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So, in deciding how one is going to
go about studying the origin of life,
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a key initial consideration
is the question -
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how hard or easy is it
for life to emerge?
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On the one extreme,
is it a case that you require
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very specific circumstances
and perhaps a very long period of time
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for life to emerge?
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Or, the other extreme -
is life a very easy phenomenon
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that you could readily study
in the laboratory?
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Of course, this isn't really a dichotomy -
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it's actually a continuous range
of possibilities.
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From the one extreme,
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in the cases where life requires
very specific circumstances
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and maybe billions of years to emerge,
to the other possibility,
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of life occurring relatively easily
in a broad range of conditions.
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And, they imply different things.
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On the one extreme,
we would expect that planets -
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even planets that seem similar to Earth
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and have been around
for a very long time -
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to not have life.
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At the other extreme,
we might imagine
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that we can see
the emergence of life
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in diverse natural
and laboratory settings.
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And so, we need to have some sense
as to where we sit on this continuum.
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So, historically, the field began
with the notion that life was hard.
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I think the assumption was
that life is a very special phenomenon
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and probably requires
very special circumstances to emerge.
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And, a key piece of evidence for this
came from looking at cellular life -
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life as we know it -
and realizing that all that complexity
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and diversity traces back
to a single common ancestor,
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suggesting that there is no evidence
for any more
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than a single origin of life
on this planet,
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despite the fact that this planet
is 4.6 billion years old,
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which would lead you to expect
that if life were easy...
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there would be multiple,
independent kinds of life on this planet.
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So, that's sort of with a prevailing view,
but over time,
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this view has become eroded
from a number of sources.
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So, the first issue that's come
to people's attention is that,
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as geologists have dug into deeper
and deeper rocks
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and developed more and more
sophisticated methods
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to look for indirect evidence
for the existence of life,
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they have continually found life
in older and older rock
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to the point where
we now don't know of any rock
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or geological formation
that doesn't have evidence of life.
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So, for example,
this is - the arrows show -
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some carbon inclusions
in a 4.1 billion year old zircon,
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and the isotopic ratios in this carbon
suggest that
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that carbon had been through life.
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So, even if one takes a view that life -
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that gave rise to modern life -
arose only once,
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it nonetheless happened
surprisingly quickly
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after the planet became hospitable,
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and given that the earliest evidence
of water on the planet
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is around this period of time,
4.1 billion years ago.
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So... to kind of help us think through
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how we can reconcile that observation
that life only arose once -
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but it arose very, very early -
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we can actually turn to Charles Darwin
who was thinking about the origin of life
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in some letters to his friend, Hooker.
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And, he wrote -
"But if (and oh! what a big if!)
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"we could conceive
in some warm little pond,
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"with all sorts of ammonia
and phosphoric salts,
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"light, heat, electricity, etc., present,
that a proteine compound
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"was chemically formed
ready to undergo
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"still more complex changes..."
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So, he's just saying - imagining -
that life could emerge.
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"...at the present day such matter
would be instantly devoured or absorbed,
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"which would,
not have been the case
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"before living creatures were formed."
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So, what Darwin noticed
was that life preempts life.
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So that, if we accept that life..
our cellular life
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originated a long time ago,
it's very plausible that
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it would make it much more difficult
for additional instances of life
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to emerge and become established
to the point
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where they could leave something
for us to see.
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So, the fact that life only arose once
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is not evidence - concrete evidence -
that life is hard.
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So, another thing
we should bear in mind is -
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do we know for sure that all life
on this planet is cellular life?
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It turns out that we don't really have
the clearest ways
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to identify kinds of life that lack
features we're familiar with -
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that lack cells, that lack DNA.
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And so, we should be open to
the possibility that -
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in some, perhaps remote,
part of this planet,
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maybe deep in the crust -
we'll find systems and structures
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that deserve to be called life,
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they just haven't yet had
the opportunity to get to the point
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where we can recognize them
as obvious instances of life.
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And, the final point to bring to bear
is actually the theory,
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which over time has led people
to be more and more confident
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that there might be
relatively diverse ways
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in which life can spontaneously emerge.
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And, as a result,
many of us nowadays
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are quite open to the possibility
that life is considerably easier
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than we used to think.
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And, this is exciting,
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because it means that,
over the next few decades,
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as origin of life researchers
and astrobiologists study,
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both in natural environments
on this planet -
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maybe visit other planets -
and conduct laboratory investigations,
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there is, I think, a very high probability
that we may be able to find
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other instances of life
or life-like systems.