In particular, compare the first paragraph with the last two
paragraphs :-).
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"Chemist Adds Missing Pieces to Theory on Life's Origins"
New York Times, July 4, 1995
Page 19, Science Section
by Malcolm W. Browne
No one has yet shown how life originated on Earth, and the chance
that science will ever unlock all the details of the process seems
vanishingly small. But as molecular biologists discover how nature
may have built unaided the maze of tiny bridges that led from inanimate
chemicals to living creatures, hopes brighten that the ancient pathway
to life may one day be at least dimly revealed.
The latest news about the quest comes from Dr. Stanley L. Miller of
the University of California at San Diego, a pioneer in the reconstruction
of "prebiotic" chemical reactions -- the primeval chemistry of the young
Earth that is presumed to have given rise to life. Dr. Miller and a
colleague, Dr. Michael P. Robertson, report in a pair of papers that
they have found a way that nature could have created several vital
building blocks that had seemed beyond the reach of prebiotic chemistry.
The achievement, they believe, strengthens the theory that the first
living creatures were able to synthesize protein and reproduce without
the help of the DNA (deoxyribonucleic acid) that makes up the genes
of advanced organisms, including human beings. Instead, scientists
theorize, primitive viruses and one-celled organisms may have depended
solely on RNA (ribonucleic acid) to guide and catalyze their growth
and reproduction. According to this theory, which has been gradually
accepted during the last 20 years by most biologists, the early "RNA
world" evolved into the present system, in which both RNA and DNA
play important roles in life processes.
Speculation that the primitive earth was an RNA world arose in 1982,
when Dr. Thomas Cech of the University of Colorado disclosed the
discovery (for which he was later awarded a Nobel prize) that RNA
molecules alone, without the help of outside enzymes, could induce
themselves to split up and splice themselves together in new
arrangements. This meant that primitive forms of life (like the
single-celled tetrahymena animals with which Dr. Cech experimented)
could have come into being without any need by nature to invent
complex protein enzymes.
Since a landmark experiment Dr. Miller performed in 1953 with the
late Dr. Harold C. Urey, the discoverer of deuterium, Dr. Miller
has devoted his life to the quest for life's origins. In the
Miller-Urey experiment, a sealed flask was filled with a mixture
of methane, ammonia, carbon dioxide, hydrogen and water vapor --
all substances presumed to have been rich in the Earth's early
atmosphere. Through this mixture an artificial lightning bolt
was passed for several weeks, at the end of which a brownish sludge
that formed in the flask was found to contain amino acids of the
kind necessary to build proteins. Since then, experimenting with
different mixtures of simple gases and conditions, Dr. Miller has
created 13 of the 20 amino acids essential to life.
But one problem the theory has had to confront was a possible
shortage in the primeval oceans of two key pieces in the structure
of an RNA molecule, known as cytosine and uracil. Dr. Miller
and Dr. Robertson believe they have solved the difficulty.
In companion pieces in the journals Science and Nature, the two
scientists report that both substances might have been produced
by the lifeless young oceans in ample quantities by a process
involving the evaporation of sea water in tropical lagoons, the
freezing of sea water in polar regions and the mixing of their
products in the open ocean.
The freezing part of the process could have increased sea water
concentrations of hydrogen cyanide, Dr. Miller believes. Cyanide
is a deadly poison to animals, but it was an essential precursor
to many of the molecules from which primitive life arose.
The evaporative part of the process, Dr. Miller said, could have
concentrated the traces of urea that accumulate in sea water as
a result of reactions in the atmosphere caused by lightning
flashes. In experiments, Dr. Miller and Dr. Robertson showed that
when the concentration of the simple chemical urea in sea water
is high enough, it reacts with another quite common component of
sea water that also owes its formation partly to lightning bolts.
Under these conditions, the scientists found, the reaction
between urea and the second chemical, known as cyanoacetaldehyde,
yields fairly large amounts of cytosine, which is one of the
nucleotide bases (or "letters") the DNA and RNA molecules use
to spell out the genetic "words" controlling protein production
and the growth and reproduction of organisms.
A bonus of this discovery is that cytosine, which is itself an
essential component of both DNA and RNA, reacts in the presence
of water to from uracil, one of the four essential "letters" in
RNA. (Uracil is not an ingredient of DNA, which uses another
nucleotide base, thymine, in its place). Relatively large amounts
of two other essential bases in the four-letter alphabets of
DNA and RNA, adenine and guanine, are believed to have formed
in ample amounts from the polymerization of naturally occurring
ammonium cyanide in a slightly alkaline solution.
Because of the new experiments, in now appears plausible that
all four RNA nucleotide bases could have been created in nature
by ordinary atmospheric, oceanic and geological processes from
simple, naturally occurring ingredients.
In another series of experiments, Dr. Miller and Dr. Robertson
have carried the discovery a step further.
One of the many chemicals that lightning bolts are believed to
have created as they ripped through the early Earth's atmosphere
is formaldehyde, which presumably rained into the primeval oceans.
When uracil reacts with formaldehyde it forms an important
intermediary substance known as hydroxymethyluracil, which in
turn can react with a large variety of chemicals that biologists
assume to have been present in the primeval oceans. The results,
Dr. Miller and Dr. Robertson report, are molecules that incorporate
"side chains", molecular branches connected to chains of atoms,
that are also found in most of the 20 amino acids used by
organisms to make proteins.
This coincidence implies, the scientists said, that the catalytic
activity of RNA -- its ability to promote the synthesis of many
different kinds of protein -- may have been much greater in the
infant Earth than had been assumed. The RNA world may therefore
have been much richer in variety than scientists had supposed.
But a daunting array of problems still blocks understanding of
the origin of life. One of these concerns the origin of the
molecular backbone of RNA -- a current concern of Dr. Miller's.
The backbones of RNA and DNA are both sugar phosphates containing
five-sided rings of carbon atoms, but there is a slight difference
between them. In the case of DNA, double strands of pintos sugar
are wrapped helically around each other and connected by pairs
of nucleotide bases. The backbone of RNA, however, is a single
strand of ribose phosphate -- a pintos sugar containing an
additional oxygen atom. To this backbone are attached the four
nucleotide bases.
The problem in imagining how RNA got started, Dr. Miller said,
is that ribose is unstable, particularly when it is warm.
Dr. Miller said he is preparing a new paper describing experiments
that show that half of any quantity ribose decays in a little
over an hour at the boiling point of water. Even at the freezing
point, the half life of ribose is only 44 years. This, he said,
implies that high-temperature settings, including a primitive
earth heated by the impact of an asteroid, seem less plausible
as cradles of life, if it is assumed that early life came about
in an RNA world -- one in which the creation and survival of the
ribose-phosphate backbone was crucial.
Some scientists, including Dr. Francis Crick, co-discoverer
of the double-helical structure of DNA, have suggested that
life had too little time to originate on the primitive earth,
given only a "prebiotic soup" of simple chemicals. They have
proposed that life might have reached the earth in the form
of spores sent out from some distant planet.
But Dr. Miller and most other molecular biologists regard this
idea, called "panspermia", as begging the question; the origin
of life on that hypothetical distant planet still must be
explained somehow.
Dr. Christian de Duve, a Belgian microbiologist awarded with
the 1974 Nobel prize for his investigation of the structures
of cells, dismisses the panspermia notion as unnecessary.
"If you equate the probability of the birth of a bacterial cell
to that of the chance assembly of its component atoms," Dr. de
Duve wrote in his textbook, "A Guided Tour of the Living Cell,"
"even eternity will not suffice to produce one for you. So you
might as well accept, as to most scientists, that the process
was completed in no more than one billion years and that it took
place entirely on the surface of our planet."
The hard part, he wrote, was getting from the simplest chemicals
to the first specialized cells, after which "it took no more
than 150,000 generations for an ape to develop into the inventor
of calculus."
As to whether some guiding hand was needed for the process, Dr.
de Duve commented: "The answer of modern molecular biology to
this much-debated question is categorical: chance, and chance
alone, did it all, from primeval soup to man, with only natural
selection to sift its effects. This affirmation now rests on
overwhelming factual evidence."
But the succession of chances that created life did not operate
in a vacuum, he said. "It operated in a universe governed by
orderly laws and made of matter endowed with specific properties.
These laws and properties are the constraints that shape
evolutionary roulette and restrict the numbers that can turn
up. Among these numbers are life and all its wonders, including
the conscious mind."
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Brian Harper |
Associate Professor | "It is not certain that all is uncertain,
Applied Mechanics | to the glory of skepticism" -- Pascal
Ohio State University |
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