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One of the unsolved puzzles of
Astrobiology is how did life originate?

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Where did the building blocks for life
come from? The central conundrum that

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astrobiologists face in trying to address
this is question is how do we go from

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basic molecules to a self-replicating
microorganism. And I should say right from

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the beginning we don't know the answer to
this problem, but many significant strides

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have been made by astrobiologists. In
understanding where those building blocks

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came from and how they might have been
assembled into the earliest life forms. So

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there are several possibilities of how
life might have originated and here are

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three of them. The first one is some
supernatural or divine intervention. Life

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was created by some Supernatural force.
Well whether you believe that or not, it

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doesn't get off very far. It doesn't
explain how the building blocks of life

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were created and how they might have
assembled into the early living organisms

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on the Earth. So even if you were to
believe this, it doesn't get away from the

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central astrobiological problem of
understanding those building blocks.

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Another response to how did life originate
might be it originate elsewhere and was

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transferred to the Earth, maybe on comets
or in asteroid impacts. This is a

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possibility, we can't prove at the current
time that life originated on the Earth,

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but it still doesn't get us away from the
problem of how did life originate in that

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distant location? Now of course there's a
problem there because if it originated on

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a planet with very different conditions,
then there had been conditions on some

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different world who were much more
conducive to the origin of life. But

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without the ability to test that at the
current time, without the ability to find

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other planets and prove that life
originated there, we're left with our

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final possibility and that's that life
originated on the earth. And, it's that

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assumption that we use in trying to
understand the origin of life, and at

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least constrain the sort of experiments 
that astrobiologists do, in trying to

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understand how building blocks were
assembled in early life and how they came

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together. So the first question we might
ask as astrobiologists is where did the

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building blocks of life come from and we
seen in an earlier lecture that life is

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constructed from building blocks such as
amino acids and sugars. One of the

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earliest experiments to look at where the
building blocks of life might have come

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from was carried out by Harold Urey and
Stanley Miller in 1952, and it's really

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become an iconic experiment. In the design
of this experiment they take a container

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with water and they boil that water, and
the water travels through a tube to

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another container that contains a
simulated early Earth atmosphere. And that

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atmosphere has methane, hydrogen, carbon
monoxide and ammonia. And across that

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chamber they have an electrical discharge
that put energy into the gasses. And these

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chemical reactions occur as the gasses are
electrified to this discharge and as the

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water passes through them. They carried
out this experiment and after a few days

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and weeks they found the production of
yellow substance almost a tarry substance

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in their experiment. And they removed that
substance and analysed it and they found

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that it contained amino acids as well as
other complex chemical compounds. This

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experiment shows that you could
potentially create the building blocks of

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life, amino acids,  the building blocks of
proteins in simple experiments such as

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this. Well, nowadays, we know that the
early Earth's atmosphere was slightly

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different from the atmosphere used by 
Urey and Miller. We think it was less

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reducing. In other words, it had less of
the compounds, ammonia, hydrogen, carbon

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monoxide. It may have been more oxidizing.
More content of carbon dioxide in

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nitrogen. But nevertheless, this
experiment was really a turning point in

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Astrobiology. Because, for the first time,
it allowed us to think about the creation

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of these, the formation of these building
blocks of life in simple chemical

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reactions. It turns out that amino acids
are really everywhere in the universe. We

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find them in meteorites that land on the
surface of the earth. This is an image of

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the Murchison meteorite that landed in the
earth in the late 1960's. And it was found

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to contain amino acids. Aspartic acid,
glutamic acid, and many other types of

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amino acids that are used in biology as
the building blocks of proteins. It turns

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out that these meteorites that also
contain amino acids that are not used by

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biology. Non-biological amino acids. It
seems that life has selected a subset of

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the available amino acids to be found in
the extraterrestrial environment. And

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since this early work, many other.
Meteorites. Carbonaceous chondrites have

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also been found to have amino acids.
Suggesting that these are common

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constituents of extraterrestrial material.
What does all this tell us? It tells us

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that the building blocks of life could
have been produced on the

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Earth, endogenously in chemical reactions
on the surface of the early Earth. And it

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also tells us that the building blocks of
life could have arrived on the Earth in

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extraterrestrial materials in
meteorites. So we have two sources of the

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building blocks of life for early chemical
reactions. Amino Acids are the building

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blocks of Proteins, and that's significant
if you think that the early history of

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life began with Proteins building up with
cellular materials and finally into life.

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Another question is where do nucleic acids
come from, such as DNA, Deoxyribonucleic

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Acid that we talked about in the earlier
lectures, as the information storage

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system of life. Well, there's another
type of Nucleic Acid called RNA,

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Ribonucleic acid it's very much like DNA.
Instead the Thymine of that four letter

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code in DNA is replaced by Uracil it has a
four letter code, Adenine, Guanine, Uracil

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and Cytosine. These four letters come
together in a chain a bit like DNA,

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forming these strands of RNA. RNA is a
very interesting molecule because it seems

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to be a little bit more primitive than
DNA. What's particularly interesting about

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RNA is it can self-assemble into molecules
called ribozymes, and these ribozymes can

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carry out chemical reactions even
including self-replication. And one idea

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is that the early life on earth began from
RNA an RNA world as it's been called.

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These RNA molecules would be floating
around perhaps in pools of water on the

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early earth carrying out chemical
reactions replicating and leading to early

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genetic systems, which would of eventually
a transition to more complex chemical

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systems and eventually to early cells. One
of the questions that astrobiologists

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address is, which came first, a protein
world proteins constructed from these

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building blocks of amino acids that may
have come from space, or endogenously

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produced in environments and earlier, or
did life start in this RNA world produced

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from these early nuclear gasses. Well it
may well be that both came together.

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Perhaps proteins and RNA were necessary to
construct the earliest cells and these

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reactions were occurring at the same time.
This is one of the questions that

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astrobiologists currently seeking to
address. What were the earliest molecules

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that led to the earliest forms of
chemistry, that led to early lifeforms,

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and in what sort of environments were
these molecules produced? We saw that in

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an earlier lecture, we need the building
blocks of life, amino acids to build

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proteins. Nucleotides to build nucleic
acids. But we also so that life needs

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membranes in order to create cells. And
one question is, how do you get membranes?

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Membranes would've been very important for
early life on earth to enclose those early

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chemical systems preventing them from
dissipating into the environment simply

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becoming diluted in the early Were early
water bodies on the earth. How would these

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early chemistries have been enclosed in
membranes? Well, experiments studying the,

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the production of early membranes have
shown that you could produce membranes

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very, very easily. Scientists have taken
materials from simulated interstellar ice

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And even materials directly from
meteorites. And they've adde d these

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materials to water, and found that
membranes can spontaneously form from the

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lipids inside these materials. It turns
out that meteorites contain lipids, and

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these lipids, When they're added to water,
the tails of the lipids are attracted

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towards each other. The heads of the
lipids, which are hydrophilic, they like

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water, are attracted towards the water,
and you get this spontaneous formation of

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vesicles in water, early membranes, which
could have enclosed early chemical

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reactions on the earlier So in a very
simplistic way we could think some sort of

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scheme of meteorites coming in from space
and other types of material, landing in

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the early oceans and dissipating in the
water. And perhaps chemical reactions on

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the early Earth as well. Forming lipids
that spontaneously form membranes,

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vehicles in which chemical reactions could
occur. Amino acids, formed from early

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chemical reactions on the Earth, and also
brought in by meteorites, would have

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provided the building blocks for ealry
protiens, and other chemical reactions

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could have provided the early building
blocks for sugars and nucleic acids, RNA

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and eventually DNA. And in these sorts of
environments, These materials would have

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assembled to form early cells. Well of
course you can tell by the way I say that

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it's very hand waving and very
speculative. In truth we really don't know

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how these membranes and these early
building blocks came together to form a

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self-replicating cell. It's one thing to
create a membrane and to use that membrane

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to enclose some basic chemical reactions.
But going from that to an organism

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self-replicating looks something like
bacteria or archaea that we're familiar

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with on today's Earth is another matter
altogether. And it remains one of the

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great challenges in Astrobiology.
Understand that transition from chemical

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world to biological world. So what do we
learn in this lecture? We have learned

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that the building blocks of life could be
produced in simulated early earth

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conditions. We have also learned those
building blocks could be found in

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extraterrestrial material. So it could
have been produced on the early earth.

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They also could have been delivered to the
surface of the early earth. In

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extraterrestrial materials. We've also
learned that primitive membranes that can

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enclose chemical reactions necessary for
the first cells to form can be easily made

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in the laboratory, from simple molecules,
even meteoritic materials, and simulated.

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Materials formed on interstellar ices. But
we've also learned that the order of the

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emergence of the building blocks is not
known. But what is important is that

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Astrobiology has now at least identified
plausible sources of these early building

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blocks and some of the processes by which
they may have come together and led to the

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first life forms on earth.