Today, oxygen fuels much of life on Earth, but it wasn’t always that way. Three billion years ago, this gas was scarce in the atmosphere and oceans. Knowing why oxygen became plentiful could illuminate the evolution of our planet’s flora and fauna, but scientists have struggled to find an explanation satisfying to all. Now a research team has proposed a novel link between how fast our planet spun on its axis, which defines the length of a day, and the ancient production of additional oxygen. Their modeling of Earth’s early days, which incorporates evidence from microbial mats coating the bottom of a shallow, sunlit sinkhole in Lake Huron, produced a surprising conclusion: as Earth’s spin slowed, the resulting longer days could have triggered more photosynthesis from similar mats, allowing oxygen to build up in ancient seas and diffuse up into the atmosphere.
That proposal, described today in Nature Geoscience, has intrigued some scientists. “The rise of oxygen [on Earth] is easily the most substantial environmental change in the history of our planet,” says Woodward Fischer, a geobiologist at the California Institute of Technology who was not involved with the work. This study offers “a totally new flavor of an idea. It’s making a connection that people haven’t made before.”
Earth was much different when life first took hold about 4 billion years ago, with vast shallow seas whose only living creatures were one-celled. Many of those early microbes were cyanobacteria, which can form mats on sediments and rock surfaces and today sometimes cause algal blooms deadly to fish and other aquatic animals. Microbes that became cyanobacteria evolved the molecular machinery for photosynthesis early on, letting them convert carbon dioxide and water into sugars and oxygen. Researchers have long thought these microbes provided Earth’s initial supply of oxygen, over the eons creating an environment that favored the evolution of aerobic life in all its forms. But they always puzzled over why about a billion years passed between the first photosynthetic microbes, which fossils indicate arose about 3.5 billion years ago, and the first good geological evidence for a buildup of oxygen.
Researchers already knew, from modeling the Moon’s distance from Earth and the resulting atmospheric and oceanic tides, that the infant Earth turned much faster on its axis than it does today. Many agree that 4.5 billion years ago, a day was only about 6 hours long. By about 2.4 billion years ago, the models predict, the pull of the Moon had slowed that spin to about a 21-hour day. Earth’s rotational speed then stayed constant for about a billion years, as its gravitational pull countered the Moon’s drag. Those forces fell out of balance about 700 million years ago, because the resonance cycle between Earth and the Moon is not completely stable, and the planet’s spin slowed to its current speed, creating a 24-hour day, according to the models
In 2016, after a chance suggestion, Judith Klatt, a biogeochemist now at the Max Planck Institute for Marine Microbiology, realized those slowdowns in Earth’s rate of spin mirrored big leaps in atmospheric oxygen. For example, oxygen first jumped during what’s called the Great Oxygenation Event, some 2.4 billion years ago, and then again during the Neoproterozoic era, more than a billion years later. During the Paleozoic, about 400 million years ago, there was a final major increase in atmospheric oxygen.
As a postdoc at the University of Michigan, Ann Arbor, Klatt had studied microbial mats growing on sediments in the Middle Island Sinkhole in Lake Huron. There, the water is shallow enough for the cyanobacteria to get enough sunlight for photosynthesis. Oxygen-depleted water and sulfur gas bubble up from the lake floor, creating anoxic conditions that roughly approximate conditions of early Earth.
Scuba divers collected samples of the microbial mats and in the lab, Klatt tracked the amount of oxygen they released under various day lengths simulated with halogen lamps. The longer the exposure to light, the more of the gas the mats released.
Excited, Klatt and Arjun Chennu, a modeler from the Leibniz Center for Tropical Marine Research, set up a numerical model to calculate how much oxygen ancient cyanobacteria could have produced on a global scale. When the microbial mat results and other data were plugged into this computer program, it revealed a key interaction between light exposure and the microbial mats.
Typically, microbial mats “breathe” in almost as much oxygen at night as they produce during the day. But as Earth’s spin slowed, the additional continuous hours of daylight allowed the simulated mats to build up a surplus, releasing oxygen into the water. As a result, atmospheric oxygen tracked estimated day length over the eons: Both rose in a stepped fashion with a long plateau.
This “elegant” idea helps explain why oxygen didn’t build up in the atmosphere as soon as cyanobacteria appeared on the scene 3.5 billion years ago, says Timothy Lyons, a biogeochemist at the University of California, Riverside. Because day length was still so short back then, oxygen in the mats never had a chance to build up enough to diffuse out. “Long daytimes simply allow more oxygen to escape to the overlying waters and eventually the atmosphere,” Lyons says.
Still, Lyons and others say, many factors likely contributed to the rise in oxygen. For example, Fischer suspects free-floating cyanobacteria, not just those in rock-affixed mats, were big players. Benjamin Mills, an Earth system modeler at the University of Leeds, thinks the release of oxygen-binding minerals by ancient volcanoes likely countered the early buildup of the gas at times and should be factored into oxygen calculations.
Nonetheless, changing day length “is something that should be considered in more detail,” he says. “I’ll try to add it to our Earth system models.”