One of the unsolved puzzles of Astrobiology is how did life originate?
Where did the building blocks for life come from? The central conundrum that
astrobiologists face in trying to address this is question is how do we go from
basic molecules to a self-replicating microorganism. And I should say right from
the beginning we don't know the answer to this problem, but many significant strides
have been made by astrobiologists. In understanding where those building blocks
came from and how they might have been assembled into the earliest life forms. So
there are several possibilities of how life might have originated and here are
three of them. The first one is some supernatural or divine intervention. Life
was created by some Supernatural force. Well whether you believe that or not, it
doesn't get off very far. It doesn't explain how the building blocks of life
were created and how they might have assembled into the early living organisms
on the Earth. So even if you were to believe this, it doesn't get away from the
central astrobiological problem of understanding those building blocks.
Another response to how did life originate might be it originate elsewhere and was
transferred to the Earth, maybe on comets or in asteroid impacts. This is a
possibility, we can't prove at the current time that life originated on the Earth,
but it still doesn't get us away from the problem of how did life originate in that
distant location? Now of course there's a problem there because if it originated on
a planet with very different conditions, then there had been conditions on some
different world who were much more conducive to the origin of life. But
without the ability to test that at the current time, without the ability to find
other planets and prove that life originated there, we're left with our
final possibility and that's that life originated on the earth. And, it's that
assumption that we use in trying to understand the origin of life, and at
least constrain the sort of experiments that astrobiologists do, in trying to
understand how building blocks were assembled in early life and how they came
together. So the first question we might ask as astrobiologists is where did the
building blocks of life come from and we seen in an earlier lecture that life is
constructed from building blocks such as amino acids and sugars. One of the
earliest experiments to look at where the building blocks of life might have come
from was carried out by Harold Urey and Stanley Miller in 1952, and it's really
become an iconic experiment. In the design of this experiment they take a container
with water and they boil that water, and the water travels through a tube to
another container that contains a simulated early Earth atmosphere. And that
atmosphere has methane, hydrogen, carbon monoxide and ammonia. And across that
chamber they have an electrical discharge that put energy into the gasses. And these
chemical reactions occur as the gasses are electrified to this discharge and as the
water passes through them. They carried out this experiment and after a few days
and weeks they found the production of yellow substance almost a tarry substance
in their experiment. And they removed that substance and analysed it and they found
that it contained amino acids as well as other complex chemical compounds. This
experiment shows that you could potentially create the building blocks of
life, amino acids, the building blocks of proteins in simple experiments such as
this. Well, nowadays, we know that the early Earth's atmosphere was slightly
different from the atmosphere used by Urey and Miller. We think it was less
reducing. In other words, it had less of the compounds, ammonia, hydrogen, carbon
monoxide. It may have been more oxidizing. More content of carbon dioxide in
nitrogen. But nevertheless, this experiment was really a turning point in
Astrobiology. Because, for the first time, it allowed us to think about the creation
of these, the formation of these building blocks of life in simple chemical
reactions. It turns out that amino acids are really everywhere in the universe. We
find them in meteorites that land on the surface of the earth. This is an image of
the Murchison meteorite that landed in the earth in the late 1960's. And it was found
to contain amino acids. Aspartic acid, glutamic acid, and many other types of
amino acids that are used in biology as the building blocks of proteins. It turns
out that these meteorites that also contain amino acids that are not used by
biology. Non-biological amino acids. It seems that life has selected a subset of
the available amino acids to be found in the extraterrestrial environment. And
since this early work, many other. Meteorites. Carbonaceous chondrites have
also been found to have amino acids. Suggesting that these are common
constituents of extraterrestrial material. What does all this tell us? It tells us
that the building blocks of life could have been produced on the
Earth, endogenously in chemical reactions on the surface of the early Earth. And it
also tells us that the building blocks of life could have arrived on the Earth in
extraterrestrial materials in meteorites. So we have two sources of the
building blocks of life for early chemical reactions. Amino Acids are the building
blocks of Proteins, and that's significant if you think that the early history of
life began with Proteins building up with cellular materials and finally into life.
Another question is where do nucleic acids come from, such as DNA, Deoxyribonucleic
Acid that we talked about in the earlier lectures, as the information storage
system of life. Well, there's another type of Nucleic Acid called RNA,
Ribonucleic acid it's very much like DNA. Instead the Thymine of that four letter
code in DNA is replaced by Uracil it has a four letter code, Adenine, Guanine, Uracil
and Cytosine. These four letters come together in a chain a bit like DNA,
forming these strands of RNA. RNA is a very interesting molecule because it seems
to be a little bit more primitive than DNA. What's particularly interesting about
RNA is it can self-assemble into molecules called ribozymes, and these ribozymes can
carry out chemical reactions even including self-replication. And one idea
is that the early life on earth began from RNA an RNA world as it's been called.
These RNA molecules would be floating around perhaps in pools of water on the
early earth carrying out chemical reactions replicating and leading to early
genetic systems, which would of eventually a transition to more complex chemical
systems and eventually to early cells. One of the questions that astrobiologists
address is, which came first, a protein world proteins constructed from these
building blocks of amino acids that may have come from space, or endogenously
produced in environments and earlier, or did life start in this RNA world produced
from these early nuclear gasses. Well it may well be that both came together.
Perhaps proteins and RNA were necessary to construct the earliest cells and these
reactions were occurring at the same time. This is one of the questions that
astrobiologists currently seeking to address. What were the earliest molecules
that led to the earliest forms of chemistry, that led to early lifeforms,
and in what sort of environments were these molecules produced? We saw that in
an earlier lecture, we need the building blocks of life, amino acids to build
proteins. Nucleotides to build nucleic acids. But we also so that life needs
membranes in order to create cells. And one question is, how do you get membranes?
Membranes would've been very important for early life on earth to enclose those early
chemical systems preventing them from dissipating into the environment simply
becoming diluted in the early Were early water bodies on the earth. How would these
early chemistries have been enclosed in membranes? Well, experiments studying the,
the production of early membranes have shown that you could produce membranes
very, very easily. Scientists have taken materials from simulated interstellar ice
And even materials directly from meteorites. And they've adde d these
materials to water, and found that membranes can spontaneously form from the
lipids inside these materials. It turns out that meteorites contain lipids, and
these lipids, When they're added to water, the tails of the lipids are attracted
towards each other. The heads of the lipids, which are hydrophilic, they like
water, are attracted towards the water, and you get this spontaneous formation of
vesicles in water, early membranes, which could have enclosed early chemical
reactions on the earlier So in a very simplistic way we could think some sort of
scheme of meteorites coming in from space and other types of material, landing in
the early oceans and dissipating in the water. And perhaps chemical reactions on
the early Earth as well. Forming lipids that spontaneously form membranes,
vehicles in which chemical reactions could occur. Amino acids, formed from early
chemical reactions on the Earth, and also brought in by meteorites, would have
provided the building blocks for ealry protiens, and other chemical reactions
could have provided the early building blocks for sugars and nucleic acids, RNA
and eventually DNA. And in these sorts of environments, These materials would have
assembled to form early cells. Well of course you can tell by the way I say that
it's very hand waving and very speculative. In truth we really don't know
how these membranes and these early building blocks came together to form a
self-replicating cell. It's one thing to create a membrane and to use that membrane
to enclose some basic chemical reactions. But going from that to an organism
self-replicating looks something like bacteria or archaea that we're familiar
with on today's Earth is another matter altogether. And it remains one of the
great challenges in Astrobiology. Understand that transition from chemical
world to biological world. So what do we learn in this lecture? We have learned
that the building blocks of life could be produced in simulated early earth
conditions. We have also learned those building blocks could be found in
extraterrestrial material. So it could have been produced on the early earth.
They also could have been delivered to the surface of the early earth. In
extraterrestrial materials. We've also learned that primitive membranes that can
enclose chemical reactions necessary for the first cells to form can be easily made
in the laboratory, from simple molecules, even meteoritic materials, and simulated.
Materials formed on interstellar ices. But we've also learned that the order of the
emergence of the building blocks is not known. But what is important is that
Astrobiology has now at least identified plausible sources of these early building
blocks and some of the processes by which they may have come together and led to the
first life forms on earth.