Ann Arbor, Michigan — Earth’s rotation has been gradually decelerating since its formation approximately 4.5 billion years ago, resulting in longer days that have profound implications for life on the planet. While this change occurs at a rate imperceptible in human lifetimes, it has influenced significant biological and atmospheric changes over millions of years, including a critical increase in oxygen levels linked to the emergence of blue-green algae, known as cyanobacteria.
Cyanobacteria, which proliferated around 2.4 billion years ago, played a pivotal role in the Great Oxidation Event—a period marked by a dramatic rise in Earth’s atmospheric oxygen. Microbiologist Gregory Dick from the University of Michigan emphasizes the longstanding scientific curiosity surrounding the origins of atmospheric oxygen and the factors that orchestrated its increase. His research suggests that the gradual lengthening of Earth’s days has been an unrecognized contributor to this vital process.
The gradual decrease in Earth’s rotational speed is primarily attributable to the gravitational influence of the Moon, which has been slowly moving away from the planet. Historical evidence reveals that, 1.4 billion years ago, days were only 18 hours long, and just 70 million years ago, they were half an hour shorter than they are today. Current estimates suggest that Earth gains about 1.8 milliseconds in the length of a day each century.
The interplay between Earth’s rotation and the emergence of life is also underscored by the competition among microbial mats found in locations like Lake Huron’s Middle Island Sinkhole. These mats include purple cyanobacteria that produce oxygen through photosynthesis and sulfur-metabolizing microbes that interact in noteworthy ways. Judith Klatt, a geomicrobiologist at the Max Planck Institute for Marine Microbiology, noted the rhythmic nature of these organisms’ activities, indicating that cyanobacteria’s oxygen production is highly dependent on specific daylight conditions.
As vibrant cyanobacteria begin their photosynthetic processes, they do so after a delay once the sun rises, lending insight into how variations in day length might have historically affected oxygen levels. Klatt and oceanographer Brian Arbic from the University of Michigan investigated how these shifts in day length contributed to the overall oxygen production, hypothesizing that a similar competitive dynamic among early microbial populations may have influenced the delay of oxygen release.
Their research included direct experiments on microbial behavior both in situ and in laboratory settings, enabling them to model the relationship between microbial activity and light availability. Marine scientist Arjun Chennu elaborated on the findings, noting that although one might assume a faster cycle of day and night would yield a proportional output of oxygen, the process instead operates under constraints defined by molecular diffusion. This unexpected disconnect between sunlight exposure and oxygen release illuminates a unique aspect of microbial ecology.
Integration of their findings into global models revealed that the gradual increase in day length is intrinsically linked to major shifts in Earth’s oxygen levels, encompassing both the Great Oxidation Event and a subsequent atmospheric oxygenation phase known as the Neoproterozoic Oxygenation Event, which occurred between 550 million and 800 million years ago.
This research underscores a profound connection between celestial mechanics and microbial life, bridging the gap between macro-scale planetary changes and micro-scale biological processes. Chennu expressed excitement at how the dynamics of Earth’s rotation influence the very building blocks of life, illustrating an intricate relationship between the solar cycle and the biochemical activities of microbes.
The study’s findings have been detailed in the journal Nature Geoscience, marking a significant contribution to our understanding of how Earth’s physical characteristics have shaped its biological history and the conditions necessary for life as we know it.