“Dead Planets” –With Low Oxygen May Actually Be Teeming with Life

Posted on Sep 10, 2018

“Understanding how Earth regulates climate both in the modern era but also in the distant past is critical for our understanding of planetary habitability,” said Noah Planavsky, a biogeochemist at Yale University. “This will help guide our search for life beyond our solar system and is an example of how the evolution of complex life fundamentally changed our planet.”

New research by a UC Riverside-led astrobiology team suggests oxygen conditions in the early surface ocean were low and unstable for most of the history of Earth and possibly delayed the emergence of animals for hundreds of millions of years,

The fact that microbial life flourished amid such low oxygen levels presents a problem for scientists hunting for extraterrestrial life, reports Lucas Joel in Scientific American. The presence of the gas in the atmosphere of a planet is considered a telltale sign that it could harbor life, explains Planavsky, a co-author of a new study, published in July in the Proceedings of the National Academy of Sciences USA.

But if environments with extremely low oxygen concentrations can still support life, space telescopes designed to detect an abundance of the gas may never find such life. “Even [if such planets are] teeming with complex life, they may appear—from a remote detectability point of view—as dead planets,” Planavsky says.

Planavsky and his team tested rocks for concentrations of the element cerium, which serves as a proxy for ancient oxygen levels 1.7 billion years ago . Oxygen binds to cerium in seawater and removes it, leaving less cerium behind to be deposited in sedimentary rock. The measured cerium levels correspond to oxygen concentrations of about 0.1 percent of present atmospheric levels, the team reported. Oxygen binds to cerium in seawater and removes it, leaving less cerium behind to be deposited in sedimentary rock. The measured cerium levels correspond to oxygen concentrations of about 0.1 percent of present atmospheric levels.

This data, Planavsky says, could help guide the construction of the next generation of telescopes designed to hunt for life on other worlds. But current technology, such as NASA’s James Webb Space Telescope (JWST) is not capable of detecting oxygen at such low concentrations, says Edward Schwieterman, an astrobiologist at the University of California, Riverside, who was not involved in the work.

Records of the earliest animals on Earth extend to roughly 700 to 800 million years ago, despite much older traces of microbial life in rocks deposited 3.7 billion years ago. Multiple factors might have contributed to this delay — but not with equal effect and in ways that are often not well understood.

A new study led by former University of California, Riverside graduate student Chris Reinhard, now an assistant professor at Georgia Tech University and member of the UC Riverside-based Alternative Earths Team of the NASA Astrobiology Institute, may have found the critical control.

Reinhard and coauthors — including Timothy Lyons, UC Riverside distinguished professor of biogeochemistry and leader of the Alternative Earths Team, and his graduate student Stephanie Olson — suggest in a paper published July 25 in the Proceedings of the National Academy of Sciences that oxygen in the surface ocean was either absent or highly variable for most of Earth history, and this combination likely disfavored the emergence of animals.

The paper, called “Earth’s oxygen cycle and the evolution of animal life,” further asserts that at about 700 to 800 million years ago, an increase in oxygen in the oceans was likely the principal trigger for the earliest evolution of animals. Understanding the significance of this result requires some context, Lyons said.

“As animals eventually became large and mobile and developed ecologies that included predator-prey relationships 500 to 600 million years ago the metabolic demands for energy rose — hand in hand with increasing needs for oxygen,” Lyons said.

There is widespread agreement on that front. But the earliest animals were small, immobile, and likely able to survive on very low levels of O2. “This resilience of our earliest ancestors has sparked a spirited debate about the importance of oxygen during the earliest chapters of animal evolution,” Lyons said.

Recent related studies published in the journals Science and Geology by the Alternative Earths Team suggest that oxygen in the atmosphere may have been very low during the billion-year interval leading up to the beginnings of animal life. At the same time, however, concentrations in the critical surface ocean, where the animals lived, could in theory have been high enough, particularly given that oxygen was produced in these shallow marine settings through the activity of photosynthetic bacteria. These details are essential fuel in the oxygen debate.

Now, in the light of the new paper, a clearer story has emerged, and it may reconcile all the previous observations.
“Our newest results based on numerical model simulations of the shallow ocean indicate that oxygen, although present, remained mostly low enough and sufficiently patchy in its distributions to make conditions inhospitable even for the earliest, low-oxygen-tolerant animals,” Olson said.

The model reveals that the highest oxygen levels occurred mostly along the margins of continents, and, most importantly, conditions in those often disconnected coastal environments varied dramatically on time scales as short as the seasons.

Olson added: “It was the overall instability of oxygen in the surface ocean that held animal evolution at bay prior to the major increase in oxygen availability about 700 to 800 million years ago.”

Lyons summarizes by noting: “The animal-oxygen debate is a bit like a tennis match that lasts for years. Recently, we provided evidence for low oxygen in the biosphere prior to the rise of animals, and those on the other side of the net countered by highlighting the likelihood of low oxygen requirements at the beginnings of animal life, suggesting that oxygen availability was not the critical factor.”

But now the Alternative Earths team has sent the ball back over the net. And if their newest results are correct, even our earliest, low-oxygen kin would have suffered in a world still very far removed from the one we enjoy today. Only when oxygen rose and stabilized did complex life find a clear path forward.

In addition to Lyons, Olson and Reinhard, Douglas Erwin of the Smithsonian National Museum of Natural History and Noah Planavsky of Yale University are co-authors of the paper.

The Daily Galaxy via Yale University
UC Riverside and Scientific American

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