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How did we get here?
How did life form?
This is one of the biggest questions we can ask ourselves.
A complete scientific picture of how this happened still eludes us, but we have solved
many pieces of the puzzle.
In a prior video we looked at how the first atoms formed, from the hot plasma of the big
bang, where the first protons and neutrons condensed from more fundamental particles
called quarks.
This led to mostly hydrogen and helium nuclei in the early universe.
And after 380,000 years, the hot universe became cold enough for electrons to become
bound to these nuclei, and thus the first hydrogen and some helium atoms were formed.
In this way we got a universe with mostly these two atoms, but nothing more.
The bigger atoms came much later, millions if not billions of years later, via fusion
processes in the core of stars, and supernovae explosions.
Many of the things you see around you are made from atoms, but to make most complex
structures in particular, living things like trees, animals, and us, you need more than
just atoms.
You need a more complex combinations of atoms.
You need molecules.
Such molecules are not produced inside stars and must have come about in entirely different
ways.
How do you go from simply atoms formed in the core of stars, to molecules that can lead
to living things like us and other chemicals and structures?
How did the first molecules form?
That’s coming up right now.
To answer the question, we must first define what we mean by a molecule.
In simply terms, you form a molecule when you have two or more atoms connected via a
chemical bond.
As shown in a previous video, molecules form when it is more energetically favorable for
two or more atoms to form chemical bonds than to remain independent.
In other words, the total energy of a system of 2 or more atoms can be in a lower energy
state than individual atoms on their own.
One way to think of this is like the economics of trade.
Countries who trade with other can both become richer and more prosperous, than countries
that remain isolated and on their own.
If we wrote the wave functions for hydrogen atoms, we would see that the
total energy of the system becomes less when they form a chemical bond.
In other words, a molecule composed of two hydrogen atoms that share their electrons
is more energetically favorable than two hydrogen atoms on their own.
Thus, hydrogen molecules would have been among the first molecules to form when the universe
was only around 380,000 years old.
And these molecules are still around today, making them among the oldest molecules in
existence.
But remarkably these molecules would not have been the very first molecules to form.
There is a molecule that formed even earlier than the hydrogen molecule.
And it is Helium Hydride, which would have formed when the universe only around 120,000
years old.
This molecule is a positively charged ion consisting of a helium atom bound to a hydrogen
atom with one electron removed.
There is a surprising but very good reason why these would have formed first.
You can only make hydrogen molecules with hydrogen atoms.
To form a hydrogen atom, you need an electron and a proton.
But electrons could only attach themselves to hydrogen nuclei when the universe was cool
enough to allow this.
This happened at something called the recombination period of the universe, which was about 380,000
years after the Big Bang.
This is also the point at which the Cosmic Microwave Background (CMB) formed because
photons could travel unimpeded through the universe, without interacting with free electrons,
which up to that point had been all over the universe unattached to protons.
The universe thus turned from opaque to transparent at this point.
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Now, back to the show.
Hydrogen molecules only formed after recombination because prior to this hydrogen atoms were
not available and you can’t make a molecule out of two protons because they repel each
other.
But prior to recombination, when the universe was only 120,000 years old, helium atoms formed.
Why did they form earlier?
Because helium can absorb electrons at higher temperatures than hydrogen.
Why?
Because it is a noble element meaning it is more stable than hydrogen, and has a higher
ionization energy.
It takes more energy to remove an electron from helium, so it tends to attract electrons
at higher energy levels. So it formed when the universe was hotter at only 120,000 years
old, whereas Hydrogen didn’t form until the universe was cooler at 380,000 years old.
But keep in mind that in the early universe, helium nuclei formed in very small quantities
compared to hydrogen nuclei, just a few percent, so that’s why we don’t have a universe
full of mostly helium atoms today.
The helium having been formed earlier, could then bond with a proton to form positively
charged helium hydride molecules, and thus these molecules would have been the first
molecules to form.
Now, you can’t make life with this molecule.
In fact, it’s a highly reactive, and only survives when isolated.
It’s expected that these original molecules from the early universe only survived around
5,000 years after the first hydrogen molecules formed.
We can’t confirm that all the helium hydride formed after the big bang is lost, but it’s
statistically very unlikely that there would be anything left today.
Now, what about the molecules necessary for life?
How did they form?
This is where the situation is murkier, and scientist aren’t completely sure of what
precisely occurred.
But with the knowledge we have, the evidence we’ve gathered, appears to point to a plausible
scenario.
The big bang mainly left us with hydrogen and helium, but for life we also need atoms
like oxygen, carbon, and nitrogen to form proteins, water, fats, and other essential
organic molecules.
Over millions of years, helium and hydrogen floating in space condensed into dense clouds.
which eventually coalesced into the first stars after about 100 million years.
And then the near complete darkness of the universe was interrupted as they burst into life and lit
up the cosmos.
There was light.
These early stars would have been 40-100 times the size of our sun.
Subsequently, more stars lit up, and via fusion processes began producing heavier and heavier
elements inside their cores.
And it was in these cores, where the first nucleons of atoms essential for life as we
know it, like oxygen, carbon, and nitrogen, were formed.
But while stellar processes do make heavier atoms, they don’t produce molecules.
The temperature inside the stars is so hot that there is only plasma, not
molecules.
It’s too hot for molecules to form.
As best as we can tell, the following is how the first molecules necessary for life appear to have
been formed.
The first stars came to the end of their life, and exploded as supernovae.
This released all the heavier atoms which had formed within their cores, like carbon,
oxygen and nitrogen into the coldness of space.
The atoms cooled down to a low enough temperature such that chemical bonds could form between
them.
These initial reaction would have produced some of the essential molecules for life,
like carbon dioxide, methane, hydroxyl and water.
This is the simplified version of events.
Reality as always, is a bit more complicated.
For example, water can be made in different ways.
In fact, most water we see in the universe today comes from reactions on dust grains.
By dust, I mean small amounts of various heavy elements.
The surface of these dust grains can contain different molecules such as just oxygen molecules.
And hydrogen, which is very abundant in space, could react with oxygen to form ice on the
dust grains.
But the early universe didn’t have such dust because this dust only formed after the first
stars exploded and spewed the universe with heavier atoms.
And this didn’t happen at least tens or hundreds of millions of years after the
big bang.
The ultimate source of most of the water on Earth would be what formed on these dust grains
in interstellar clouds that I just described.
There is in fact a lot of water in space just floating around as ice from this process.
Water is essential, but we need more than just water for life. We need more complex
organic molecules.
Where did they come from?
The first more complex organic molecules could have formed in the earth's atmosphere around
4 billion years ago, a few hundred million years after the earth formed.
There is scientific evidence supporting this idea.
And it could be how organic molecules came to earth, which led the way to the formation
of life which would appear about 500 million years after this process.
There is also the possibility that the first organic molecules necessary for life could
have come from outside Earth, and not via terrestrial chemical processes.
The most recent observational evidence shows that organic compounds seem to be ubiquitous
in space: they are found in diffuse clouds, around evolved stars, in dense star-forming
nebulae, in protoplanetary disks, in comets, in meteorites and interplanetary dust particles.
This evidence seems to indicate that organic molecules may not be all that special in the
universe.
They may be everywhere.
But this picture is not quite complete because we don’t yet fully understand how these
interstellar organic molecules were formed.
One hypothesis is that radiation from nearby stars knocks out electrons of heavier atoms,
making them more reactive.
So, if there is something nearby to react with, a chemical reaction will take place
forming more complex organic molecules.
So newly formed stars could be accelerating the formation of complex molecules essential
for life by ionizing atoms with radiation causing chemical reactions.
This would explain the organic molecules found in star forming nebulae, and protoplanetary
disks around newer stars.
So although this is not settled science, we have a lot of observational evidence and hypotheses
for the mechanism.
The James Webb Telescope will perhaps help us solve some of these mysteries.
The future will be exciting and there is more reason than ever to study science.
And if you want more videos like this, please subscribe to our channel.
It really helps us out.
And If you have any question, please put them in the comments section, because I try to
answer all of them.
I’ll see you in the next video my friend.