Recent findings from researchers at MIT indicate that certain life forms may have begun utilizing oxygen hundreds of millions of years prior to the Great Oxidation Event (GOE). This discovery could provide some of the earliest evidence of aerobic respiration on our planet.
In a study published in Palaeogeography, Palaeoclimatology, Palaeoecology, geobiologists from MIT examined the origins of a vital enzyme that enables organisms to metabolize oxygen. This enzyme is found in the majority of aerobic life forms today. The research team concluded that it first emerged during the Mesoarchean era, which occurred hundreds of millions of years before the GOE.
This research may shed light on a longstanding enigma in Earth's history: if oxygen-producing microbes existed so early, why did it take so long for oxygen to build up in the atmosphere?
Cyanobacteria and Early Oxygen Production
The earliest known oxygen producers are cyanobacteria, which developed the ability to utilize sunlight and water for photosynthesis, releasing oxygen as a byproduct. Scientists estimate that cyanobacteria appeared approximately 2.9 billion years ago, suggesting they may have been generating oxygen for hundreds of millions of years prior to the GOE.
But what happened to all that early oxygen?
Researchers have long theorized that chemical reactions with rocks absorbed much of the oxygen from the environment. The latest study from MIT proposes that living organisms may have also played a role in consuming it.
The team discovered evidence that certain microbes evolved the enzyme that utilizes oxygen well before the GOE. Organisms living in proximity to cyanobacteria might have used this enzyme to quickly metabolize small amounts of oxygen as it was produced. If this is the case, early life could have contributed to the slow accumulation of oxygen in the atmosphere over hundreds of millions of years.
"This significantly alters our understanding of aerobic respiration," states Fatima Husain, a postdoctoral researcher in MIT's Department of Earth, Atmospheric and Planetary Sciences (EAPS). "Our study supports the emerging narrative that life may have been utilizing oxygen much earlier than previously assumed, highlighting the remarkable adaptability of life throughout Earth's history."
Co-authors of the study include Gregory Fournier, an associate professor of geobiology at MIT, alongside Haitao Shang and Stilianos Louca from the University of Oregon.
Tracing the Origins of Aerobic Respiration
This research builds on years of work at MIT aimed at reconstructing the history of oxygen on Earth. Earlier studies established that cyanobacteria began oxygen production around 2.9 billion years ago, while oxygen did not permanently accumulate in the atmosphere until roughly 2.33 billion years ago during the GOE.
For Husain and her colleagues, this lengthy gap raises an important question.
"We know that oxygen-producing microorganisms existed well before the GOE," Husain notes. "It was natural to inquire whether any life forms at that time could have utilized that oxygen for aerobic respiration."
If some organisms were already consuming oxygen, even in modest quantities, they might have played a role in keeping atmospheric levels low for an extended period.
To investigate this hypothesis, the researchers focused on heme copper oxygen reductases, enzymes critical for aerobic respiration as they convert oxygen into water. These enzymes are present in most oxygen-breathing organisms today, from bacteria to humans.
"We concentrated on the core of this enzyme in our analyses because that's where the reaction with oxygen occurs," Husain elaborates.
Mapping Enzymes on the Tree of Life
The team aimed to determine when this enzyme first appeared. They identified its genetic sequence and searched extensive genome databases containing millions of species for matching sequences.
"The most challenging aspect of this work was managing the vast amount of data," Fournier explains. "This enzyme is ubiquitous and found in most modern organisms. Consequently, we needed to sample and filter the data to create a representative dataset that was manageable for computation."
After refining the data to several thousand species, the researchers placed the enzyme sequences onto an evolutionary tree of life, allowing them to estimate when various branches emerged.
When fossil evidence was available for specific organisms, the scientists used their estimated ages to anchor that branch of the tree. By applying multiple fossil-based time points, they enhanced their estimates for when the enzyme evolved.
Their analysis traced the enzyme back to the Mesoarchean era, which spanned from 3.2 to 2.8 billion years ago. The researchers believe this is when the enzyme, and the capacity to utilize oxygen, first emerged, predating the GOE by several hundred million years.
The findings suggest that shortly after cyanobacteria began producing oxygen, other organisms evolved the mechanisms to consume it. Microbes living near cyanobacteria may have quickly absorbed the oxygen being released, potentially hindering the atmospheric buildup for hundreds of millions of years.
"Collectively, MIT's research has filled in the gaps in our understanding of how Earth's oxygenation unfolded," Husain concludes. "The pieces of the puzzle are coming together, emphasizing how life managed to diversify and thrive in this newly oxygenated environment."
This research received partial support from the Research Corporation for Science Advancement Scialog program.
