Meteorites bring destruction from the sky, and, possibly, microbial life.

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Panspermia: Life from Outer Space

The principle behind panspermia is that life originated outside of Earth and traveled to our planet, finding a hospitable climate in which to thrive and eventually evolve into life on Earth.

Panspermia is an old concept, dating back as far as the concept of taxonomy, when French historian Benoit de Maillet proposed that life on Earth was the result of germs "seeded" from space [source: Panspermia-Theory]. Since then, researchers from Stephen Hawking to Sir Francis Crick (who abandoned his early support for the RNA world hypothesis) have held the belief that life on Earth originated away from this planet.

The theory of panspermia falls into three broad categories. Life traveled via space debris from somewhere outside our solar system, the concept of lithopanspermia, or from another planet in our solar system, ballistic panspermia. The third hypothesis, directed panspermia, holds that life on our planet was spread purposefully by already established and intelligent life [source: Panspermia-Theory].

As panspermia hypotheses go, ballistic panspermia (also called interplanetary panspermia) enjoys the widest acceptance in the scientific community. Chunks of other planets have long bombarded Earth in the form of meteorites. In fact, one meteorite, ALH84001, discovered in Antarctica in 1984, bears what some scientists take as the traces of life or the precursors to life like amino acids. It's been calculated to have broken from Mars more than 4 billion years ago [source: Thompson].

Upon examination of ALH84001, astrobiologists -- scientists who study the potential for life in space -- found that at least four traces of ancient life, from what appeared to be fossilized microbes to a form of magnetic bacteria [source: Schirber]. Since the findings were published in 1996, three of the traces of life found in the meteorite have been discounted. But whether the last trace, chains of magnetite, are mineral or were biologically produced by ancient Martian bacteria remains under debate.

Mars is the likeliest candidate for ballistic panspermia. The arrangement of the orbits of Mars and Earth around the sun make it about 100 times easier for a rock to travel from Mars to Earth than vice versa [source: Chandler]. And over the course of Earth's history, about 5 trillion rocks are estimated to have made the journey [source: NASA]. What's more, in their early histories, Earth and Mars were similarly suited to hosting life, both featuring wet atmospheres and water on their surfaces.

Despite all of this evidence, the jury is still out on how life began on Earth. Read some criticisms of panspermia and abiogenesis on the next page.