The invention :
Amethod for synthesizing amino acids by combining
water, hydrogen, methane, and ammonia and exposing the
mixture to an electric spark.
The people behind the invention :
Stanley Lloyd Miller (1930- ), an American professor of
chemistry
Harold Clayton Urey (1893-1981), an American chemist who
won the 1934 Nobel Prize in Chemistry
Aleksandr Ivanovich Oparin (1894-1980), a Russian biochemist
John Burdon Sanderson Haldane (1892-1964), a British scientist
Prebiological Evolution
The origin of life on Earth has long been a tough problem for scientists
to solve. While most scientists can envision the development
of life through geologic time from simple single-cell bacteria
to complex mammals by the processes of mutation and natural selection,
they have found it difficult to develop a theory to define
how organic materials were first formed and organized into lifeforms.
This stage in the development of life before biologic systems
arose, which is called “chemical evolution,” occurred between
4.5 and 3.5 billion years ago. Although great advances in
genetics and biochemistry have shown the intricate workings of
the cell, relatively little light has been shed on the origins of this intricate
machinery of the cell. Some experiments, however, have
provided important data from which to build a scientific theory of
the origin of life. The first of these experiments was the classic
work of Stanley Lloyd Miller.
Miller worked with Harold Clayton Urey, a Nobel laureate, on the
environments of the early earth. John Burdon Sanderson Haldane, a
British biochemist, had suggested in 1929 that the earth’s early atmosphere
was a reducing one—that it contained no free oxygen. In
1952, Urey published a seminal work in planetology, The Planets, in
which he elaborated on Haldane’s suggestion, and he postulated
that the earth had formed from a cold stellar dust cloud. Urey
thought that the earth’s primordial atmosphere probably contained
elements in the approximate relative abundances found in the solar
system and the universe.
It had been discovered in 1929 that the Sun is approximately 87
percent hydrogen, and by 1935 it was known that hydrogen encompassed
the vast majority (92.8 percent) of atoms in the universe.
Urey reasoned that the earth’s early atmosphere contained mostly
hydrogen, with the oxygen, nitrogen, and carbon atoms chemically
bonded to hydrogen to form water, ammonia, and methane. Most
important, free oxygen could not exist in the presence of such an
abundance of hydrogen.
As early as the mid-1920’s, Aleksandr Ivanovich Oparin, a Russian
biochemist, had argued that the organic compounds necessary
for life had been built up on the early earth by chemical combinations
in a reducing atmosphere. The energy from the Sun would
have been sufficient to drive the reactions to produce life. Haldane
later proposed that the organic compounds would accumulate in
the oceans to produce a “dilute organic soup” and that life might
have arisen by some unknown process from that mixture of organic
compounds.
Primordial Soup in a Bottle
Miller combined the ideas of Oparin and Urey and designed a
simple, but elegant, experiment. He decided to mix the gases presumed
to exist in the early atmosphere (water vapor, hydrogen, ammonia,
and methane) and expose them to an electrical spark to determine
which, if any, organic compounds were formed. To do this,
he constructed a relatively simple system, essentially consisting of
two Pyrex flasks connected by tubing in a roughly circular pattern.
The water and gases in the smaller flask were boiled and the resulting
gas forced through the tubing into a larger flask that contained
tungsten electrodes. As the gases passed the electrodes, an electrical
spark was generated, and from this larger flask the gases and any
other compounds were condensed. The gases were recycled through
the system, whereas the organic compounds were trapped in the
bottom of the system.
Miller was trying to simulate conditions that had prevailed on
the early earth. During the one week of operation, Miller extracted
and analyzed the residue of compounds at the bottom of the system.
The results were truly astounding. He found that numerous organic
compounds had, indeed, been formed in only that one week. As
much as 15 percent of the carbon (originally in the gas methane) had
been combined into organic compounds, and at least 5 percent of
the carbon was incorporated into biochemically important compounds.
The most important compounds produced were some of
the twenty amino acids essential to life on Earth.
The formation of amino acids is significant because they are the
building blocks of proteins. Proteins consist of a specific sequence of
amino acids assembled into a well-defined pattern. Proteins are necessary
for life for two reasons. First, they are important structural
materials used to build the cells of the body. Second, the enzymes
that increase the rate of the multitude of biochemical reactions of life
are also proteins. Miller not only had produced proteins in the laboratory
but also had shown clearly that the precursors of proteins—
the amino acids—were easily formed in a reducing environment
with the appropriate energy.
Perhaps the most important aspect of the experiment was the
ease with which the amino acids were formed. Of all the thousands
of organic compounds that are known to chemists, amino acids
were among those that were formed by this simple experiment. This
strongly implied that one of the first steps in chemical evolution was
not only possible but also highly probable. All that was necessary
for the synthesis of amino acids were the common gases of the solar
system, a reducing environment, and an appropriate energy source,
all of which were present on early Earth.
Consequences
Miller opened an entirely new field of research with his pioneering
experiments. His results showed that much about chemical
evolution could be learned by experimentation in the laboratory.
As a result, Miller and many others soon tried variations on
his original experiment by altering the combination of gases, using
other gases, and trying other types of energy sources. Almost all
the essential amino acids have been produced in these laboratory
experiments.
Miller’s work was based on the presumed composition of the
primordial atmosphere of Earth. The composition of this atmosphere
was calculated on the basis of the abundance of elements
in the universe. If this reasoning is correct, then it is highly likely
that there are many other bodies in the universe that have similar
atmospheres and are near energy sources similar to the Sun.
Moreover, Miller’s experiment strongly suggests that amino acids,
and perhaps life as well, should have formed on other planets.
See also : Artificial hormone; Artificial kidney .
Further Reading :
