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