Another Strike Against the Panspermia Hypothesis

The Earth's oldest ice has been thawed, and its drippings contained fragments of very, very old DNA. Scientists recently brought this DNA back to life.

As he describes in a recent PNAS paper, microbiologist Kay Bidle and colleagues at Rutgers discovered DNA pieces in thawed chunks of Antarctic ice that ranged in age from 100,000 to eight million years old.

When they tried to make the bacteria "viable" again—that is, when they tried to get it to grow and reproduce in lab cultures—the researchers found that the older samples were much more fragmented than the newer ones. From these samples, they calculated a "DNA half-life:" The length of DNA fragments in the ice breaks in half about every 1.1 million years.

They attributed the DNA breakdown to its long-term exposure to cosmic radiation, which digs another nail into astrobiology's Panspermia Hypothesis. The idea, which dates back to the writings of the 5th Century Greek philosopher Anaxagoras, is that the "seeds of life" are found throughout the universe. In 1973, the co-discoverer of DNA Francis Crick proposed a "directed panspermia" theory: An advanced alien civilization, perhaps facing its imminent demise, intentionally spread small grains of DNA in random directions through the universe, some of which landed on Earth.

Panspermia, though an immensely popular idea in science fiction, has been widely criticized in the astrobiology field, mostly because the specific combination of carbon, nitrogen, and oxygen necessary for life isn't found widely in the universe.

Moreover, stellar winds and cosmic rays make space is a harsh environment for poor little traveling microbes. Biddle's latest research seems to support this latter point. As he recently told Nature News: "If you take the speed of a comet and take the distance it would need to travel it would take longer than eight million years to do that. In a comet the DNA would be completely deteriorated."

(For more astrobiology goodness, check out my Master's thesis about the hunt for life on Mars.)