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From the tallest Sequoia, to a smartest human, to the most poisonous mushroom, to the tiniest
bacterium…
There is one thing that unites all living things on earth, and that is the fact that
it’s all life is based on carbon chemistry, This element is so important in fact that
it has its own branch of chemistry – organic chemistry.
But carbon is not the most abundant element on earth.
That would be oxygen.
And it is not the most stable, like Helium.
On the surface, it doesn’t appear particularly special.
Yet if we look at our body composition, we find carbon everywhere in our cells.
20% of our body is made up of carbon, but it comprises less than 1% of the mass of the
earth’s atmosphere, oceans and crust.
Why did life go to the trouble of concentrating Carbon 20 fold in our bodies, when other more
abundant elements, like Oxygen and Silicon were available, or even Nitrogen which makes
up 78% of our atmosphere?
The big question is why? We are going to examine that, and we’regoing to trace the logic that nature used
in choosing this all important element - Carbon.
That’s coming up right now…
There are 94 naturally occurring elements on the periodic table.
Why is everything based on carbon and not something else when the choices are so many?
The answer boils down to three things: complexity, abundance and stability.
What do I mean by this?
Let’s start with complexity.
Carbon is able to form the complex molecular structures needed for the complex chemistry
that life requires.
Why is carbon able to do this?
To understand this, we have to look at the foundation of chemistry, which is physics,
particularly quantum mechanics.
Quantum mechanics tells us that some electron orbital configurations are more energetically
favorable than others, making them more stable.
And the most stable configuration of electrons are those of the Noble gases on the far right
side of the periodic table.
These are special elements that have a complete outer shell of electrons.
This is the lowest energy configuration that an atom can have.
This makes them chemically stable because they do not typically need to share, gain,
or lose electrons to form bonds with other atoms to become more stable.
This is also why they are chemically inert, since they do not generally interact with
other atoms to form molecules.
Why are only the Noble gases this way and not other elements?…because they happen
to have the right number of protons to have the right number of electrons to fill their
shell fully.
The driving force behind chemistry is really a hunt to get to the most energetically efficient
state in a system, the state of lowest energy.
And for the most part, this lowest energy state occurs when an atom is able to fill
its outermost shell fully, like the noble elements.
And some atoms can do this by sharing their electrons with other atoms to form what’s
called covalent chemical bonds.
The periodic table of elements is arranged in such a way that we can easily tell by column,
the maximum number of bonds that an element can form with other elements.
Starting from the left column of the table, the first group of atoms can form a maximum
of one bond.
The second column of elements can form 2 bonds.
Third column 3 bonds.
This trend continues up to the fourth column which can form 4 bonds.
After this, the maximum number of bonds decreases, so the fifth column of elements can only form
3 maximum bonds.
Then 2 bonds, one bond, and finally zero bonds for the right most column, which are the noble
elements, which are already the most stable.
What you’ll notice is that Carbon is in the group of elements which can form the maximum
number of bonds.
It has a total of 6 electrons, 4 of which are in its outer shell.
In order fill its outermost shell, it needs 4 more electrons to make 8 like the noble
gas Neon.
So it can share up to 4 additional electrons from other atoms to form covalent bonds, making
it more stable.
This makes Carbon very versatile.
Imagine if Lego bricks could be connected on four sides instead of two, it would make
building things easier.
Each carbon atom can form a strong stable bond with up to 4 other atoms including other
carbon atoms.
This feature of carbon gives it the ability to form complex molecules, which is necessary
for the complex chemical functions that life requires.
Carbon based molecules can form long non-repetitive chains of polymers, they can form closed rings,
and they can form single, double or triple bonds with other elements.
There are millions of possible configurations.
Taken together, this makes Carbon uniquely able to take part in a vast multitude of chemical
processes.
It can easily form long stable polymer chains that can, for example, carry a lot of information.
This is the case for DNA.
DNA after all, carries ALL the information that makes up living things, including us.
And the 4 nucleotides that make up the building blocks of DNA are complex.
Carbon is the backbone partly because it can handle this complexity.
The other elements are not as interesting.
Take for example oxygen with is the most abundant element on earth.
It can form only 2 bonds.
This means that once it bonds with 2 other atoms, it’s done.
It can’t really form interesting scaffolds of complex molecules, like carbon can.
Boron could be interesting because it can form 3 bonds, so it’s molecular structures
could also be fairly complex.
The problem is that it’s extremely rare, so it’s just not very available for life
to have chosen it as its backbone.
This brings us to the second factor that made carbon attractive for life to latch onto,
abundance.
Not only is carbon versatile, it is also abundant.
If we look at the top 5 most abundant elements in our solar system, what we will see is the
following in order of abundance: 1. Hydrogen
2. Helium
3. Oxygen
4. Carvon
5. Nitrogen
Now if we look at the top 5 of elements in
our body, what we will see is the following in order of abundance:
1. Oxygen
2. Carbon 3. Hydrogen 4. Nitrogen
and 5. Calcium - So what we see is that 4 out of the top 5
elements of the solar system are also among the top 5 elements making up the human body.
This give us another clue about why life is based on Carbon.
There is plenty of it in the universe.
It is very abundant.
It’s easier to build something that you have a lot of.
You can’t build a castle if you don’t have enough Lego bricks.
At this point, you might say, if abundance is so important then what about Silicon which
is abundant and can also form 4 bonds, or Nitrogen which forms only 3 bonds, but makes
up 78% of our atmosphere.
This brings us to the third factor in determining the most suitable element that nature chose
for life, stability.
What I mean by this is bond stability.
Let’s look at that.
As I said earlier, the structure of the periodic table is such that as a rule of thumb, all
elements in each column have the same general properties.
Carbon turns out to be the lightest element in group 14 or 4 depending on how you count.
Therefore, we would expect that the sister elements like silicon and germanium would
have similar chemical capabilities.
Silicon is the next lightest element in the row.
Its position on the periodic table tells us that, like Carbon, it also has four valence
electrons.
This means it can also make four covalent bonds.
For every molecule made out of carbon, there can be an analogous molecule with silicon
in its place.
Silicon also happens to be quite abundant on earth.
In fact, there is more silicon on earth than carbon.
It’s just locked in rocks within the earth’s crust.
Chemically, Silicon has 4 unpaired electrons in its outer orbital, just like carbon.
The main difference is silicon has its unpaired electrons farther away from its nucleus, on
its third shell, whereas carbon’s electrons are on its second shell, closer to the nucleus.
This makes Silicon’s electrons more weakly bound to its nucleus.
The consequence of this is that when silicon bonds with other atoms including itself, the
bonds formed are weaker, and thus less stable.
To give you some numbers, the silicon-silicon bond strength is 196kJ/mol.
In contrast, carbon-carbon bonds are stronger at 334 kJ/mol.
This bond strength factor is also a reason Nitrogen is not well suited to be the backbone
of organic chemistry, as its bond strength is roughly half of carbon.
You can’t make a sky scraper with a foundation built from cardboard.
The structural support has to be strong to hold the walls, windows and doors.
Organic molecules have the same need.
The backbone has to be strong enough to withstand the conditions under which other parts of
the molecule break their bonds and react chemically with other molecules.
So the carbon-carbon scaffold needs to remain intact while its functional components break
apart.
A molecular scaffold made from nitrogen or silicon would more easily break apart.
So if we go back now to the periodic table, and look at the first 3 rows of elements.
If we remove all the chemically inert elements, and then remove elements that can’t from
more than 2 bonds, then we remove all the elements that are exceedingly low in abundance,
we end up with Carbon, Nitrogen and Silicon.
Then if we remove those that cannot form strong single bonds to themselves to create a strong
molecular backbone, this leaves pretty much only Carbon as the best choice.
It is uniquely suited for life because of the best combination of abundance, ability
to form complex structures, and stable bonds with other carbon atoms.
Now, having said all that, there is nothing that would preclude silicon-based life forms
from existing on an extraterrestrial planet, if the conditions are right for it.
By this logic, we can also imagine life based on germanium, tin or even super-dense creatures
made of lead.
But we have to keep in mind that as elements get heavier, they tend to be more rare.
So each of these are less plausible than the last.
For example, Germanium is at least five orders of magnitude less abundant than carbon, and
tin is even rarer.
It so happens that the pressures and temperatures on earth works well for life forms that use
liquid water as the solvent, and carbon based molecules that can form a stable backbone
on which biological chemistry can take place.
If it gets too cold, water turns to ice and we’ll have problems with chemical transport.
If it gets too hot, we will get issues with carbon polymer chains breaking too easily.
We may never know for sure, but it's perfectly possible that carbon trumps everything else
except in extreme environments.
But we can’t rule anything out and we need to be open minded about the conditions in
which life could hypothetically occur.
For example, there are lakes of ethane and methane on Titan.
These liquids, like water on earth, could be used as a solvent by a different type of
lifeform.
So at different temperatures and pressures, it is conceivable that a different element
may be more conducive to life.
One of my patreon supporters pointed out that we Carbon life forms may right now be in the
process of creating Silicon artificial life forms.
And this process is accelerating.
This form of life could very likely to exist in the near future right here on earth.
But there is no evidence for life outside of earth.
It could be quite rare in the universe.
So you can have all the various elements available in abundance, but the interactions or chemistry
of these various elements is the critical process needed for life to happen.
That process is organic chemistry.
And for further study, I’d highly recommend a great course I recently watched on Wondrium,
the sponsor of today’s video, called, “Foundations of Organic Chemistry.”
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material I covered in this video.
This is a 36 lecture course that takes you on a learning journey from the basics of atoms,
to the formation of the most important organic chemicals, to the mechanisms behind organic
chemical reactions.
The first courses is even called, “Why Carbon?”
It’s taught by professor Ron Davis of Georgetown University, whose lucid explanations makes
this subject relevant and easily graspable.
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