New Origin of Life Proposed: Zinc & Zap

http://www.livescience.com/space/090903-am-zinc-life.html

"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.

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.

"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.

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."