Re: NEWS: How Life Began: New Research Suggests Simple Approach

On Sun, 11 Jun 2006 00:56:39 GMT,
NashtOn <nana@xxxxx> wrote:
Ye Old One wrote:
How Life Began: New Research Suggests Simple Approach

Michael Schirber
Special to LiveScience
LiveScience.com Sat Jun 10, 9:00 AM ET
http://news.yahoo.com/s/space/20060610/sc_space/howlifebegannewresearchsuggestssimpleapproach

Somewhere on Earth, close to 4 billion years ago, a set of molecular
reactions flipped a switch and became life. Scientists try to imagine
this animating event by simplifying the processes that characterize
living things.

New research suggests the simplification needs to go further.

All currently known organisms rely on DNA to replicate and proteins to
run cellular machinery, but these large molecules?intricate weaves of
thousands of atoms?are not likely to have been around for the first
organisms to use.

"Life could have started up from the small molecules that nature
provided," says Robert Shapiro,a chemist from New York University .
Life's Big Questions

When? The oldest known fossils, called stromatolites, are about 3.5
billion years old. Although debated, these colonial structures appear
to have been formed by photosynthesizing cyanobacteria (blue-green
algae). Simpler organisms likely came earlier.

Where? The main competing theories are hot start vs. cold start. The
one claims that the first life fed off the sulfur chemistry near a hot
volcanic vent, while the other says that temperatures had to be cooler
to have stable bio-molecules.

What? Genetic analysis shows that hyperthermophiles sit near the root
of the tree of life, implying an ancient origin. But this does not
mean these hot-loving microbes were the first to breathe life; they
may simply have survived meteorite impacts that wiped out everything
else on the primordial Earth. What's more certain is that the first
organisms were anaerobic, as there was little oxygen in our planet's
early atmosphere.

Shapiro and others insist that the first life forms were
self-contained chemistry experiments that grew, reproduced and even
evolved without needing the complicated molecules that define biology
as we now know it.

Primordial soup

An often-told origin-of-life story is that complex biological
compounds assembled by chance out of an organic broth on the early
Earth's surface. This pre-biotic synthesis culminated in one of these
bio-molecules being able to make copies of itself.

The first support for this idea of life arising out of the primordial
soup came from the famous 1953 experiment by Stanley Miller and Harold
Urey, in which they made amino acids?the building blocks of
proteins?by applying sparks to a test tube of hydrogen, methane,
ammonia, and water.

If amino acids could come together out of raw ingredients, then
bigger, more complex molecules could presumably form given enough
time. Biologists have devised various scenarios in which this
assemblage takes place in tidal pools, near underwater volcanic vents,
on the surface of clay sediments, or even in outer space.

But were the first complex molecules proteins or DNA or something
else? Biologists face a chicken-and-egg problem in that proteins are
needed to replicate DNA, but DNA is necessary to instruct the building
of proteins.

Many researchers, therefore, think that RNA?a cousin of DNA?may have
been the first complex molecule on which life was based. RNA carries
genetic information like DNA, but it can also direct chemical
reactions as proteins do.

Metabolism first

Shapiro, however, thinks this so-called "RNA world" is still too
complex to be the origin of life. Information-carrying molecules like
RNA are sequences of molecular "bits." The primordial soup would be
full of things that would terminate these sequences before they grew
long enough to be useful, Shapiro says.

"In the very beginning, you couldn't have genetic material that could
copy itself unless you had chemists back then doing it for you,"
Shapiro told LiveScience.

Instead of complex molecules, life started with small molecules
interacting through a closed cycle of reactions, Shapiro argues in the
June issue of the Quarterly Review of Biology. These reactions would
produce compounds that would feed back into the cycle, creating an
ever-growing reaction network.

All the interrelated chemistry might be contained in simple membranes,
or what physicist Freeman Dyson calls "garbage bags." These might
divide just like cells do, with each new bag carrying the chemicals to
restart?or replicate?the original cycle. In this way, "genetic"
information could be passed down.

Moreover, the system could evolve by creating more complicated
molecules that would perform the reactions better than the small
molecules. "The system would learn to make slightly larger molecules,"
Shapiro says.

This origin of life based on small molecules is sometimes called
"metabolism first" (to contrast it with the "genes first" RNA world).
To answer critics who say that small-molecule chemistry is not
organized enough to produce life, Shapiro introduces the concept of an
energetically favorable "driver reaction" that would act as a constant
engine to run the various cycles.

Driving the first step in evolution

A possible candidate for Shapiro's driver reaction might have been
recently discovered in an undersea microbe, Methanosarcina
acetivorans, which eats carbon monoxide and expels methane and acetate
(related to vinegar).

Biologist James Ferry and geochemist Christopher House from Penn State
University found that this primitive organism can get energy from a
reaction between acetate and the mineral iron sulfide. Compared to
other energy-harnessing processes that require dozens of proteins,
this acetate-based reaction runs with the help of just two very simple
proteins.

The researchers propose in this month's issue of Molecular Biology and
Evolution that this stripped-down geochemical cycle was what the first
organisms used to power their growth. "This cycle is where all
evolution emanated from," Ferry says. "It is the father of all life."

Shapiro is skeptical: Something had to form the two proteins. But he
thinks this discovery might point in the right direction. "We have to
let nature instruct us," he says.


I thought that the beginnings of life didn't interest evolutionary biology.

You thought wrong, as usual.

--
Aaron Clausen
mightymartianca@xxxxxxxxx

.

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