You might not be able to tell just by looking, but carbon is the fundamental element to life on Earth. DNA and RNA are made of carbon. Not only carbon, of course, but carbon can bond with other molecules to make up the long chains that are necessary for life to exist. Not just human life, but every single kind of life from plants to fungus to mammals, insects, you name it. Everything on our planet that is alive is carbon-based.
In technical terms, life anywhere should be carbon based. It’s conceivable that another element could replace carbon in the formation of these molecular chains that make up the building blocks of life, but that’s all theoretical. We’ve never actually seen a non-carbon-based life form before, mostly we just throw them into science fiction and then make them look kind of weird.
If all life is truly carbon-based, then that does lead to the question of why. What makes carbon the ideal element for formula life? And how likely are any of these theories we’ve had about other elements? Which elements could form life and how do we even know that? Let’s take a look!
What Allows Carbon to Form Life?
Even though hydrogen is the most abundant element in the universe, and oxygen is necessary for life, carbon is the thing that forms life and is the most important element in that equation. That’s because carbon has some unique properties that many other elements don’t. Specifically, carbon is really good when it comes to teamwork. Carbon easily bonds with other elements and those bonds are strong.
Because carbon bonds are so strong it allows for the formation of very complicated molecules. DNA, for instance, is an extremely complicated molecule. But it works because the carbon bonds hold it all together. It can easily form long polymer chains which allow for DNA and proteins to exist because they can get extremely long.
The molecular formula for DNA is C15H31N3O13P2. As you can see, carbon has a strong presence in the molecule. It bonds here to hydrogen, nitrogen, oxygen, and phosphorus. Those are the basic elements of life. Everything else is structured around that so carbon is kind of like a scaffolding, or skeleton on which life is based.
Our world is absolutely full of carbon, especially relative to many other elements. Carbon is actually the fourth most common element in the entire universe. The first three are hydrogen, helium, and oxygen, all of which are gases. Carbon is therefore the most common solid you’re going to find. It makes sense then that it might be integral to life, given its abundance.
Because the atoms are so small, it makes it much easier to form extremely complicated molecular chains. Like we mentioned, carbon can form proteins, as well as sugars. The things life needs for energy are carbon-based just the way that life itself is. That means living things can break down other carbon-based molecules to get energy from them in order to sustain life. This is a kind of micro-scale “you are what you eat” scenario.
The Carbon Cycle
Earth enjoys something called the carbon cycle. This is a macro-scale process that covers everything in the world. It allows the entire planet to recycle carbon, spread it around, and, in some ways, ensure that life can keep happening.
The world has a finite amount of carbon. We’re not destroying it, but we’re not creating it either. That’s not a thing we can really do, certainly not at scale. So we only have as much carbon as we have ever had. That means everything that uses it will need to share it at some point. It’s entirely possible some of the carbon inside of you right now was once inside of Abraham Lincoln, a tyrannosaurus rex, and maybe the first handful of single-celled organisms that were paddling around in those pools when life began on our planet.
Plants draw carbon dioxide from the air. It’s stored in the ground and in root systems and can be released again when these plants die. The oceans absorb carbon and living organisms like animals exhale it into the atmosphere to exchange with oxygen.
Much of the carbon cycle is concerned with the atmosphere and how carbon dioxide affects the air and global temperatures. But you can see how the carbon also spreads other ways when it is absorbed by plants, the plant is consumed by an animal; the animal is consumed by us, and these carbon atoms are constantly being used in exchange in various forms. It’s not just carbon dioxide, it’s those proteins, and sugars, and other molecules that are shared between organisms in the cyclical dance of life.
Are Other Biochemistries Possible?
So if carbon can form the basics of life because it bonds easily with other elements, are any other elements able to do the same task? As it happens, while carbon is well suited to this task, it’s not alone. Keep in mind, this is all entirely theoretical, but what is science if not asking questions to see if something is possible?
Silicon
Silicon is located on the periodic table of elements right below carbon. It is usually the most frequently cited element when people are discussing other potential forms of life. The reason for this is that silicon mimics carbon in a lot of ways when it comes to forming molecules. It is not as good at forming complicated bonds with other elements, but it’s still pretty good.
Structurally, a lot of silicon bonds and molecules resemble those made with carbon. Silicon can make strong bonds with oxygen as well, which would be essential for creating the elements of life, at least as we understand them. But there are some potential drawbacks to this as well.
Not all the silicone bonds are going to be as strong as carbon bonds. Silicon is ideal for high temperatures, but at normal surface temperatures on earth, carbon bonds are much stronger. As well, silicon atoms are much larger than carbon atoms. That makes them a little ungainly in certain molecules and reduces the potential of creating some of the most complicated polymers that carbon makes.
Researchers have managed to manipulate microbes in labs to create organic compounds out of silicon, something that doesn’t happen normally in nature. This gives a lot of strength to the idea that perhaps one day we could discover silicon life in the universe.
Methane
A little more theoretical than silicon is the idea of potential methane-based life forms. This is where things get much more sciencey and hard to understand. At least carbon and silicon are single elements. Methane is a molecule made of carbon and oxygen so maybe this is technically carbon-based life again? Nevertheless, methane is not something that most life on Earth can thrive in. You certainly wouldn’t want to be trying to breathe it in all the time.
Researchers at NASA studying one of Saturn’s moons, Titan, discovered vinyl cyanide in the atmosphere. They believe that this organic compound could provide cell membranes for methane-based lifeforms. It would likely just be microbial life in the methane oceans that cover the moon, but theoretically, it could work.
The membranes on Titan would have to be formed from nitrogen, carbon, and hydrogen unlike phosphorus and oxygen like you’d find here on Earth.
Sulfur
The oldest fossils we have ever found on Earth date back 3.4 billion years. That was when life was just getting started and we didn’t have a planet that looked anything like it does right now. There was no oxygen in the atmosphere, no photosynthesis happening to convert carbon dioxide into something breathable. The world was hot and gassy and extremely inhospitable to anything that’s alive now. So how did life form in the first place and what was it doing?
Fossils discovered by scientists show evidence of sulfur-based microbes. They used sulfur as an energy source that helped them grow and reproduce. These fossils also gave some hope to the idea of one day finding life, or at least evidence of past life, on Mars.
In addition, it’s believed that some sulfur compounds that helped seed life on earth came from space to begin with. If they landed here and gave rise to dinosaurs and pigeons and soccer players, who knows what might have happened on another planet?
Ammonia
Like methane, ammonia is another substance that most living things on earth won’t thrive in, to say the least. Ammonia-based life has been theorized but, in this case, it’s less about ammonia replacing carbon, which it can’t do since it’s a molecule and not an atom, but replacing water.
Ammonia is not entirely antithetical to life. We produce ammonia in our urine, so It is a part of our biology even though it is not considered organic. That’s only because it’s a nitrogen-based compound and there’s no carbon in it. Carbon-based life is what is organic. That said, if we could discover non-carbon-based life, you would have to tweak your definition of organic.
Because so few planets in the solar system, and arguably the universe at large, have liquid water, it’s not unreasonable to wonder what other liquids could still be used in the formation of life. Life as we know it demands water, but who says life has to be the same as us? That’s where ammonia comes in.
There are only so many liquids you are likely to find in any abundance even in our own solar system. Mercury and Venus have sulfuric acid. While we have water, and water has been discovered on some moons out there, the colder planets likely are going to have ammonia which has a lower freezing point. If you get colder, methane liquefies, and then even nitrogen can be present in liquid form at the outer reaches of the system.
Just because there’s a liquid press it doesn’t mean it can work in a life form. It needs to meet a number of criteria including whether it works as a solvent, has low viscosity, moderates temperature well, and other factors. Given everything that needs to be considered, if we eliminate liquid water which is, as we know, ideal, then ammonia is our next best option.
Ammonia is the 4th most abundant molecule in the universe and is chemically very similar to water. It doesn’t meet water head to head in all the ways we need a life solvent to operate, but we’re also basing our knowledge on what we know and have seen, so it’s hard to know if we’re missing details or not.
Cosmic Necklace Life
There is another theory about how life could form somewhere in the universe that leaves the idea of carbon-based life way back in the rearview mirror. This is even beyond the idea of silicon life, ammonia-based life, or anything like that. This is cosmic necklace life, and it is the most extreme of extremes.
If you consider what you know about extreme life on Earth, you might come up with something like the tardigrade. These tiny little organisms can exist in extreme cold, extreme heat, even the vacuum of space. There are also life forms that exist around incredibly hot, toxic hydrothermal vents that are erupting on the ocean floor. Although these things are outliers, they show that there are occasions when life can flourish in ways that defy our expectations. Where 99.9% of everything else would die, these things are living as happy as clams. Now let’s imagine that on a cosmic scale.
If we eliminate what we think we know about what’s necessary for life, which is essentially carbon, oxygen, water, the right gravity, the right distance from a star, we can start looking in some really exotic places. Like inside of stars themselves.
These are extremely theoretical, but the idea is that there could be a particle, not unlike the Higgs Boson, called the magnetic monopole. We don’t even know if this particle is real yet. Like the Higgs Boson, it has been theoretical for a long time. While the Higgs Boson was eventually proven real, this one still eludes us.
If it is real, it may be able to thread together on cosmic strings to form DNA-like structures inside of stars. If the ability to replicate itself is encoded in RNA and DNA, then these particles may be able to perform a similar function inside of stars, encoding and reproducing creating a kind of life unlike anything that exists in our world. This would be particle-based life, at least the building blocks of it.
As for whether these necklaces could ever form into something with a consciousness, now we’re really in the realm of speculation and who’s to say?