How Life Began
(Top Posts - Science - 122604)

... Good article summarizing, briefly, what we know,
what we don't know yet, and what our grandchildren
may know, regarding how life began on this 3rd
rock from an average star in a nondescript galaxy ...


Beginning & Ending excerpts:

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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 conspic-
uous 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 bac-

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 organ-
isms 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 inter-
acting 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

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 defin-

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 sis-

And once you have that variation, then natural sel-
ection can actually select, by either differential
birth or death, some of the variants that function

That may turn out to be a fairly general definition
of life wherever we might find it.


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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 frustrat-
ingly 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 mys-
teries and causes for wonder and, most import-
antly, 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 genealogic-

And chemists will get at it by doing new experi-
ments 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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