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 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 today (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, is a co-author of the paper.
Source: UC Riverside