Several key moments in Earth’s history help us humans answer the question, “How did we get here?” These moments also shed light on the question, “Where are we going”? – Offering scientists a deeper insight into how organisms adapt to physical and chemical changes in their environment. Among them is an extended evolutionary event 2 billion years ago, known as the Great Oxidation Event (GO). It marked the first time that oxygen produced by photosynthesis – essential for the survival of humans and many other forms of life – began to accumulate in significant quantities in the atmosphere.
If you traveled back in time before Go (4 2.4 billion years ago), you would encounter a largely anoxic (oxygen-free) environment. Organisms that flourished at this time were anaerobic, meaning they did not need oxygen and relied on processes such as fermentation to produce energy. Some of these organisms still exist today in extreme environments such as highly acidic hot springs and hydrothermal vents.
Go triggered one of the most profound chemical changes in Earth’s surface history. This marked the transition from a planet effectively devoid of atmospheric oxygen – and inhospitable to complex life – to an oxygenated atmosphere that supports life as we know it today.
Scientists have long been interested in identifying the timing and causes of large changes in atmospheric oxygen because they are fundamental to understanding how complex life, including humans, evolves. Although our understanding of this critical period is still taking shape, a team of researchers from Syracuse University and MIT is digging deep—literally—in ancient rock cores from beneath South Africa for clues about the time of the GO. Their work provides new insights into the pace of biological evolution in response to rising oxygen levels.
The study, published in the journal Proceedings of the National Academy of Sciences, was led by Benjamin Uvage 18 Ph.D., who completed the project as a postdoctoral associate at MIT and collaborated with Syracuse University Earth Sciences Professor Christopher Junium on the chemical analyses.
The answers are embedded in the rock
To step back in time, the research team analyzed sedimentary rock cores collected from several locations in South Africa. These sites were carefully selected because their rocks, which are 2.2 to 2.5 billion years old, fall within the ideal age range for preserving GOE evidence. By analyzing the stable isotopic ratios embedded in these rocks, the team uncovered evidence of oceanic processes that required the presence of nitrate.
To analyze the ancient sediments, Uvages worked with Junium, an associate professor of earth and environmental sciences at Cirques University. Junem specializes in studying how past environments evolved to better understand future global change. His sophisticated equipment was essential for accurate readings of trace nitrogen levels.
“The rocks we analyzed for this study had very low nitrogen concentrations, too low to measure with the traditional instruments used for this work,” Uvages says. “Chris has built one of only a handful of instruments in the world that can measure nitrogen isotope ratios in samples that contain 100 to 1,000 times less nitrogen than the normal minimum.”
In Junem’s lab, the team analyzed nitrogen isotope ratios from rock samples from South Africa using an isotope ratio mass spectrometer (IRMS). The samples were first crushed into a powder, chemically treated to extract specific components, then turned into a gas. This gas was ionized (turned into charged particles) and accelerated by a magnetic field, which separated their mass isotopes. IRMs then measured the ratio of ⁵n to ⁴n, indicating how nitrogen had been processed in the past.
So how does this process reflect past oxygen levels? Microbes (short for microorganisms) affect the chemical makeup of rocks before they become rocks, leaving behind isotopic signatures of how nitrogen is being processed and used. Tracking changes from ⁵n to ⁴n over time helps scientists understand how Earth’s atmosphere, especially oxygen levels, evolved.
Rewriting the Oxygen Timeline
According to UVGs, the most striking finding is a change in the timing of the ocean’s aerobic nitrogen cycle. The evidence suggests that nitrogen cycling became sensitive to dissolved oxygen about 100 million years earlier than previously thought – indicating a significant delay between the build-up of oxygen in the ocean and its accumulation in the atmosphere.
Jonim notes that these findings mark a major tipping point in the nitrogen cycle, when organisms had to update their biochemical machinery to process nitrogen in a more oxidized form that was harder for them to absorb and use.
“All of this fits with the emerging idea that the GOE was a long experiment where organisms had to find a balance between exploiting the energy yield of oxygenic photosynthesis, and gradually adapting to deal with its byproduct, oxygen,” Junium says.
As oxygen produced by photosynthesis began to accumulate in the atmosphere, this increase in oxygen led to the extinction of many anaerobic organisms and set the stage for the evolution of aerobic respiration—a process that uses oxygen to break down glucose and provide the energy necessary for functions such as countermovement, brain activity, and cellular repair.
“For the first 2-plus billion years of Earth’s history, there was very little free oxygen in the oceans or atmosphere,” Uvages says. “By contrast, today oxygen makes up one-fifth of our atmosphere and basically all complex multicellular life as we know it depends on it for respiration. So, in a way, studying the rise of oxygen and its chemical, geological and biological effects is really studying how life on the planet and life have been adapted to the current situation.”
Their findings reshape our understanding of when Earth’s surface atmosphere became rich in oxygen after the evolution of oxygen-producing photosynthesis. The research also identifies an important biochemical milestone that can help scientists model how different forms of life evolved before and after the GOE.
“I hope our findings will encourage further research into this fascinating time period.” “By applying new geochemical techniques to the geochemical techniques we’ve studied, we can develop an even more detailed picture of GO and its effects on life on Earth.”
This work was funded by grants: an NSF Career Award (Syracuse University – Christopher Junium) and a Simons Foundation Origins of Life Cooperation Award (MIT – Benjamin Yus, Garrett Ezon and Roger Summon).
