13.7 Billion Years: The Shadow Lives of Others

Tuesday, March 3, 2009

The Shadow Lives of Others

The exact age of the Earth is unknown, but modern geologists agree that it's somewhere in the range of 4.5 billion years, based on the oldest-known terrestrial samples -- small zircon crystals found in the Jack Hills of Western Australia.

Single-celled prokaryotes are the first known living things on Earth, but we don't know how they came to be. One reigning theory drives abiogenesis, the study of how life arose from inanimate matter like amino acids. Another leading concept is panspermia, the hypothesis that the seeds of life exist throughout the universe, which includes exogenesis, the idea that the earliest microbes on Earth may have arrived from outer space, carried on an asteroid. The oldest-known microbial fossils are around 3.5 billion years old, which leaves the Earth lifeless for its first billion years -- or so we think.

The scientific community generally agrees that all life on Earth derived from a "Last Universal Common Ancestor" (LUCA), a single-celled organism whose genetic code was based on DNA. But there is another idea about the origin of life on Earth that gaining ground in the scientific community -- that of "alien" life on Earth.

In a February 15 lecture at the annual meeting of the American Association for the Advancement of Science entitled "Shadow Life: Life As We Don't Yet Know It," esteemed theoretical physicist and Arizona State University cosmologist Paul Davies asked, "Life as we know it appears to have had a single common ancestor, yet, could life on Earth have started many times? Might it exist on Earth today in extreme environments and remain undetected because our techniques are customized to the biochemistry of known life?"

What we do know is that we are living in a bacterial world. "For every cycle of a biologically important element, bacteria are necessary; organisms like ourselves are optional," says Andrew Knoll, Fisher Professor of Natural History at Harvard University. "Animals might be evolution's icing, but bacteria are really the cake."

"If a biochemically weird microorganism should be discovered," wrote Davies in the November 19, 2007, issue of Scientific American, "its status as evidence for a second genesis, as opposed to a new branch on our own tree of life, will depend on how fundamentally it differs from known life."

Davies, who is also the chair of the Search for Extraterrestrial Intelligence (SETI) at the International Academy of Astronautics, says that this "shadow life" may be lurking in deep-sea hydrothermal vents like the one where LUCA is believed to have been born. He suggests a "mission to Earth" to investigate this possibility. He could well be right -- we all know that so often what you're looking for is right under your nose.

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