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Earth’s atmosphere, ocean, and life have interacted over the past more than 500 million years in ways that improved the conditions for early organisms to flourish. Now, an interdisciplinary team of scientists has produced a perspective paper on this coevolutionary history, published in National Science Review.

“One of our tasks was to summarize the most important findings about carbon dioxide and oxygen in the atmosphere and ocean over the past 500 million years,” said Syracuse University geochemistry professor Xunli Lu, lead author of the paper. “We looked at how these physical changes affected the evolution of life in the ocean. But it’s a two-way street. The evolution of life also affected the chemical environment. Figuring out how to build a habitable Earth on long time scales is not a trivial task.”

The team from Syracuse University, Oxford University and Stanford University investigated the complex feedbacks between ancient life forms, including plants and animals, and the chemical environment in the current Phanerozoic Eon, which began approximately 540 million years ago.

At the beginning of the Phanerozoic, carbon dioxide levels in the atmosphere were high and oxygen levels were low. Such a condition would be difficult for many modern organisms to thrive in. But ocean algae changed that. They absorb carbon dioxide from the atmosphere, lock it in organic matter and produce oxygen through photosynthesis.

The ability of animals to live in ocean environments is affected by oxygen levels. Lu studies where and when ocean oxygen levels may have risen or fallen during the Phanerozoic using geochemical proxies and model simulations. Co-author Jonathan Payne, a professor of earth and planetary sciences at Stanford University, compared the estimated metabolic needs of an ancient animal to places where it survived or went extinct in the fossil record.

As photosynthetic algae remove atmospheric carbon in sedimentary rocks to reduce carbon dioxide and increase oxygen levels, the algae’s enzymes become less effective at fixing carbon. Algae therefore had to create more complex ways to carry out photosynthesis at lower levels of carbon dioxide and higher levels of oxygen, and achieved this by creating chemically controlled internal photosynthetic compartments.

“For algae, it’s changes in the ecological ratio of O₂/CO₂ which appear to be key to driving improved photosynthetic efficiency,” says co-author Rosalind Rickaby, who is professor of geology at Oxford. “What’s really intriguing is that these improvements in photosynthetic efficiency may have expanded the chemical envelope of habitability for many forms of life.”

Ancient photosynthesizers had to adapt to changes in the physical environment they themselves had created, Lu notes. “The first part of the Phanerozoic story is an increase in habitability for life, and the second part is adaptation.”

If scientists want to further understand this interaction between life and the physical environment, as well as the drivers and constraints of habitability, the authors suggest that mapping the spatial patterns of ocean oxygen, biomarkers of photosynthesis and metabolic tolerance of animals shown in fossils will be a key future research direction.