So, in everyday discourse the term "life" is not ambiguous.
When we point at something and label it "alive,"
or an "instance of life," we usually know what we mean.
But, when it comes to the science and the philosophy of life,
this is not so trivial.
And, in fact,
scientists and philosophers have debated for a long time
what we really mean by the term "life."
And, of course, we have a very clear reference point.
The reference point is cellular life - life as we know it on this planet,
which shares a number of distinctive features.
And, we can make a very long list of the features
that all life as we know it share,
and some of them are very specific.
So, for example -
they all use a nucleic acid code with the same bases;
they all use the same 20 amino acids to make their proteins;
and they're always the left-handed variants.
They will have a very similar genetic code,
similar machinery - ribosomes - for making proteins,
and they have similar biochemistry, and even they share particular genes.
So, on the one hand, it's kind of exciting to realize
that all life has these unique and shared properties,
because it tells us something fundamental about life as we know it,
which is that life as we know it traces back to a single common ancestor.
So, there was, at some point in the past,
an ancestral lineage that gave rise to all three branches of life
that we know of today - bacteria, archaea and eukaryotes.
And, the traits that we see in all life as we know it -
that list I just gave you and more -
all predate the last universal common ancestor,
the last organism that was ancestral to all three of those lineages.
So, that's an exciting and important insight,
but it also poses a slight problem, which is -
if life as we know it has a very long list of shared traits.
But, it doesn't necessarily guide us as to ask the question -
suppose you found some other lifelike system,
maybe on another planet or in some strange environment,
you wanted to decide -
"Is this thing alive? Is it an instance of life?" -
you presumably wouldn't care about this full laundry list.
These particular traits are the result of the historical factors
that occurred in the origins of this life.
and we want some more general understanding of life
to answer the question -
is something else another instance of life?
So, people have tried to approach this by whittling down those specific traits
to generalities - general features - that life seems to have
that distinguish these instances of life
from instances of non-life - inanimate matter.
And if, you open any sort of biology textbook,
you'll have a list of the features of life,
and they vary in number from five, seven, eight, whatever.
And, this has been the starting point for a process in the origin of life field
of thinking about - how low can we go?
What, at core, are the essential features of something to be considered to be
an instance of life in the general sense?
Now, there might be some difference of opinion,
but, by and large, the field has converged
on two key features, which are captured
in this definition I've given here.
This definition is based on one that was kind of established by NASA
to help guide them in the question of looking on other planets
and deciding whether there is life there.
And so, the two key features of this definition are -
first, that life is a self-propagating chemical system,
meaning it's a system that makes more of itself over time - or least can;
and, secondly, we expect that this system
is capable of undergoing adaptive evolution.
So, let's look at those two pieces in turn.
The idea of self-propagation is that you have some kind of chemical system
that can make more of itself and occupy additional areas of space.
It doesn't much matter whether we're thinking about growth,
which is where we have a single, living protoplasm
that expands to encompass more space,
or if we're thinking about the process of making multiple cells
from a single parent cell.
The only difference between these phenomena
is whether or not the newly formed protoplasm - living material -
either is not sort of packaged up into cells.
And so, what we expect of anything that we would label to be "alive" -
is that it has some capacity to propagate itself spatially.
But, of course, that isn't really enough
because we know of quite a few systems that can self-propagate,
but we usually wouldn't call them "alive" -
for example, a crystallization process.
And, that's where the second part of the definition comes in -
and that is this idea that things that we want to consider to be alive
also have the capacity for adaptive evolution...
which means that they have the capacity
to get better at self-propagating over time.
This is an important idea and something that we don't see in,
for example, crystallization processes, because it explains
how it is that life as we know it became so complicated
and out of equilibrium with their chemical environments.
Cells and living systems that we know are quite sophisticated
and certainly far from being typical of the chemical environment around them.
And, this is possible because, during the adaptive process,
the variants arise, and the fate of those variants
is independent of whether they are of raised or lower complexity.
Natural selection, the driving force of adaptive evolution,
will select the trait that confers higher fitness
whether or not it's a more complicated trait.
Furthermore, there are many instances in which we know
that the more complicated variants have an advantage.
As a result, over long, long periods of time,
adaptive evolution can explain how a simple self-propagating system
can become more and more complex.
So, putting these two pieces together,
it's fair to say that the origin of life field,
in general, has a great deal of attention...
that it pays to these two properties -
self-propagation and adaptive evolution.
We try and understand how they come about,
and we try and understand the kinds of planetary or chemical conditions
in which we might expect self-propagation and evolution
to emerge spontaneously.