If you like the smell of spring roses, the sounds of summer birdsong and the colors of fall foliage, you have the stabilization of the ozone layer to thank for it. Located in the stratosphere where it shields the Earth from harmful ultraviolet radiation, the ozone layer plays a key role in preserving the planet’s biodiversity.

Now, we may have a better idea of why that took more than 2 billion years to happen.

Michigan State University researcher Dalton Hardisty contributed to a new Yale University-led study finding that Earth’s early atmosphere hosted a battle royal between iodine and oxygen. This war effectively delayed the creation of a stable ozone layer that would shield complex life from much of the sun’s ultraviolet radiation, or UVR.

The new theory, described in a study in the journal may solve a mystery that has puzzled scientists for hundreds of years.

“This was a fun interdisciplinary team that really challenged what we thought we knew about the establishment of Earth’s atmospheric ozone layer, which has wide-ranging implications,” said Hardisty, assistant professor in the College of Natural Science’s Department of Earth and Environmental Sciences. “A major contribution from MSU includes my research group’s work constraining the geochemical cycling of iodine in seawater today and across Earth history. Marine iodine cycling had not been previously considered in ancient ozone models despite its known importance for ozone chemistry today. Our work at MSU was an important context for bridging this gap.”

Indeed, scientists have long wondered why land plants did not emerge on Earth until 450 million years ago, even though their progenitors, cyanobacteria, had been in existence for 2.7 billion years. Likewise, there are no fossils for complex land animals or plants before the Cambrian era (541 to 485 million years ago) despite the evidence of much older microfossils.

“The origin and diversification of complex life on Earth remains one of the most profound and enduring questions in natural science,” said Jingjun Liu, a Yale doctoral student and first and corresponding author of the study.

The new study suggests that something beyond the need for time was responsible: the delayed stabilization of Earth’s ozone layer, caused by elevated marine iodine concentrations that prevented a protective UVR shield from forming in the atmosphere.

Ozone production depends on atmospheric oxygen and background UVR. It has been widely accepted by scientists that once Earth established a substantial concentration of atmospheric oxygen, the planet formed an ozone layer that allowed for biological evolution to proceed unimpeded.

“We challenge this paradigm by considering how Earth’s evolving iodine cycle may have influenced ozone abundance and stability,” Liu said.

For the study, Hardisty collaborated with a research team that analyzed multiple lines of independent geological evidence and developed an ocean-atmosphere model to reconstruct the iodine-ozone dynamics for the early Earth. The researchers found that elevated marine iodide content compared to today’s levels prevailed through most of Earth’s history, which would have led to significant inorganic iodine emissions into the atmosphere after the rise of oxygen — with the potential for disrupting ozone.

The mechanism of ozone destruction by iodine is similar to the process by which chlorofluorocarbons, or CFCs, created the “ozone hole” over Antarctica. When CFCs undergo photolysis, they release reactive chlorine, which catalytically destroys ozone in the stratosphere — and that’s what led to as much as a 50% depletion in the ozone layer over continental Antarctica at the peak of the problem.

Liu noted that at a global scale, unstable and low ozone levels likely persisted from 2.4 billion years ago until roughly half a billion years ago.