Today, oxygen is essential to life and constantly present in the air we breathe. But for most of Earth’s early history, that was not true. Oxygen did not become a stable part of the atmosphere until about 2.3 billion years ago, during a transformative period known as the Great Oxidation Event (GOE). That shift permanently altered the planet and paved the way for oxygen breathing organisms to evolve and thrive.
Now, researchers at MIT report evidence that some forms of life may have learned to use oxygen hundreds of millions of years before the GOE. Their findings could represent some of the earliest signs of aerobic respiration on Earth.
In research published in Palaeogeography, Palaeoclimatology, Palaeoecology, MIT geobiologists investigated the origins of a crucial enzyme that allows organisms to consume oxygen. This enzyme is present in most aerobic, oxygen breathing life today. The team determined that it first evolved during the Mesoarchean, a geological era that occurred hundreds of millions of years before the Great Oxidation Event.
Their results may help answer a long standing mystery in Earth’s history. If oxygen producing microbes appeared so early, why did it take so long for oxygen to accumulate in the atmosphere?
Cyanobacteria and Early Oxygen Production
The first known oxygen producers were cyanobacteria. These microbes developed the ability to harness sunlight and water through photosynthesis, releasing oxygen as a byproduct. Scientists estimate that cyanobacteria emerged around 2.9 billion years ago. That means they were likely generating oxygen for hundreds of millions of years before the Great Oxidation Event.
So what happened to all that early oxygen?
Researchers have long suspected that chemical reactions with rocks removed much of it from the environment. The new MIT study suggests living organisms may also have been consuming it.
The team found evidence that certain microbes evolved the oxygen using enzyme long before the GOE. Organisms living near cyanobacteria could have used this enzyme to rapidly consume small amounts of oxygen as it was produced. If so, early life may have slowed the buildup of oxygen in the atmosphere for hundreds of millions of years.
“This does dramatically change the story of aerobic respiration,” says study co-author Fatima Husain, a postdoc in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS). “Our study adds to this very recently emerging story that life may have used oxygen much earlier than previously thought. It shows us how incredibly innovative life is at all periods in Earth’s history.”
Other co-authors include Gregory Fournier, associate professor of geobiology at MIT, along with Haitao Shang and Stilianos Louca of the University of Oregon.
Tracing the Origins of Aerobic Respiration
This work builds on years of research at MIT aimed at reconstructing the history of oxygen on Earth. Previous studies helped establish that cyanobacteria began producing oxygen around 2.9 billion years ago, while oxygen did not permanently accumulate in the atmosphere until roughly 2.33 billion years ago during the Great Oxidation Event.
For Husain and her colleagues, that long gap raised an important question.
“We know that the microorganisms that produce oxygen were around well before the Great Oxidation Event,” Husain says. “So it was natural to ask, was there any life around at that time that could have been capable of using that oxygen for aerobic respiration?”
If some organisms were already using oxygen, even in small amounts, they might have helped keep atmospheric levels low for a significant stretch of time.
To explore this idea, the researchers focused on heme copper oxygen reductases. These enzymes are essential for aerobic respiration because they convert oxygen into water. They are found in most oxygen breathing organisms today, from bacteria to humans.
“We targeted the core of this enzyme for our analyses because that’s where the reaction with oxygen is actually taking place,” Husain explains.
Mapping Enzymes on the Tree of Life
The team set out to determine when this enzyme first appeared. They identified its genetic sequence and then searched massive genome databases containing millions of species to find matching sequences.
“The hardest part of this work was that we had too much data,” Fournier says. “This enzyme is just everywhere and is present in most modern living organism. So we had to sample and filter the data down to a dataset that was representative of the diversity of modern life and also small enough to do computation with, which is not trivial.”
After narrowing the data to several thousand species, the researchers placed the enzyme sequences onto an evolutionary tree of life. This allowed them to estimate when different branches emerged.
When fossil evidence existed for a particular organism, the scientists used its estimated age to anchor that branch of the tree. By applying multiple fossil based time points, they refined their estimates for when the enzyme evolved.
Their analysis traced the enzyme back to the Mesoarchean, which spanned from 3.2 to 2.8 billion years ago. The researchers believe this is when the enzyme, and the ability to use oxygen, first arose. That timeframe predates the Great Oxidation Event by several hundred million years.
The findings suggest that soon after cyanobacteria began producing oxygen, other organisms evolved the machinery to consume it. Microbes living near cyanobacteria could have quickly absorbed the oxygen being released. In doing so, these early aerobic organisms may have helped prevent oxygen from accumulating in the atmosphere for hundreds of millions of years.
“Considered all together, MIT research has filled in the gaps in our knowledge of how Earth’s oxygenation proceeded,” Husain says. “The puzzle pieces are fitting together and really underscore how life was able to diversify and live in this new, oxygenated world.”
This research was supported, in part, by the Research Corporation for Science Advancement Scialog program.
