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 - "But if (and oh! what a big if!)
"we could conceive in some warm little pond,
"with all sorts of ammonia and phosphoric salts,
"light, heat, electricity, etc., present, that a proteine compound
"was chemically formed ready to undergo
"still more complex changes..."
So, he's just saying - imagining - that life could emerge.
"...at the present day such matter would be instantly devoured or absorbed,
"which would, not have been the case
"before living creatures were formed."
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.