Geobiology by University of Copenhagen researchers, working together with colleagues from the Woods Hole Oceanographic Institution, the University of Southern Denmark and Lund University among others.

By studying the chemical composition of ancient rock samples from an Omani mountain range, which was on the seabed during the Avalon explosion, the researchers have been able to ‘measure’ oxygen concentrations in the world’s oceans from when these multicellular organisms appeared. Defying expectations, the result shows that Earth’s oxygen concentrations had not increased. Indeed, levels remained 5–10 times lower than today, which is roughly how much oxygen there is at twice the height of Mount Everest.

“Our measurements provide a good picture of what average oxygen concentrations were in the world’s oceans at the time,” said Copenhagen Associate Professor Christian J Bjerrum, who has been quantifying the conditions surrounding the origin of life for the past 20 years. “And it’s apparent to us that there was no major increase in the amount of oxygen when more advanced fauna began to evolve and dominate Earth. In fact, there was somewhat of a slight decrease.”

The researchers have had their findings confirmed in fossils from three different mountain ranges around the world: Oman’s Hajar Mountains, Canada’s Mackenzie Mountains and the Three Gorges region along China’s Yangtze River. The result seems to put to rest a 70-year-old theory that advances the centrality of higher oxygen concentrations in the development of more advanced life on our planet.

“The fact that we now know, with a high degree of certainty, that oxygen didn’t control the development of life on Earth provides us with an entirely new story about how life arose and what factors controlled this success,” Bjerrum said.

“Specifically, it means that we need to rethink a lot of the things that we believed to be true from our childhood learning. And textbooks need to be revised and rewritten.”

So, if not extra oxygen, what triggered the era’s explosion of life? Perhaps the exact opposite, Bjerrum suggested.

“It’s interesting that the explosion of multicellular organisms occurs at a time with low concentrations of atmospheric and oceanic oxygen,” he said. “That indicates that organisms benefited from lower levels of oxygen and were able to develop in peace, as the water chemistry protected their stem cells naturally.”

According to Bjerrum, the same phenomenon has been studied in cancer research, in the stem cells of humans and other animals. Here, colleagues at Lund University observed that low oxygen levels are crucial for keeping stem cells under control until an organism decides that the cell ought to develop into a specific type of cell, such as a muscle cell.

“We know that animals and humans must be able to maintain low concentrations of oxygen in order to control their stem cells, and in so doing, develop slowly and sustainably,” Bjerrum said. “With too much oxygen, the cells will develop, and in the worst case, mutate wildly and perish. It is far from inconceivable that this mechanism applied back then.”

Bjerrum hopes that the new findings can spur other researchers around the world to reconsider their previous results in a new light. He concluded, “There are many research sections around the world, including in the United States and China, that have done lots of research on this topic, whose earlier results may shed important new details if interpreted on the basis that oxygen didn’t drive the development of life.”

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