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There are multiple origin of life theories.

by: JT

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In a special "Collectors Edition" of Astronomy entitled 50 Greatest Mysteries of the Universe, the 43rd question is "Did comets bring life to Earth." The following is extracted from that article:

Understanding how life began on Earth engages many fields of science. It's a complex question involving related bits of physics, chemistry, astronomy, and biology.

...

Based on the realization that life could get going on early Earth despite hostile conditions, scientists have developed several theories about how it first emerged. One idea is the so-called RNA world hypothesis. This suggests that RNA molecules could have formed spontaneously and catalyzed their own replication.

So-called metabolism-first models came next. These suggest that a primitive metabolism arose and led to the development of RNA. The so-called bubble theory suggests organic molecules concentrated on ocean shores just as bubbles concentrate in breaking waves. When enough prebiotic material came together to form the right chemical reactions, the development of living systems began.

Other models include physicist Thoms Gold's "deep-hot biosphere," which posits that biomolecules formed several miles below Earth's surface.

Some suggest that comets may have delivered Earth's organic materials. In the solar system's early days, Earth underwent a period of heavy bombardment. Hundreds of thousands of small bodies crashed into the planets and their moons. Vast numbers of comets and asteroids struck Earth and they left behind incredible amount of water... But comets likely left other important chemicals behind. Complex organic molecules like amino acids exist on asteroids, and probably on comets too...

... How significant were comets in seeding the young Earth with organic chemicals? No-one knows.

The full article is well worth reading, as are all the articles in this particular edition of Astronomy magazine. It should be available for about another month on newsstands, and can also be ordered from the Astronomy.com website.