Between 720 and 635 million years ago, snow and ice blanketed much of the land on planet Earth. This was the Cryogenian, one of our world's two hypothesized "Snowball Earth" periods. Yet out of such brutally cold and desolate conditions launched perhaps the greatest proliferation of life to play out on Earth. Immediately after the Cryogenian, during the Ediacaran period, the first definitive multicellular life arose, taking the form of worms, fronds, and disks that stretched up to meters-long. Following the Ediacaran, the Cambrian Explosion took place, in which trilobites, crustaceans, and echinoderms emerged over a period of roughly 20 million years, a relative blink of an eye in the 4.54-billion-year history of our planet.

Precisely what caused this sudden and rapid diversification of complex life has long been debated. All sorts of explanations have been put forth, from a sudden rise in oxygen, to the formation of the ozone layer, to a surge of calcium in Earth's oceans. Now, scientists from the Institute of Geographic Sciences and Natural Resources Research in China present new evidence suggesting that worldwide volcanic eruptions helped bring about the end of "Snowball Earth" and, in the process, seeded the oceans with nutrient trace elements that effectively served as fertilizer for the future rise of early complex life.

Scientists Jie Long, Shixi Zhang, and Kunli Luo surveyed volcanic rock layers in Central China dated to the Tonian, Cryogenian, and Ediacaran periods, collecting a total of 254 samples. They found that rock layers towards the end of the Cryogenian and continuing into the Ediacaran were enriched in elements like cobalt, chromium, bismuth, nickel, selenium, gallium, copper, barium, phosphorus, and cadmium compared to earlier layers. Phosphorus and cadmium stood out with concentrations 26.4 and 12 times higher. Many of these microelements are now essential to complex life.

This chart shows the rise in five elements between the Tonian and Ediacaran periods, and also describes the state of life during each time.

As the scientists wrote, this sudden wealth of elements suggests "a global magmatic and volcanic event occurred that not only guaranteed longtime-favorable and warm-water conditions for the survival of certain oceanic organisms but also carried an essential concentration of nutrient trace elements to the oceans."

Prior research from 2014 also turned up evidence of volcanic activity just before the Cambrian Explosion. Geologists from the University of Texas examined ancient rock layers and noted that zircons, crystals that form within volcanic magma, are rare during most of the Cryogenian layers but become common a hundred million years later.

The "Snowball Earth" during the Cryogenian was a harsh time during which the evolution of life sputtered and stalled. But this extreme bottleneck may very well have provided a blank slate of sorts into which life, when fertilized with a volcanic shower of trace elements, exploded to fill the copious available ecological niches.

Source: Jie Long, Shixi Zhang & Kunli Luo. "Cryogenian magmatic activity and early life evolution." Scientific Reports volume 9, Article number: 6586 (2019). https://doi.org/10.1038/s41598-019-43177-8