The
Miller-Urey experiment, conducted by chemists Stanley Miller and Harold Urey in
1953, is the classic experiment on the origin of life. It established that the
early Earth atmosphere, as they pictured it, was capable of producing amino
acids, the building blocks of life, from inorganic substances.
Now, more
than 55 years later, two scientists are proposing a hypothesis that could add a
new dimension to the debate on how
life on Earth developed.
Armen
Mulkidjanian of the University of Osnabrueck, Germany and Michael Galperin of
the U.S. National Institutes of Health present their hypothesis and evidence in
two papers published and open for review in the web site Biology Direct.
The
scientists suggest that life on Earth originated at photosynthetically active porous
structures, similar to deep-sea hydrothermal vents, made of zinc sulfide (more
commonly known as phosphor). They argue that under the high pressure of a
carbon-dioxide-dominated atmosphere, zinc sulfide structures could form on the
surface of the first continents, where they had access to sunlight. Unlike many
existing theories that suggest UV radiation was a hindrance to the development
of life, Mulkidjanian and Galperin think it actually helped.
"The
problem of the origin of life is such that you have to answer a set of
different questions to explain how life has originated," says lead author Mulkidjanian.
"We just provide answers to the problem of energetics of the origin of life."
Altering
the Early Atmosphere
According
to Mulkidjanian, the debate about whether life could arise from chemical
reactions began to change when scientists started to question the atmospheric
conditions used by Miller and Urey. In their famous experiment, Miller and Urey
replicated the early Earth atmosphere with a mixture of methane, hydrogen,
ammonia and water vapor. This mixture, along with some "sparks" which simulated
lightning, led to the formation of amino acids. With this setup, Miller and
Urey assumed that the early Earth had a reducing atmosphere, which meant it had
large amounts of hydrogen and almost no oxygen.
However,
many scientists have now abandoned the notion of a reducing early Earth
atmosphere. Instead, they believe Earth had a neutral atmosphere, composed
primarily of carbon dioxide, with smaller amounts of nitrogen and hydrogen, similar
to the modern atmospheres of Mars and Venus. Researchers who have repeated the
Miller-Urey experiment under the new atmospheric assumptions, including Miller,
have shown that this new mixture does not produce amino acids.
"After it
became clear that the origin of the atmosphere was made of carbon dioxide,"
says Mulkidjanian, "there was no physically or chemically plausible hypothesis
of the origin of life."
Living
organisms can exist only if there is some form of energy flow—solar radiation
or chemical reactions, for example.
"If you
have an atmosphere of carbon dioxide, you need, in addition, a source of
electrons to reduce carbon dioxide if you want to make complex compounds,"
Mulkidjanian explains.
From A-biotic
to Zinc
Mulkidjanian's
"Zn world" hypothesis presents a different version of the prebiotic Earth
atmosphere—one in which zinc sulfide plays a major role in the development of
life. In nature, zinc sulfide particles precipitate only at deep-sea hydrothermal
vents. Its unique ability to store the energy of light has made it popular in
many modern-day devices, from various types of television displays to
glow-in-the-dark items (and zinc oxide is used in sunscreen).
Its ability
to store light makes zinc sulfide an important factor in the discussion on
life's origin. Mulkidjanian explains that, once illuminated by UV
light, zinc sulfide can efficiently reduce carbon dioxide, just as plants
do.
To test the
hypothesis, Mulkidjanian and Galperin analyzed the metal content of modern
cells and found "surprisingly high levels of zinc," particularly in the
complexes of proteins with DNA and RNA molecules.
"We have
found that proteins that are considered 'evolutionarily old' and particularly
those related to handling
of RNA specifically contain large amounts of zinc," Mulkidjanian
says.
The
scientists say the result is evidence that the first life forms evolved in a
zinc-rich environment. But as the authors indicate in their paper, acceptance
of a new hypothesis for the origin of life will likely require more work,
particularly to further describe the nature of life and the chemical reactions
in these zinc-rich communities.
"We cannot
explain fully the properties of modern organisms unless we understand how life
has originated," says Mulkidjanian.
For
astrobiologists, this new hypothesis presents a considerable shift in the
debate on the origin
of life.
"If this
hypothesis is adopted in the origins of life community, it would represent a
real conceptual shift, and so it would be significant," says NASA
astrobiologist Max Bernstein. "Whether it will be adopted or not eventually I
cannot say, but I expect that many will want to see experimental evidence of
the viability of reactions consistent with the hypothesized scheme under
prebiotic conditions."
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