so one of the critical questions of
astrobiology is how do we model life and
we can think about abstract or general
models for life. The reason that we're
after those is because we want to not
only explain life on earth but we want
to be able to explain life on other
worlds and so we need to come up with as
general a principle as possible. So far a
lot of astrobiology has focused on the
idea of trying to define life, so we have
come up with a lot of different
definitions for life
from numerous different perspectives and
this word cloud is just showing some examples, words,
definitions for life and there are so many different definitions for life that
has really become quite a muddle as to
how we should be thinking about it in a more rigorous way.
And so one of the things that when you think about as
astrobiologists is really what's the
value of a definition versus a theory or
model and a lot of the traditional
literature and origins of life has been
focused on this idea of defining life so
that we can actually be able to identify
life on other worlds but the definitions
for life are really rather ad-hoc and in
some sense they're they're derived from
observations of life on Earth so for
example we know that life on Earth
evolved so we might have an evolutionary
definition of life or we know that life
on Earth is cellular so we might assume
that all life requires cells but what we
ultimately really need to be aiming for
in the field of astrobiology is to build
better models and theories which might be more general and allow us to move
beyond definitions of life that are
anthropocentric to our own life but
actually become predictive theories for
how life might look on other worlds the
challenge that we face with anything
trying to get beyond an anthropocentric
or human-centered or earth-centered
viewpoint is that we only have a single
example of life on Earth so despite all
the diversity of life forms that we see
trees cats people bacteria in your gut
all of that life is related by a common ancestry
and the way that astrobiologists talk about that is to talk about
something called the last Universal common ancestor
so if we look at the tree of life as shown here and we trace
the evolution of all the life-forms that
exist today backwards in time they all
converge on what's called this last
Universal common ancestor which is a
population of cells that lived on the primitive earth that we think that all
modern life descended from and the
properties that that life-form would
have had would have had to have DNA and
a translation machinery proteins and
cellular architecture much like a modern
cell so it was actually a very advanced life-form it doesn't take us all the way
back to the origins of life on earth but
the fact that all life shares is kind of
common biochemical architecture is
actually really limiting because it
means that we only have one example of
life to go on and extrapolating any kind of general principles from one example
is actually rather hard so what people
have done traditionally in the origin of life field is to try to come up with
models that are based on sort of the core
components of that architecture of life
that we know today and two of those core
components that have been dominant
models for origins of life are what are
called genetics first and metabolism
first so we know that cells metabolize
they need to acquire food from their
environment or you or I need to acquire
food in order to survive in order to
reproduce so metabolism is obviously a
critical component and then in the
genetics first view we also know that
life requires genetic information in
order to be able to reproduce and to
evolve over many generations and so if
you split these two kinds of core
components of biology and to just what
that essential thing is we have this
sort of genetics idea where people have
proposed that the first living entities
might have just been molecules like RNA
that could copy themselves and here on
the slide is shown sort of an
abstract model that kind of process
where you might just talk about the the
binary digits in a sequence if it was an
RNA molecule it would be the sequence of
ribonucleotides in the actual RNA so
that actual basis and you can actually
talk about reproducing that information
via copying and then the reason that the
RNA world
has been so popular as an idea is that
RNA also has a catalytic function
associated with it whereas in modern
organisms we have DNA and protein and
DNA controls genetic heredity and
proteins are primarily the catalysts
that actually execute reactions in the
cell, RNA can do both those functions so
genetics first has emerged of this idea
that you can model quite simply with
these kind of models where you talk
about copying and heredity and this idea
of evolution through this kind of
process as the core thing that emerged
first as a first living entity and the
metabolism first perspective there's an
alternative view that the first kinds of
living entities were not individual
molecules that could replicate but sets
of molecules that were reacting together
and could collectively reproduce due to
the organizational patterns of their
reactions, and that idea is called
autocatalytic set theory and there's an
example shown here about a catalytic
sets, using that same kind of
representation of representing molecules
as binary strings, just strings of zeros
and ones which is a way of modeling
these kinds of processes in artificial
chemistries. And so in this metabolism-
first view the first kinds of living
systems would have been these organized
patterns of chemical reactions. And so
both of these perspectives allow one to
model certain attributes of living
systems. But it's really nice if you
actually put them side-by-side and look
at something like the binary polymer
representation of them because you start
to see that both of them are different
ways of propagating information in
chemical systems, and a theme starts to
emerge about what kinds of theories
might unify different approaches to
origins of life. And so this gets back to
the idea that what we need to start
doing to move forward in origins of life
whereas traditionally we've had these
models like genetics first, metabolism
first, and there's other models like
compartment first, where we're talking about
mineral surfaces and all kinds of
things and that we need to really start thinking about what are the theories for
life, and how do we actually develop more
predictive models that are more generic
too different chemistries and allow us to actually go to the lab and predict
under what circumstances we should start getting things that look more lifelike.
And a nice example of the need for
theories and thinking about origins of
life was given by Carol Cleland and
Chris Chyba and a paper that they wrote on
defining life where they talked about
trying to define water and how difficult
it was to actually define water before
we had a molecular theory for water so
you might describe water as a clear liquid, you might describe it based on
the fact that it's liquid at a certain
range of temperatures that it you know
doesn't have a strong odor there's a lot of different ways that you could
describe what water that might lead to a
definition of water but none of them are really exclusive to water because
there's other clear liquids that you
might describe there's other things that are also a liquid at room temperature
and so the way that we really precisely
define what water is is actually to have
an atomic theory that describes
molecules and their interactions and we
can precisely define water now as h2o
and so their thought was that what we do
now is sort of phemenologically define
life, we have a lot of heuristics or a lot of ideas about what we think life
might be but ultimately what we need is
a theory and that our definition should
derive from the theory not the other way around.
And one way I like to think about
that is actually to think about like the emergent properties of life
so water for example, one of the defining properties
that we think water has is that it's wet
but wetness of water is an emergent
property it requires many many many
millions of water molecules potentially
to be wet although there's actually an active debate
about how many water molecules
if it's a few hundred a few
thousand and people have been working to
develop models to quantify when water gets wet
likewise if we're thinking about
emergent properties of life evolution is
often considered to be a defining
property of life but evolution exists at
the level of population so it requires many interacting individuals
in order to be an evolutionary
system and so in some sense evolution
that we use as a defining property of life is also an emergent property of
life and so one of the things that we
really need to challenge ourselves with
is to try to find the underlying theory
that explains that emergent property in the same way that we have an atomic
theory for water that explains some of its emergent properties
and so one of the ways that we might think about that
is actually to think about life as an
information processing system so this is
kind of a newer proposal about trying to
unify different properties of life that
a lot of people have been very enthusiastic about in the field
and a lot of people are working on from
different perspectives but if we go back
to thinking about that genetics versus metabolism picture
and we had the binary polymer model.
Both of those were kind of
a representation of an informational
system that was capable of reproducing
itself they were just very different
architectures for that kind of thing and
one might think of genetics first as a digital type of information processing
and metabolism first as an analogue type
of information processing and so so
there is this idea in the biological
community and also emerging in
astrobiology about information possibly
being a unifying principle for how we
should think about life across all
scales and that may be organisms are
really organized by flows of information
so one way to think about the origin of
life potentially as kind of a new
perspective is to think about it as a
transition and how information is stored
propagated and used and this might be
sufficiently general to be able to
predict properties of alien chemistries
that can also process information in a
similar way potentially to Earth's
biology but might allow different
chemistries than that biochemical
architecture that we have on earth as characteristic
of the last Universal common ancestor
so if we could
understand how information works in
biochemical networks on earth we could
potentially understand what other
possible chemistries could enable those
same kinds of emergent properties on
other worlds and maybe predict alien
chemistries