Life, as far as we know, has existed on Earth for about 3.5 billion years. At some point early on in that history, living things evolved the ability to breathe oxygen.
What factors opened the door for this evolution? The answers have been hazy, but in new research, geoscientists may have unearthed a key piece of the puzzle: Nitrous oxide (N2O), more popularly known as laughing gas, once made up a significant part of Earth’s atmosphere. This gas helped lay the groundwork for organisms to eventually evolve the ability to breathe oxygen, researchers hypothesize.
It’s likely that “life respired other things, like nitrous oxide, before oxygen,” said Jennifer Glass, a biogeochemist at the Georgia Institute of Technology in Atlanta who led the new work. Although scientists had suspected this before, through Glass’s and her team’s research, we now have a mechanism for how nitrous oxide could have first existed in the ancient atmosphere.
And in clearing the air surrounding the oxygen breathing mystery, the current research also helps solve another geological riddle: the “faint young Sun paradox.”
The Irony of It
For their study, Glass and her team sought to recreate ancient seawater chemistry from a stretch of time between 1.6 and 1.0 billion years ago. Back then, the team reported last month in Geobiology, the oceans were different from today’s in a key way: They had lots of dissolved iron.
In their lab experiments, the team filled bottles with artificial seawater that was devoid of oxygen. They then added iron to the seawater and found that the iron provided the impetus for N2O to form. Their tests showed that in a process called denitrification, the iron reacted with nitrogen in the seawater to form N2O gas, which then bubbled out of the water and into the air. Up until now, denitrification—the conversion of dissolved nitrogen ions into gas—was thought to be facilitated only by organisms, but Glass’s team discovered a way that the process could proceed in an abiotic way.
This abiotic denitrification explains “how you could’ve gotten a lot of the nitrous oxide around for the organisms to be breathing,” Glass said. Such a chemical reaction would not be very common today because the oceans don’t contain nearly enough dissolved iron. The reason there is no iron, Glass added, is because today there is plenty of oxygen in the atmosphere. This oxygen reacts with iron in seawater (in the same reaction that creates rust), causing solid iron oxides to form and settle to the seafloor.
A Fuzzy Timeline
But before oxygen in the atmosphere climbed to its current levels, N2O built up in the atmosphere. By calculating the rate at which N2O formed in their lab experiments, Glass and her team determined that levels of the gas in the ancient air were maybe 10 times greater than today’s atmospheric levels of about 0.00003%.
Such levels would have been helpful to early microbes, which scientists think breathed N2O, just as many microbes still do today. Two enzymes that enable N2O-respiring organisms to breathe “came together and created what became cytochrome c oxidase,” Glass said, which is the enzyme that enables oxygen-respiring organisms like us to breathe.
In other words, somehow, “the key enzyme for us to breathe oxygen came from denitrification,” Glass said.
Solidifying the evidence that this was exactly how oxygen breathing evolved, however, is not so straightforward, explained Aubrey Zerkle, a biogeochemist at the University of St. Andrews in Scotland who was not involved in the new work.
“We have indisputable evidence that there was already oxygen around during this time period,” she said. From the fossil record and other studies, she explained, we know that “life was undoubtedly already producing and respiring oxygen.”
If oxygen was already in the air and if life was already breathing it, the ability to breathe oxygen might have evolved before N2O came onto the scene. Whether this was the case is “very difficult to test,” Zerkle said. “The question is the sequence of evolutionary events—whether this [N2O respiration] evolved before oxygen respiration or after.”
Under a Faint Young Sun
These evolutionary changes occurred in the midst of the faint young Sun paradox. First coined by the astronomer Carl Sagan in 1972, the term refers to how the Sun was about 10% less luminous than it is today. Under such low-light conditions, Earth should have been mostly frozen, scientists have argued, but it wasn’t. Why?
The answer again may lie with N2O, Glass explained. N2O is an extremely potent greenhouse gas that, at even minute levels, can help to warm a planet. And the N2O concentrations that the team calculated equate, she added, to a rise in atmospheric temperatures by about 3°C to 5°C: a heat increase great enough to help moderate the planet’s temperatures and prevent global ice ages.
“It’s an interesting ingredient to the cocktail of what kept Earth habitable,” said Noah Planavsky, a biogeochemist at Yale University who was not involved in the work. Planavsky added that figuring out how Earth remained “clement” while basking under the rays of a dim sun has been a lingering problem for researchers but this new research reveals one of the mechanisms that was likely at work.
N2O as a warming mechanism under a faint young Sun “is something that people haven’t considered in the past,” said Zerkle. “I don’t think it’s a smoking gun,” she added, as it is very hard to say how much any one factor helped keep Earth warm. But at the least for now, a part of the paradox appears to be demystified.
—Lucas Joel (email: [email protected]), Freelance Journalist