For around 4 billion years, life chugged along as single-celled microbes dominated the oceans. Then at some point roughly 541 million years ago, something extraordinary happened. The oceans suddenly filled with complex multicellular organisms as the Cambrian explosion got into full swing.
The fact that it took such a long time for this diversification to occur, not to mention that it happened so rapidly, has always been a bit of a puzzle. The general consensus has been that the Cambrian explosion was triggered by a rise in atmospheric oxygen, but the evidence for this has been a bit shaky.
“A heated hunt for the geochemical evidence that oxygen increased when animals diversified goes on but, after decades of discussion, it seems worthwhile to consider the development of multicellularity also from other angles,” explains Emma Hammarlund, lead author of the study published in Nature Ecology & Evolution, in a statement.
To do this, Hammarlund turned to an unconventional branch of science for a geobiologist: oncology.
You see, tumors often start off as a single mutated cell that then grows and develops into a multicellular mass of tissue, during which they have to deal with low-oxygen concentrations, known as hypoxia. The team, along with some tumor biologists, decided to look into whether the same molecular tools used by tumors to grow in these environments might also be relevant to the development of multicellularity during the Cambrian explosion.
“Hypoxia is generally seen as a threat, but we forget that oxygen shortage in precise periods and settings also is a prerequisite for multicellular life,” says tumor biologist Professor Sven Påhlman. “Our stem cells are the ones that form new tissue, and they are extremely sensitive to oxygen. The stem cells therefore have various systems for dealing with the effects of both oxygen and oxygen shortage, which is clear in the case of tumors."
Paradoxically, stem cells actually need low-oxygen environments to maintain their ability to self-renew and differentiate. This means that if life did diversify because oxygen concentrations increased as is usually thought, it likely would have impacted the ability of these early organisms to maintain their stem cells and they would have matured too early.
The authors of this latest study therefore argue that the increase in atmospheric oxygen during the Cambrian can’t have been what caused the sudden explosion of life. They instead suggest that it resulted from a revolutionary change within the animals’ own biology, rather than being driven by environmental factors.
This is supported by the fact that there were earlier times in the history of Earth that oxygen levels were "enough" and yet animals did not diversify.