What is it about?

Our new study reveals a previously unknown chapter in Earth's natural history and offers a solution to “Darwin’s dilemma” regarding the sudden appearance of complex life, including terrestrial plants and animals, in the early Phanerozoic. For life on land, we suggest that the delayed stabilization of Earth’s ozone layer—linked to elevated marine iodide concentrations—prevented an essential protective UV shield from forming for approximately two billion years until the early Phanerozoic. The destruction of atmospheric ozone today is inherently linked to the cycling of marine and atmospheric iodine. At the molecular level, iodine is approximately 1000 times more efficient at destroying ozone than chlorine derived from the CFCs, the latter of which is responsible for the well-known "ozone hole" over Antarctica. Supported by multiple independent lines of geological evidence and independently validated through an iodine mass balance model, we find that elevated marine iodide content prevailed through most of Earth’s history. Since the rise of oxygen 2.4 billion years ago, high marine iodide concentrations would have led to significant inorganic iodine emissions to the atmosphere, facilitating catalytic ozone destruction and resulting in atmospheric ozone instability with periodic or persistently lower ozone levels. Even under high levels of oxygen production, atmospheric ozone could have been very low—potentially as little as 0.1% of modern levels—and was likely unstable. At a global scale, this unstable and low ozone levels likely persisted until ~500 million years ago, exposing Earth’s surface to elevated fluxes of solar UVR for nearly two billion years.

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Why is it important?

The origin and diversification of complex life on Earth remains one of the most profound and enduring questions in natural science. Meanwhile in "On the Origins of Species", Darwin expressed deep concern over the absence of progenitor fossils for plants and animals predating the Cambrian strata, a challenge now known as “Darwin’s dilemma”. Our research demonstrates that the long-term chemical evolution of the oceans would have driven major changes in the atmospheric chemistry, ultimately enabling the emergence of complex life on land. This progression helps explain why life remained largely confined to the ocean for billions of years and provides a compelling solution to "Darwin’s dilemma".

Perspectives

Solar UVR is detrimental to biomolecules such as lipids, proteins and nucleic acids, even short exposure to a high dose of solar UVR can be detrimental. Carl Sagan has even suggested that the deleterious effects from high solar UVR would cause natural selection itself to become inoperative, as surface organisms would mutate too fast to establish a selected characteristic within the population. While microbial life may have relied on additional UV shields such as water to survive high levels of solar UVR, the evolution of complex life on land required an effective ozone layer to mitigate high UVR exposure.

Jingjun Liu
Yale University

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This page is a summary of: Evolution of the iodine cycle and the late stabilization of the Earth’s ozone layer, Proceedings of the National Academy of Sciences, January 2025, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2412898121.
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