Today, oxygen is important to life and is continuously present within the air we breathe. But for many of Earth’s early history, this was not true. Oxygen didn’t develop into a stable a part of the atmosphere until about 2.3 billion years ago, during a period of change often known as the Great Oxidation Event (GOE). This change permanently modified the planet and paved the way in which for oxygen-breathing organisms to evolve and flourish.
Now, MIT researchers provide evidence that some life forms can have learned to make use of oxygen thousands and thousands of years before the GOE. Their findings may represent a few of the earliest signs of aerobic respiration on Earth.
In research published in , MIT geologists investigated the origins of a key enzyme that permits organisms to make use of oxygen. This enzyme is present in most aerobic, oxygen-breathing life today. The team determined that it first evolved through the Mesoarchean, a geological era that occurred thousands and thousands of years before the Great Oxidation Event.
Their findings could help answer a long-standing mystery in Earth’s history. If oxygen-producing microbes appeared so quickly, why did it take so long for oxygen to build up within the atmosphere?
Cyanobacteria and elementary oxygen production
The first known oxygen producers were cyanobacteria. These microbes developed the power to make use of sunlight and water through photosynthesis, releasing oxygen as a byproduct. Scientists estimate that cyanobacteria emerged about 2.9 billion years ago. This means they were likely producing oxygen for lots of of thousands and thousands of years before the Great Oxidation Event.
So what happened to that initial oxygen?
Researchers have long suspected that chemical reactions with rocks remove much of it from the atmosphere. A brand new MIT study shows that living organisms are using it, too.
The team found evidence that some microbes produced oxygen using the enzyme long before the GOE. Organisms living near cyanobacteria could use this enzyme to rapidly use small amounts of oxygen because it was developed. If so, youth would have slowed the build-up of oxygen within the atmosphere over lots of of thousands and thousands of years.
“This dramatically changes the story of aerobic respiration,” said study co-author Fatima Hussain, a postdoctoral fellow in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS). “Our study adds to the recently emerging story that life may have used oxygen earlier than previously thought. It shows us how incredibly innovative life has been at every period of Earth’s history.”
Other co-authors include Gregory Fournier, associate professor of geobiology at MIT, together with Haitao Shang and Stelianos Louka of the University of Oregon.
Tracing the origin of aerobic respiration
The work builds on years of research at MIT aimed toward reconstructing the history of oxygen on Earth. Previous studies helped establish that cyanobacteria began producing oxygen about 2.9 billion years ago, while oxygen didn’t accumulate permanently within the atmosphere until about 2.33 billion years ago through the Great Oxidation Event.
For Hussain and his colleagues, this long hiatus raised a vital query.
“We know that oxygen-producing microorganisms existed before the Great Oxidation Event,” Hussain says. “So it was natural to ask, was there any life around at that time that might have been able to use that oxygen for aerobic respiration?”
If some organisms were already using oxygen, even in small amounts, they may have helped keep atmospheric levels low for a certain time frame.
To explore this concept, the researchers focused on heme copper oxygen reductases. These enzymes are essential for aerobic respiration because they convert oxygen into water. They are present in most oxygen-breathing organisms today, from bacteria to humans.
“We targeted the core of this enzyme for our analyses, as this is where the reaction with oxygen is taking place,” Hussain explains.
Mapping enzymes on the tree of life.
The team set out to find out when the enzyme first appeared. They identified its genetic sequence after which searched large genome databases containing thousands and thousands of species to seek out matching sequences.
“The most difficult part of this work was that we had a lot of data,” says Fournier. “This enzyme is ubiquitous and present in most modern organisms. So we had to sample and filter the data into a dataset that was representative of the diversity of modern life and small enough to do the calculations with it, which is not trivial.”
After narrowing down the information to several thousand species, the researchers placed the enzyme sequences on an evolutionary tree of life. This allowed them to predict when the various branches emerged.
When there was fossil evidence for a specific organism, scientists used its estimated age to anchor that branch of the tree. By applying multiple fossil-based time points, they refined their estimates for enzyme evolution.
Their evaluation traced the enzyme back to the Mesoarchean, which spanned from 3.2 to 2.8 billion years ago. Researchers imagine this happened when enzymes, and the power to make use of oxygen, first evolved. This timeframe is several hundred million years before the Great Oxidation Event.
The results show that soon after cyanobacteria produced oxygen, other organisms developed the machinery to make use of it. Microbes living near the cyanobacteria could quickly absorb the released oxygen. By doing so, these early aerobic organisms can have helped prevent oxygen from accumulating within the atmosphere for thousands and thousands of years.
“Taken together, the MIT research fills a gap in our knowledge of how Earth’s oxygen evolves,” Hussain says. “The pieces of the puzzle are fitting together and really indicate how life was able to diversify and survive on this new, oxygenated world.”
This research was supported, partially, by the Research Corporation for the Advancement of Science Schlag Program.