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What are the origins of life? How did things go from non-living to
living? From something that could not reproduce to something that
could? One person who has exhaustively investigated this subject is
paleontologist Andrew Knoll, a professor of biology at Harvard and
author of
Life on a Young Planet: The First Three Billion Years of Life. In this wide-ranging interview, Knoll explains, among other
compelling ideas, why higher organisms like us are icing on the cake
of life, how deeply living things and our planet are intertwined,
and why it's so devilishly difficult to figure out how life got
started.
A bacterial world
NOVA: When people think of life here on Earth, they think of
animals and plants, but as you say in your book, that's really not
the history of life on our planet, is it?
Knoll: It's fair to say when you go out and walk in the woods
or on a beach, the most conspicuous forms of life you will see are
plants and animals, and certainly there's a huge diversity of those
types of organisms, perhaps 10 million animal species and several
hundred thousand plant species. But these are evolutionary
latecomers. The history of animals that we've recorded from fossils
is really only the last 15 percent or so of the recorded history of
life on this planet. The deeper history of life and the greater
diversity of life on this planet is microorganisms—bacteria,
protozoans, algae. One way to put it is that animals might be
evolution's icing, but bacteria are really the cake.
NOVA: So we live in their world rather than the other way
around?
Knoll: We definitely live in a bacterial world, and not just
in the trivial sense that there's lots of bacteria. If you look at
the ecological circuitry of this planet, the ways in which materials
like carbon or sulfur or phosphorous or nitrogen get cycled in ways
that makes them available for our biology, the organisms that do the
heavy lifting are bacteria. For every cycle of a biologically
important element, bacteria are necessary; organisms like ourselves
are optional.
NOVA: What is your definition of life?
Knoll: I think you can say that life is a system in which
proteins and nucleic acids interact in ways that allow the structure
to grow and reproduce. It's that growth and reproduction, the
ability to make more of yourself, that's important. Now, you might
argue that that's a local definition of life, that if we find life
on Europa at some time in the future, it might have a different set
of interacting chemicals.
“The short answer is we don’t really know how life
originated on this planet.”
People have tried to find more general, more universal definitions
of life. They're speculative, because we don't know about any life
other than ourselves. But one definition that I kind of like says
life is a system that's capable of Darwinian evolution. What does it
require to have a system that evolves in a Darwinian fashion? First,
you have to be able to reproduce and make more of yourself, so that
fits with our local definition. You also need a source of variation
so that all of the new generation is not identical either to the
previous generation or to all its brothers and sisters. And once you
have that variation, then natural selection can actually select, by
either differential birth or death, some of the variants that
function best. That may turn out to be a fairly general definition
of life wherever we might find it.
Launching life
NOVA: What do you think was the first form of life?
Knoll: It's pretty clear that all the organisms living today,
even the simplest ones, are removed from some initial life form by
four billion years or so, so one has to imagine that the first forms
of life would have been much, much simpler than anything that we see
around us. But they must have had that fundamental property of being
able to grow and reproduce and be subject to Darwinian evolution.
So it might be that the earliest things that actually fit that
definition were little strands of nucleic acids. Not DNA
yet—that's a more sophisticated molecule—but something
that could catalyze some chemical reactions, something that had the
blueprint for its own reproduction.
NOVA: Would it be something we would recognize under a
microscope as living, or would it be totally different?
Knoll: That's a good question. I can imagine that there was a
time before there was life on Earth, and then clearly there was a
time X-hundred thousand years or a million years later when there
were things that we would all recognize as biological. But there's
no question that we must have gone through some intermediate stage
where, had you been there watching them, you might have placed your
bets either way.
So I can imagine that on a primordial Earth you would have
replicating molecules—not particularly lifelike in our
definition, but they're really getting the machinery going. Then
some of them start interacting together and pretty soon you have
something a little more lifelike, and then it incorporates maybe
another piece of nucleic acid from somewhere else, and by the
accumulation of these disparate strands of information and activity,
something that you and I would look at and agree "that's biological"
would have emerged.
NOVA: In a nutshell, what is the process? How does life form?
Knoll: The short answer is we don't really know how life
originated on this planet. There have been a variety of experiments
that tell us some possible roads, but we remain in substantial
ignorance. That said, I think what we're looking for is some kind of
molecule that is simple enough that it can be made by physical
processes on the young Earth, yet complicated enough that it can
take charge of making more of itself. That, I think, is the moment
when we cross that great divide and start moving toward something
that most people would recognize as living.
Recipe for life
NOVA: Is this an inevitable consequence of the conditions and
chemicals and stuff that existed on early Earth?
Knoll: We don't know whether life is an inevitable
consequence of planetary formation. Certainly in our solar system
there is no shortage of planets that probably never had life on
them. So it's a hard question to answer. I think the way I'd be most
comfortable thinking about it is that you probably have to get the
recipe right. That is, you need a planet that has a certain range of
environments, certain types of gases in the atmosphere, certain
types of geological processes at work, that when you have the right
conditions, life will emerge fairly rapidly. I don't think we need
to think about inherently improbable events that eventually happen
just because there are huge intervals of time. My guess is that it
either happens or it doesn't.
NOVA: Has there been a change in thinking about this over the
years?
Knoll: People's ideas on the circumstances under which life
might emerge have really changed and developed over the last 30 or
40 years. I think it's fair to say that when I was a boy those few
people who thought about the origin of life thought that it probably
was a set of improbable reactions that just happened to get going
over the fullness of time. And I think it's fair to say that most of
those people probably thought that we would find out what those
reactions were, that somehow we would nail it in a test tube at some
point.
“To a first approximation you’re just a bag of carbon,
oxygen, and hydrogen.”
Now I think, curiously enough, both of those attitudes have changed.
I think that there's less confidence that we're really going to be
able to identify a specific historical route by which life emerged,
but at the same time there's increasing confidence that when life
did arise on this planet, it was not a protracted process using a
chemistry that is pretty unlikely but rather is a chemistry that,
when you get the recipe right, it goes, and it goes fairly quickly.
NOVA: What is the recipe for life?
Knoll: The recipe for life is not that complicated. There are
a limited number of elements inside your body. Most of your mass is
carbon, oxygen, hydrogen, sulfur, plus some nitrogen and
phosphorous. There are a couple dozen other elements that are in
there in trace amounts, but to a first approximation you're just a
bag of carbon, oxygen, and hydrogen.
Now, it turns out that the atmosphere is a bag of carbon, oxygen,
and hydrogen as well, and it's not living. So the real issue here
is, how do you take that carbon dioxide in the atmosphere (or
methane in an early atmosphere) and water vapor and other sources of
hydrogen—how do you take those simple, inorganic precursors
and make them into the building blocks of life?
There was a famous experiment done by Stanley Miller when he was a
graduate student at the University of Chicago in the early 1950s.
Miller essentially put methane, or natural gas, ammonia, hydrogen
gas, and water vapor into a beaker. That wasn't a random mixture; at
the time he did the experiment, that was at least one view of what
the primordial atmosphere would have looked like.
Then he did a brilliant thing. He simply put an electric charge
through that mixture to simulate lightning going through an early
atmosphere. After sitting around for a couple of days, all of a
sudden there was this brown goo all over the reaction vessel. When
he analyzed what was in the vessel, rather than only having methane
and ammonia, he actually had amino acids, which are the building
blocks of proteins. In fact, he had them in just about the same
proportions you would find if you looked at organic matter in a
meteorite. So the chemistry that Miller was discovering in this
wonderful experiment was not some improbable chemistry, but a
chemistry that is widely distributed throughout our solar system.
NOVA: So life is really chemistry.
Knoll: Life really is a form of chemistry, a particular form
in which the chemicals can lead to their own reproduction. But the
important thing, I think, is that when we think about the origin of
life this way, it isn't that life is somehow different from the rest
of the planet. Life is something that emerges on a developing
planetary surface as part and parcel of the chemistry of that
surface.
“Life is really part of the fabric of a planet like
Earth.”
Life is also sustained by the planet itself. That is, all of the
nutrients that go into the oceans and end up getting incorporated
into biology, at first they're locked up in rocks and then they are
eroded from rocks, enter the oceans, and take part in a complex
recycling that ensures that there's always carbon and nitrogen and
phosphorous available for each new generation of organisms.
The most interesting thought of all is that not only does life arise
as a product of planetary processes, but in the fullness of time, on
this planet at least, life emerged as a suite of planetary processes
that are important in their own right. We're sitting here today
breathing an oxygen-rich mixture of air. We couldn't be here without
that oxygen, but that oxygen wasn't present on the early Earth, and
it only became present because of the activity of photosynthetic
organisms. So in a nutshell, life is really part of the fabric of a
planet like Earth.
Building a being
NOVA: To get back to these basic chemistry building blocks,
is everything from a mouse to a bacterium to you and me made from
this simple set of ingredients?
Knoll: All life that we know of is fundamentally pretty
similar. That's why we think that you and I and bacteria and
toadstools all had a single common ancestor early on the Earth. If
you look at the cell of a bacterium, it has about the same
proportions of carbon and oxygen and hydrogen as a human body does.
The basic biochemical machinery of a bacterium is, in a broad way at
least, similar to the chemistry of our cells.
The big difference between you and a bacterium in some ways is that
your body consists of trillions of cells that function in a
coordinated manner. Bacteria are single cells, although they're not
free agents. In fact, bacteria working in a sediment or in the sea
actually live in consortia as well. They're not really lone
operators. They work in these very, very highly coordinated
communities of organisms that help each other to grow and prosper.
NOVA: Is it hard to go from these little building blocks to a
full-fledged organism?
Knoll: Well, we don't know how hard it is to go from the
simplest bricks, if you will, in the wall of life to something that
is complicated, like a living bacterium. We know that it happened,
so it's possible. We don't really know whether it was unlikely and
just happened to work out on Earth, or whether it's something that
will happen again and again in the universe.
My guess is it's not too hard. That is, it's fairly easy to make
simple sugars, molecules called bases which are at the heart of DNA,
molecules called amino acids which are at the heart of proteins.
It's fairly easy to make some of the fatty substances that make the
coverings of cells. Making all of those building blocks individually
seems to be pretty reasonable, pretty plausible.
The hard part, and the part that I think nobody has quite figured
out yet, is how you get them working together. How do you go from
some warm, little pond on a primordial Earth that has amino acids,
sugars, fatty acids just sort of floating around in the environment
to something in which nucleic acids are actually directing proteins
to make the membranes of the cell?
Somehow you have to get all of the different constituents working
together and have basically the information to make that system work
in one set of molecules, which then directs the formation of a
second set of molecules, which synthesizes a third set of molecules,
all in a way that feeds back to making more of the first set of
molecules. So you end up getting this cycle. I'm not sure we've
gotten very far down the road to understanding how that really
happens.
Through a glass darkly
NOVA: In your book, you liken the study of the origin of life
to a maze.
Knoll: Yes. There are multiple doors that enter the maze, but
there's really only one historical path that life took. I think that
while we've had some very clever entryways into several of these
doors, at this point we still don't know which of these pathways
ultimately will thread us through the maze and which end up in a
blind alley.
NOVA: So at this point we're seeing the origins of life
through a glass darkly?
Knoll: If we try to summarize by just saying what, at the end
of the day, do we know about the deep history of life on Earth,
about its origin, about its formative stages that gave rise to the
biology we see around us today, I think we have to admit that we're
looking through a glass darkly here. We have some hints, we have a
geologic record that tells us that life formed early on the planet,
although our ability to interpret that in terms of specific types of
microorganisms is still frustratingly limited.
“I imagine my grandchildren will still be sitting around
saying that it’s a great mystery.”
There are still some great mysteries. People sometimes think that
science really takes away mystery, but I think there are great
scientific mysteries and causes for wonder and, most importantly,
things that will, I hope, stimulate biologists for years to come. We
don't know how life started on this planet. We don't know exactly
when it started, we don't know under what circumstances.
It's a mystery that we're going to chip at from several different
directions. Geologists like myself will chip at it by trying to get
ever clearer records of Earth's early history and ever better ways
of interrogating those rocks through their chemistry and
paleontology. Biologists will chip at it by understanding at an ever
deeper level how the various molecular constituents of the cell work
together, how living organisms are related to one other
genealogically. And chemists will get at it by doing new experiments
that will tell us what is plausible in how those chemical
correspondences came to be.
NOVA: Will we ever solve the problem?
Knoll: I don't know. I imagine my grandchildren will still be
sitting around saying that it's a great mystery, but that they will
understand that mystery at a level that would be incomprehensible to
us today.
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