- There isn't really a very good definition of life.
What is the difference between something that's alive,
and something that isn't alive?
You might think this is a rather simple question to answer,
but in fact, it's not so easy to answer.
Should be said, I'm by no means the first person
to have considered this question.
And if you look in Wikipedia or in a dictionary,
you often get rather complex answers.
And I wanted a go answering
what I think is one of the key questions in biology.
And the way I approached it
was by exploring five of the great ideas of biology.
My name is Paul Nurse.
I'm a geneticist and cell biologist,
and I've just written a book-
it's my first book, and it's called, "What Is Life?"
The cell is really critical for the way I think about life
because it's the simplest entity
that expresses characteristics of life.
It can grow, it can divide, it can reproduce.
And really, all living things-
myself, a plant, an insect-
is either a single cell
or made up of groups of cells acting together.
Most people are not excited by yeast.
I have to say, I am excited by yeast.
They just think, "Well, it's good for making beer
and wine and bread maybe,"
but actually, it's very good as a model for other cells
in all sorts of more complicated living things
like ourselves.
In my laboratory, we found a human gene
that was similar to the yeast gene
that controls the reproduction of a cell from one to two.
And it was clear that that human gene
could completely substitute for the yeast gene,
and control the reproduction of a yeast cell just as well
as it could control the reproduction of a human cell.
If the same process
was controlling the reproduction of a yeast cell
as controls the reproduction of a human cell,
then this means this was extremely ancient
because yeast and human beings diverged
back in the depths of time, deep time,
somewhere between 1,000 million years ago,
and 1,500 million years ago.
This meant that every other living thing that we could see-
every plant, every animal, and every fungus-
was controlled by the same mechanism.
A monk called Gregor Mendel was interested
in looking at the genetics of peas-
and what he did is he crossed different peas
with different characteristics.
And he counted the different types of offspring
that were produced from these pea plants.
And he began to notice
there were very clear ratios:
famous ratios like three to one,
and nine to three, to three to one.
And he realized that maybe
what was responsible for inheritance could be described
as sort of unitary particle
which could be passed down from one plant to another.
But it was the first evidence
that there was something which we now would call a gene.
The idea of evolution by natural selection is, for me,
probably the most beautiful idea in biology.
Charles Darwin, who went around the world
and collected animals and plants and birds,
came up with the idea of evolution by natural selection.
He speculated that all living things
have hereditary material;
that if this hereditary material had some differences
that resulted in the living thing being different,
and if it was advantageous
in the environment in which it found itself,
then it would eventually take over the whole population.
This has been a revolutionary change
in our understanding in biology
because what it leads to is a better-designed living thing-
but without having a designer.
Life is made up of molecules and chemicals.
There are many thousands of chemical reactions
going on all the time in a little cell.
Chemistry is responsible for the growth of the cell,
the reproduction of that cell,
for its ability to capture energy and use energy.
One of the ways in which cells
can carry out all of this chemistry
is that the cell is made up of lots and lots
and lots of little compartments,
and it's compartmentation
that allows all these different chemistry
to occur simultaneously in such a small space.
I want to emphasize how fantastic that is.
I want you to imagine in a tiny, tiny cell,
there's thousands of chemical reactions,
all very different going on all the time
very, very close to each other.
So life is chemistry, but life is also built on information-
because life has to constantly manage information.
Let's take what I said about chemistry
and all those different compartments:
That can only occur if there's an information transmission
to keep the whole thing coordinated.
Let's say a gene is making a substance,
and it wants to keep that substance at a constant level.
As that substance increases, it switches off the gene,
and when it decreases, it turns the gene on.
But you only really understand that when you realize
there's information management going on here.
The cell is measuring how much stuff it's got,
and then using that to determine
whether the gene should be switched on or switched off.
If we take DNA,
we could understand the structure of DNA
in terms of how one base is related to another base,
but it only makes biological sense
when seen to be a digital information storage device.
Life is and can only operate through information.
It permeates every aspect of how living things work.
Putting all these five ideas together,
I think some principles emerge.
Living things are bounded physical entities.
The bounded entity is the chemical
and informational machine.
And then critically, that informational chemical machine
in a bounded entity has a hereditary system
that determines how it works,
a system which has variability,
and therefore, the whole thing can evolve
by natural selection.
And that means that the living thing can acquire purpose:
Purpose to be better adapted
in the life state it finds itself,
and so we can evolve living things
from one type into another.
And for me, that bounds everything together
that emphasizes the core principles underlying life.
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