So, in deciding how one is going to
go about studying the origin of life,

a key initial consideration
is the question -

how hard or easy is it
for life to emerge?

On the one extreme,
is it a case that you require

very specific circumstances
and perhaps a very long period of time

for life to emerge?

Or, the other extreme -
is life a very easy phenomenon

that you could readily study
in the laboratory?

Of course, this isn't really a dichotomy -

it's actually a continuous range
of possibilities.

From the one extreme,

in the cases where life requires
very specific circumstances

and maybe billions of years to emerge,
to the other possibility,

of life occurring relatively easily
in a broad range of conditions.

And, they imply different things.

On the one extreme,
we would expect that planets -

even planets that seem similar to Earth

and have been around
for a very long time -

to not have life.

At the other extreme,
we might imagine

that we can see
the emergence of life

in diverse natural
and laboratory settings.

And so, we need to have some sense
as to where we sit on this continuum.

So, historically, the field began
with the notion that life was hard.

I think the assumption was
that life is a very special phenomenon

and probably requires
very special circumstances to emerge.

And, a key piece of evidence for this
came from looking at cellular life -

life as we know it -
and realizing that all that complexity

and diversity traces back
to a single common ancestor,

suggesting that there is no evidence
for any more

than a single origin of life
on this planet,

despite the fact that this planet
is 4.6 billion years old,

which would lead you to expect
that if life were easy...

there would be multiple,
independent kinds of life on this planet.

So, that's sort of with a prevailing view,
but over time,

this view has become eroded
from a number of sources.

So, the first issue that's come
to people's attention is that,

as geologists have dug into deeper
and deeper rocks

and developed more and more
sophisticated methods

to look for indirect evidence
for the existence of life,

they have continually found life
in older and older rock

to the point where
we now don't know of any rock

or geological formation
that doesn't have evidence of life.

So, for example,
this is - the arrows show -

some carbon inclusions
in a 4.1 billion year old zircon,

and the isotopic ratios in this carbon
suggest that

that carbon had been through life.

So, even if one takes a view that life -

that gave rise to modern life -
arose only once,

it nonetheless happened
surprisingly quickly

after the planet became hospitable,

and given that the earliest evidence
of water on the planet

is around this period of time,
4.1 billion years ago.

So... to kind of help us think through

how we can reconcile that observation
that life only arose once -

but it arose very, very early -

we can actually turn to Charles Darwin
who was thinking about the origin of life

in some letters to his friend, Hooker.

And, he wrote -
<i>"But if (and oh! what a big if!)</i>

<i>"we could conceive
in some warm little pond,</i>

<i>"with all sorts of ammonia
and phosphoric salts,</i>

<i>"light, heat, electricity, etc., present,
that a proteine compound</i>

<i>"was chemically formed
ready to undergo</i>

<i>"still more complex changes..."</i>

So, he's just saying - imagining -
that life could emerge.

<i>"...at the present day such matter
would be instantly devoured or absorbed,</i>

<i>"which would,
not have been the case</i>

<i>"before living creatures were formed."</i>

So, what Darwin noticed
was that life preempts life.

So that, if we accept that life..
our cellular life

originated a long time ago,
it's very plausible that

it would make it much more difficult
for additional instances of life

to emerge and become established
to the point

where they could leave something
for us to see.

So, the fact that life only arose once

is not evidence - concrete evidence -
that life is hard.

So, another thing
we should bear in mind is -

do we know for sure that all life
on this planet is cellular life?

It turns out that we don't really have
the clearest ways

to identify kinds of life that lack
features we're familiar with -

that lack cells, that lack DNA.

And so, we should be open to
the possibility that -

in some, perhaps remote,
part of this planet,

maybe deep in the crust -
we'll find systems and structures

that deserve to be called life,

they just haven't yet had
the opportunity to get to the point

where we can recognize them
as obvious instances of life.

And, the final point to bring to bear
is actually the theory,

which over time has led people
to be more and more confident

that there might be
relatively diverse ways

in which life can spontaneously emerge.

And, as a result,
many of us nowadays

are quite open to the possibility
that life is considerably easier

than we used to think.

And, this is exciting,

because it means that,
over the next few decades,

as origin of life researchers
and astrobiologists study,

both in natural environments
on this planet -

maybe visit other planets -
and conduct laboratory investigations,

there is, I think, a very high probability
that we may be able to find

other instances of life
or life-like systems.