What makes Earth special? Our planet is the only one we know of where life persists, thanks to a perfect marriage of chemistry and physics — and one of the key ingredients in that happy mix is oxygen.
We need oxygen to breathe, and thus, to live. The element is perhaps as vital to life thriving on Earth as is the Earth’s distance from the Sun — it is reasonable to assume that this lifeforce would be part of the sustenance of any life in the Universe. But while we need a precise amount of oxygen in our atmosphere to survive (19.5 percent), the amount of oxygen needed for any form of life to survive on another planet may not be the same.
The reason why scientists think this might be the case is, somewhat counterintuitively, to do with our own planet.
In a 2021 study, scientists tried to reconstruct the oxygen levels on Earth more than 2 billion years ago during the Proterozoic Eon. This time period, about a mid-point in the Earth’s total lifespan so far, comes much later than when life originally emerged on our planet.
Surprisingly, they found that the gas was at much lower levels in the atmosphere than today. And yet, life still made it.
As we hunt for signs of life on other planets, the findings shed light on exactly how much oxygen scientists need to look for in the atmospheres of foreign worlds to deem whether or not a planet might be habitable by life similar to that found on Earth.
Let’s rewind the clock — When Earth formed around 4.5 billion years ago, there was no oxygen. Zilch.
In fact, the oxygen in Earth’s atmosphere is thought to have been created by clever primordial lifeforms that somehow figured out how to produce oxygen using sunlight and water.
Stephen Zinder, professor of microbiology at Cornell University, explains to Inverse that oxygen is not necessary during the early stages of the development of life.
“As far as anyone can say, there was no more than traces of oxygen for the first two and a half billion years of planet Earth,” Zinder says.
“So all of these early steps of the RNA world and development of cells must’ve taken place in the total absence of oxygen.”
What Zinder means by the “RNA world” is this: There’s a theory that in the very earliest days of life emerging on Earth, there were self-replicating RNA molecules. RNA is similar to DNA in that it acts as a set of blueprints for the cell to produce proteins from genes, but unlike DNA, RNA is single-stranded, it can act similarly to a protein, and it can regulate genes. In this hypothesis, scientists believe RNA molecules would have emerged before DNA, and even given rise to DNA molecules which we today recognize as part of the building blocks of life.
Did early life use oxygen?
More than 3 billion years ago, tiny primordial organisms inhabited a vastly different Earth. As these organisms began to evolve and change, they transformed into cyanobacteria — essentially algae. Cyanobacteria are the first knowns lifeforms to develop photosynthesis, using energy from the Sun to produce oxygen.
After this early life began to sprout, oxygen levels still didn’t match up with what they are today. In fact, during the crucial time in Earth’s history when animals, plants, and fungi began to develop and thrive, oxygen levels in the atmosphere were at a fraction of today’s levels.
But these tiny creatures’ work slowly changed the course of life on Earth. Here’s how they did it:
- Over the course of a few hundred million years, oxygen levels in Earth’s atmosphere increased to make up about 18 percent of the atmosphere.
- As oxygen rose, some microorganisms actually died out — they couldn’t take it. But others thrived, multiplied, and evolved into even more complex life forms.
- The initial spike in oxygen levels is known as the Great Oxidation Event.
- After the initial spike, the amount of oxygen in the Earth’s atmosphere began to decrease.
- But over time, life on Earth became intertwined with oxygen levels — essentially, life on Earth developed a dependency on the chemical.
Taken together, all this knowledge begs the question: Was oxygen that critical for the development of life on Earth at all?
And similarly, is it that critical for the development of life anywhere in the cosmos?
How oxygen shapes life
In the 2021 study, the researchers looked at oxygen levels during a period spanning between 2.5 billion years ago and 541 million years ago.
During that time, oxygen became an integral part of Earth’s atmosphere and possibly contributed to developing the diverse lifeforms on Earth we know today — including us. But exactly how much oxygen was needed for life to survive and thrive?
Interestingly, the answer lies in the rocks — the researchers looked at nine geological formations formed during a 1 billion year span of the Proterozoic Eon. By analyzing organic matter in the rock from that era, they confirmed that the Earth’s atmosphere did contain oxygen at that time — but they were between two and 24 percent of the levels that they are today.
If oxygen levels now are at 21 percent, then that means the total oxygen levels in the Earth’s atmosphere might have been less than one percent during this period.
This suggests that the evolution of Earth’s earliest animals took place under conditions with far less oxygen available compared to today. In other words, the evolution of life was not necessarily hindered by deficient levels of atmospheric oxygen.
While humans and other animals thrive on oxygen, our atmosphere is actually 78 percent nitrogen compared to just 21 percent oxygen. If oxygen dipped below 19 percent, we’d be in trouble as a species. Earth’s ozone layer, which protects our planet from high levels of ultraviolet light and heat from the Sun, is also made up of oxygen.
Where is oxygen found in the Universe?
Oxygen is the third most common element in the Universe after hydrogen and helium.
But molecular oxygen, the stuff we breathe, is rarely ever detected in the atmosphere of planets or moons other than Earth. In turn, Earth is the only planet we know of with organisms that produce oxygen as waste. In fact, 200 million years ago, when the Earth had far higher levels of oxygen than it did today, dragonflies, for example, could grow to gigantic sizes, according to Zinder.
This makes Earth the planet with the highest concentration of oxygen in its atmosphere known to science.
Part of the reason is that oxygen bonds easily with other elements, like carbon, to produce carbon dioxide, while molecular oxygen requires two oxygen atoms to bond together.
There are only two other places in the cosmos where scientists have detected molecular oxygen — the Orion Nebula and the Rho Ophiuchi cloud.
Mercury and Mars also have traces of oxygen in their atmosphere, but not enough to sustain a human presence without serious technological help. Oxygen on Mars makes up just about 0.145 percent of its atmosphere — this figure is close to the oxygen levels on early Earth.
Zinder explains that while oxygen is not necessarily required for life to develop on a planet, it is necessary for the later stages of life — complex, multi-cellular life needs oxygen in some capacity to evolve. Similarly, if there is oxygen detected in an exoplanet’s atmosphere, it is good evidence there is life of some kind on the planet.
“It’s been thought that if there is an oxygen atmosphere, there’s a good chance then there was photosynthesis,” Zinder says.
“[Complex, multi-cellular life] is very hard to do without a lot of oxygen.”
Are there exoplanets with oxygen?
In the ongoing quest to discover exoplanets with potential habitability, scientists have detected some with traces of oxygen in their atmosphere. But none of them have an oxygen-rich atmosphere like that of Earth’s own.
Many astrophysicists look specifically for planets in other star systems that bear similarities to Earth: The ideal exoplanet, the theory goes, looks like Earth in both size and composition and has similar oxygen levels in its atmosphere. But perhaps other planets don’t need that much oxygen to sustain and develop life after all.
Just two percent of Earth’s current oxygen levels would still be enough for life to develop on a planet. Another way to think about this is that if humans require 19 percent of the atmosphere to contain oxygen to survive, two percent of that is about 0.4 percent. If we found an exoplanet with that or a slightly higher molecular oxygen level in its atmosphere, then it may not be enough for human-like life, but it might be enough for something.
Ultimately, oxygen remains one of the most important factors in developing life past a relatively primitive stage. Oxygen, more than any other gas used for respiration, has a lot more energy, and it is also a nutrient.
“I never say never, but it’s just easier to imagine [finding life on another planet] with oxygen, which is such a strong oxidant and especially if you're on land,” Zinder says.
“But maybe that’s just a failure of imagination; maybe there will be multicellular, intelligent forms of life that make methane out of carbon dioxide.”