A new study suggests that ancient oxygen levels didn’t limit the evolution hundreds of millions of years ago of ancestors to organisms like the sea sponge shown here. Credit: Twilight Zone Expedition Team 2007, NOAA-OE

Roughly 700 million years ago, animals that need oxygen began to evolve, although simpler forms of life fueled mainly by chemicals or sunlight had already been thriving on Earth for billions of years. Given how much oxygen animals today require to walk, swim, slither, or fly, scientists have long thought that low oxygen levels on early Earth prevented the earlier proliferation of species that require oxygen. A new international study, however, suggests that sufficient oxygen levels prevailed on our planet long before the evolutionary kickoff.

“You can’t completely decouple oxygen from the story, but I would say, with at least the initial evolution of what we recognize as [early] animals, oxygen wasn’t the problem.”

The researchers, led by Donald Canfield of the University of Southern Denmark in Odense, Denmark, analyzed core samples from the Xiamaling Formation in China to investigate Proterozoic oxygen levels. From considering both trace metals and biomarkers, the researchers found that a layer of oxygenated water sat beneath anoxic waters in the Mesoproterozoic oceans 1.4 billion years ago. Because the oxygen would have cycled down from the ocean’s surface where water and atmosphere mixed, Canfield and his colleagues used ocean models to calculate that atmospheric oxygen levels were no less than 4% of what they are today.

That’s low, but not low enough to have prevented simple forms of oxygen-dependent life from evolving, Canfield said. Those would include creatures such as ancestors of today’s sponges and jellyfish. “You can’t completely decouple oxygen from the story, but I would say, with at least the initial evolution of what we recognize as [early] animals, oxygen wasn’t the problem.”

The team reported its findings on 4 January in Proceedings of the National Academy of Sciences of the United States of America.

Tracing Metals in the Primordial Oceans

The researchers picked the Xiamaling Formation because it’s extremely well preserved, so it can provide a window back into the poorly understood Mesoproterozoic period of Earth’s past, about 1–1.6 billion years ago, said Canfield. He and his team used the trace metals uranium, vanadium, and molybdenum and biomarkers within the formation’s rocks to piece together this ancient environment’s characteristics. Those metals could have come from the seawater itself or eroded from land and then entered the ocean.

In the Xiamaling Formation samples, the scientists saw enrichments of molybdenum and uranium but not of vanadium. The discrepancy suggested that there had been a layer of oxygenated water at the sea bottom sediment surface that prevented the vanadium from increasing. This finding enabled the team to estimate the water layer’s oxygen concentration, Canfield explained.

Ancient Membrane Structures Built Up the Evidence

Biomarkers also helped the researchers piece together this early environment. Canfield said they looked at a whole spectrum of biomarkers, but the one that stood out the most was green sulfur bacteria. Biomarkers are organic compounds that sit within the cell membranes of bacteria and multicellular microorganisms called eukaryotes—analogous to cholesterol in human cells, said Gordon Love, an organic biogeochemist from the University of California, Riverside (UC Riverside), who was not involved in the study. They are typically complex hydrocarbon structures of about 27 to 35 carbon atoms, and that hydrocarbon part of the molecule survives in the rock record, said Love. Biomarkers left behind by green sulfur bacteria indicated that some of the ocean water wasn’t oxygenated 1.4 billion years ago because these bacteria require unoxygenated water, said Canfield.

Oxygen Didn’t Restrict Animal Evolution

The team’s evidence leaves some geochemists unconvinced.

Knowing that water above the oxygenated layer was anoxic, and how high the likely oxygen concentration near the bottom was, allowed the researchers to use ocean models to calculate how much oxygen must have been in the atmosphere to oxygenate this deep layer of water. They could backtrack to atmospheric oxygen levels because the gas could only reach the depths by entering the water at the surface and cycling down. They estimated that the oxygen level was 4% of today’s levels, an amount that would have supported simple and early forms of life, Canfield said.

Because of that, it stands to reason that the amount of oxygen didn’t limit when animals first evolved, he added, although low oxygen may have later acted as an evolutionary brake when more complex and active animals began to emerge. Canfield speculated that the main limiting factor on animal evolution was time. It’s difficult to appreciate how much time it takes to develop cell differentiation, circulatory systems, and all of the things that go into making an animal, he said.

Questions Remain

The team’s evidence leaves some geochemists unconvinced. Using sedimentary vanadium records to track local oceanic oxygen levels can be tricky because there’s a large amount of uncertainty regarding the erosion-produced detrital vanadium levels, said Noah Planavsky, a geochemist at Yale University in New Haven, Conn., who did not take part in the study. This uncertainty in detrital vanadium isn’t unique to this study. The large levels of uncertainty make it difficult to conclude that the vanadium is actually decreasing, he said.

For UC Riverside’s Love, the biomarkers raised warning flags. He said he’s seen similar biomarker patterns from previous studies in which the biomarkers were initially thought to be endogenous to rocks but later turned out to be contamination. “I don’t think they’ve done sufficient analytical tests,” he said.

Precautions Against Contaminants

Canfield acknowledged that biomarkers can pose challenges to researchers because contaminants can enter samples at any stage in a rock’s history or during analysis. However, he and his team took multiple precautions to minimize contamination, he noted, such as using water as drilling fluid, disposing of surface contamination, examining only the interior regions of core samples, and looking at formations in different parts of the stratigraphy to see if the biomarkers were present all the way through—a sign of contamination—or occupied only specific layers, as he said was the case in this study.

Those precautions satisfy Benjamin Gilbert, a geochemist at Virginia Polytechnic Institute and State University in Blacksburg, Va., who was not part of the study. In his view, Canfield and his colleagues did all they could to limit contamination in the biomarker samples and evaluated the trace metals appropriately.

—Cody Sullivan, Writer Intern

Citation: Sullivan, C. (2016), Ancient start of animal evolution wasn’t delayed by low oxygen, Eos, 97, doi:10.1029/2016EO043301. Published on 11 January 2016.

Text © 2016. The authors. CC BY-NC 3.0
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