A new study led by Yale University offers insight into why it took over 2 billion years for this protective layer to stabilize. Researchers propose that a prolonged interaction between iodine and oxygen in Earth's early atmosphere delayed the formation of a stable ozone layer. This delay hindered the emergence of complex life by allowing ultraviolet radiation (UVR) to permeate the planet's surface.
The study, published in the journal Proceedings of the National Academies of Science, addresses a long-standing scientific mystery. "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 in Earth and planetary sciences and the study's first and corresponding author.
Although cyanobacteria existed as early as 2.7 billion years ago, land plants did not appear until 450 million years ago. Similarly, no fossils of complex land animals or plants have been found predating the Cambrian period (541 to 485 million years ago), despite evidence of much older microfossils.
"The only existing explanation states that this delay is an intrinsic characteristic of evolution - that an enormous amount of time is required," said Noah Planavsky, a professor at Yale's Department of Earth and Planetary Sciences and senior author of the study. "Yet that notion fails to explain how and why complex life originated and diversified."
The researchers suggest that beyond evolutionary timelines, unstable ozone levels caused by elevated marine iodine concentrations played a major role. High levels of iodine emissions disrupted the formation of the protective ozone shield, allowing harmful UVR to penetrate the atmosphere.
Ozone generation relies on atmospheric oxygen and UVR. Previously, scientists believed that once Earth's atmosphere contained sufficient oxygen, a stable ozone layer naturally formed, fostering biological evolution. The new study challenges this assumption.
"We consider how Earth's evolving iodine cycle may have influenced ozone abundance and stability," Liu explained. The team analyzed geological data and developed an ocean-atmosphere model to simulate iodine-ozone dynamics in Earth's early history. They found that elevated marine iodide levels persisted for much of Earth's history. These levels caused inorganic iodine emissions into the atmosphere, disrupting ozone formation after oxygen levels rose.
The destructive effects of iodine on ozone are comparable to the damage caused by chlorofluorocarbons (CFCs) during the "ozone hole" crisis over Antarctica. Similar to how reactive chlorine from CFCs depletes ozone, iodine-driven reactions accelerate the breakdown of ozone at a much faster rate.
"Our calculations indicate that even a moderate increase in marine inorganic iodine emission could lead to atmospheric ozone depletion tens or hundreds of times greater than modern levels," Planavsky said.
Liu noted that globally unstable and low ozone levels likely persisted from 2.4 billion years ago until about 500 million years ago. "During this period, even with high oxygen production, atmospheric ozone levels were likely low and unstable, allowing periodic or persistent surges of solar UVR at Earth's surface," Liu said.
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