If you appreciate the fragrance of spring roses, the melody of summer birds, and the vibrant hues of autumn leaves, it’s due to the stabilization of the ozone layer. This layer, found in the stratosphere, protects our planet from harmful ultraviolet (UV) radiation and is crucial for maintaining Earth’s biodiversity. Recent findings may shed light on why it took over 2 billion years for a stable ozone layer to form.
If you appreciate the fragrance of spring roses, the melody of summer birds, and the vibrant hues of autumn leaves, it’s the stabilization of the ozone layer that you have to thank. This layer, situated in the stratosphere, shields our planet from harmful ultraviolet radiation and plays an essential role in maintaining Earth’s biodiversity.
Now, researchers have a clearer understanding of the lengthy process that spanned over 2 billion years.
A new study, led by Yale, reveals that Earth’s early atmosphere was the scene of a competitive struggle between iodine and oxygen, delaying the formation of a stable ozone layer that could protect complex life from much of the sun’s ultraviolet radiation (UVR).
This fresh hypothesis, detailed in the journal Proceedings of the National Academies of Sciences, could explain a longstanding mystery that has intrigued scientists for centuries.
“The origin and diversification of complex life on Earth is one of the most profound and enduring questions in natural science,” remarked Jingjun Liu, a PhD student in Earth and planetary sciences at Yale, who is the primary and corresponding author of the study.
Scientists have been puzzled as to why land plants did not appear on Earth until around 450 million years ago, despite the presence of their ancestors, cyanobacteria, for 2.7 billion years. Similarly, fossils of complex land animals and plants are absent before the Cambrian period (541 to 485 million years ago), even though much older microfossils exist.
Noah Planavsky, a professor of Earth and planetary sciences and senior author of the study, stated, “The only existing explanation suggests the delay is an inherent part of evolution—requiring an immense amount of time. However, this idea does not clarify how or why complex life began and diversified.”
The new research indicates that factors beyond mere time were at play: the delayed stabilization of Earth’s ozone layer, influenced by high levels of marine iodine, hindered the formation of a protective UVR shield in the atmosphere.
The formation of ozone relies on atmospheric oxygen and background UVR. Scientists have generally accepted that once Earth achieved a significant level of atmospheric oxygen, it created an ozone layer that enabled biological evolution to flourish.
“We challenge this conventional understanding by exploring how the changing iodine cycle on Earth may have impacted the abundance and stability of ozone,” Liu explained.
For this research, the Yale team examined various geological evidence and created a model to simulate iodine-ozone interactions in the early Earth environment. They discovered that elevated levels of marine iodide (which occurs when iodine combines with another element to form a salt) were prevalent throughout most of Earth’s history. This would have led to considerable releases of inorganic iodine into the atmosphere following the increase of oxygen, potentially disrupting ozone formation.
The way iodine destroys ozone parallels the mechanism that caused the “ozone hole” over Antarctica due to chlorofluorocarbons (CFCs). When CFCs break down through photolysis, they emit reactive chlorine that catalytically breaks down ozone in the stratosphere, leading to substantial depletion over Antarctica at its peak.
“Iodine-driven catalytic cycles for ozone destruction function similarly and operate much faster than those involving reactive chlorine,” noted Planavsky. “Our photochemical models suggest that even a moderate rise in marine inorganic iodine emissions could cause dramatic loss of ozone across the atmosphere, potentially many times more than current levels.”
Liu added that, on a global scale, unstable and low ozone levels likely continued from 2.4 billion years ago until about 500 million years ago. “During this time, even with high oxygen production, atmospheric ozone could have been very minimal and unstable, resulting in periodic or persistent high levels of solar UVR reaching Earth’s surface,” Liu stated.
Co-authors of the study include Dalton Hardisty from Michigan State University, James Kasting from Pennsylvania State University, and Mojtaba Fakhraee from Yale.
