Translator: Rhonda Jacobs Reviewer: Denise RQ
When you look out into the Universe and see something as remarkable as this,
billions of stars, each potentially hosting worlds that we know nothing about,
it's only natural to wonder what's going on out there.
And to me, the most essential, most fundamental question
is whether or not there's life beyond Earth.
To answer that question,
we need to know what exactly we're talking about.
What is life?
Figuring that out is actually kind of hard,
which is surprising if you think about it.
We see life all around us every day.
We all have long-term, up-close introspection
into what it's like to be a living being.
We are inextricably tied to one for literally our entire lives,
and we carry billions of other organisms around in our microbiomes
wherever we go.
Nonetheless, we still don't have a very good definition
of what life really is.
Most definitions are descriptive,
and they tell us the laundry list of properties of life,
but they don't necessarily get at its essence.
It's hard to come up with a definition
that encompasses everything that we know about life,
but excludes phenomena that we know to be abiotic.
So, for example, we'll look at a couple of examples
of what some of these properties are.
Structure - we know that life builds up structures
by resisting the entropic trend toward disorder
to create different domains, physically separating locations.
But, the same could be said about minerals
that precipitate along molecular-templated boundaries.
Energy - life uses chemistry to move electrons around
and dissipate energy in a structured way in order to move and to spread.
But the same could be said about fire.
Self-propagation - biology makes more biology
and passes down an instruction manual
in the form of a genetic code to subsequent generations.
But, this is also true with computer viruses
which propagate through the digital realm.
So each of these individual criteria has some problems,
but when we start to combine them,
we can weed out most of the false positives.
So this is NASA's current working definition of life,
"A self-sustained chemical system
capable of undergoing Darwinian evolution".
And this seems pretty good when we think about the long-term extended process
like billions of years of life on Earth,
but it also breaks down when we think about snapshots,
when we only have limited amounts of time to observe the system
which is what we would expect
with some sort of astrobiological space mission.
For example, a deer running through a rain forest
has all the properties of life,
but it is not undergoing Darwinian evolution in a visible sense.
We need time, we need generations, and we need heredity to see that process.
So, all of these discussions about definitions and descriptions
lead to a really interesting discussion
of whether or not this even a useful process.
Should we be coming up with a definition of life?
Is that actually a productive thing to be doing?
What maybe would be more useful is to come up with an underlying theory
of what life actually means.
That gets us to an interesting question of what definitions do.
They tell us about the meaning of a word,
but they don't really convey a deeper understanding of the world.
For example, water used to be defined
as a clear liquid that can dissolve various substances,
but it was only with molecular theory that we could tell what it really is:
two hydrogen atoms bonded to one oxygen atom.
In the case of coming up with a similar overarching theory for life,
we're still in the middle of that process,
grasping for the contours of what those ideas really mean,
so it's unclear what that looks like.
But already we know that life is capably of amazing things,
and that's because in parallel with this theoretical discussion
there have been scientific expeditions all over the world
that are pursuing life in its most extreme habitats
and expanding our understanding of what that actually means.
And that's where most of my research has taken place.
I'm going to describe a couple of locations
where we're probing the boundaries of the biosphere
in order to hopefully get some more insight
into the essence of life.
This is a patch of the sea floor about 100 miles off the coast of Oregon
called Hydrate Ridge.
And here, there's a lot of methane
that's streaming up out of the Earth's crust and into the sea water.
And from a biological perspective, methane is a tough nut to crack.
It's very energy poor, especially in these anoxic environments,
and it's a stable molecule.
But if you pair that metabolism with a different one from another organism
that's using sulfate in the sea water, the whole thing becomes more possible.
So this symbiosis is dragging one metabolism along
on the strength of another.
Energetically extreme locations like this are telling us
that thermodynamics might not be as deterministic as we think,
that you can bring something that might be in the red along,
if you have something that's sufficiently energetic to do so.
Another way to think about the limits of life
is to start with a completely sterile surface,
something that has no biological material in it.
And that's something that we're able to do in lava lakes in volcanoes,
like this one, which is the largest and most active one in the world
in the Pacific island nation of Vanuatu.
And here on an expedition led by Sam Cossman late last year,
we were able to descend down into the crater and collect samples
immediately after they had erupted.
This way, we know that they're starting at zero.
These materials, once they solidify
and cool to below about 120 degrees Celsius,
become habitable environments.
So by tracking this time series from about 30 second old
up to minutes, hours, days, years, we're able to see
what is the limiting factor for biology at each of those time points,
and if we can extend that idea to other locations.
This idea of coming up with an overarching theory of life
has two experimental dimensions to it.
One is the idea of going out into the world
to these extreme locations as I've been describing;
but the other is looking more at the techniques we're using to do so.
And whether it's microscopy, or spectroscopy,
or different types of genetic analyses,
we're getting better at looking for things that we might have missed before.
In the last few years, there's been a whole new class of giant viruses
that have been discovered in even the most prosaic of samples.
So these are new types of grey area continuum types of life
or pseudo-life that we hadn't seen before.
And it's this kind of analysis, this analytical power
that is allowing us to spread that continuum
and see how things really sort out.
And I think that we're not really going to find a bright dividing line
between living and non-living,
that this continuum is probably more the rule rather than the exception.
So as of right now it's kind of hard
to say that we have a strong, iron-clad definition of life,
and I don't really think we need one, but we do need something,
and that's because the search is already happening.
We need some kind of quantifiable criteria
to use to develop instruments that we're sending out into space,
and to interpret the data that they're sending back.
So this idea of coming up with a theory of life is critical,
but it still has a little ways to go.
And we know that if we look at different types of life on Earth,
and by looking for new ways
of doing those same kind of traits that we had imagined,
there might actually be a sort of alien life on Earth.
If we think about DNA for example,
we know there are the As, Ts, Gs and Cs,
but there could be other letters that do similar processes
of passing along information in a similar way
that we're just missing because of the way that we look at DNA right now.
This could be a whole different type of life
that has been there all along
and I could even be in this room right now.
So as we look outward into the Universe,
as we look deep into the bottom of the ocean,
into the more perilous pits on the planet, we're really learning more about ourselves
and our lived biological experience, how it relates to the environment,
and what life really means in our particular context.
At the moment, terrestrial biology is unique.
But is that the case, or is it just because we haven't found anything else?
If we really hope to go out, and recognize, and understand
life beyond Earth, we need to know more about ourselves and biology on Earth.
Thank you.
(Applause)