The Hunt for E.T.

Image: Lava pillars near the Axial volcano, which erupted in 1998. A new microbe was found there in in 2006. NOAA Vents Program, via Science-AAAS.

CHICAGO — They might not be green or spit out cute catchphrases, but there could be forms of "alien" life right here on Earth.

If life arose not just once, but multiple times on Earth, life as we don't know it could be here on our own planet, perhaps using different chemical processes than we've ever seen before. And because scientists have only studied a tiny slice of the world's microbes in depth, the microscopic remnants of a second (or third or fourth) biogenesis could be hiding right beneath our noses.

"If life did happen many times, there could be something like a shadow biosphere that either was, or is, all around us," Arizona State Univeristy astrobiologist Paul Davies said here Sunday at the American Association for the Advancement of Sciences meeting. "It's entirely possible that some fraction of microbial life could turn out to be alien or 'weird' life as we prefer to call it."

Davies' contention challenges the relatively accepted orthodoxy that life arose once on Earth and colonized the entire planet. This weird life would the best possible analog for extraterrestrial life. Finding it, or even creating it in a laboratory, would give researchers clues about both how life began on Earth and how common life is on other planets. If a second sample of life on Earth exists, it would raise the probability for extraterrestrial life and help provide knowledge about other plausible structures for life in the universe.

This strange life could be far more simple than the life that we know after 4 billion years of evolution, or it could use different chemical machinery to carry out the processes of life, like using arsenic in the same way that all living things we know use phosphorous.

But how do you find life as we don't know it? The easiest method would be to discover an ecologically isolated region, where no life that we currently recognize does or could live, and stumble upon a whole community of new life forms.

"One thing we could do would be to draw up a wish list of horrible places and go there to look for weird life," Davies said.

In some ways, that's what happened when researchers looked at hydrothermal vents in the ocean and found sulphur-eating microbes thriving in the absence of sunlight. But those creatures already have a spot on the tree of life and, if you go far enough back, share a common ancestor with all other life that we know on Earth.

Truly "weird" life would have to be much weirder. It would function using different elements or have different basic genetic material. Stumbling upon this life could be quite difficult, as the likeliest spot for one of these life forms would appear to be one of the thousands of unexplored deep sea vents. But Davies thinks a fairly simple endeavor could determine whether arsenic-using life exists: Find a virus that incorporates arsenic and you have suggestive evidence that the cells exist.

"The idea I did have is that if there are weird cells lurking somewhere, that they've probably got weird viruses that prey on them. Viruses get everywhere. The oceans are like virus soup," Davies said. "Just looking for viruses with arsenic in there seems fairly straightforward."

And as Davis points out, "You just need to find one of these things somewhere, and it makes the point."

But if nature doesn't offer it up, there's another route to understanding weird life on Earth or elsewhere. We can try to recreate life as we've never seen it before in a laboratory. Davies' sometime collaborator Steven Benner, a former University of Florida chemist, revealed at the conference this weekend that he had created a self-replicating chemical system that can evolve along Darwinian lines. It's not quite artificial life, but it's close. At first blush, it might sound like Gerry Joyce's work creating a similar but more self-contained system.

Joyce's system, however, took the existing RNA chemistry as a given, whereas Benner's work added letters, i.e. amino acids, to the familiar adenine, thymine, guanine and cytosine. The RNA in Benner's experiment uses six amino acids. His work attempts more radical changes to what we think of as life than does the work of synthetic biologists like Craig Venter or chemists like Joyce.

"One of your problems in synthetic biology, is that you have to decide where your interchangeable parts are going to come from," Benner said. "You have Craig Venter, he's going to try to shuffle genes that nature has produced. Jerry is the next level down. He's trying to use the ATGs and Cs and he'll do selections and cycles. Very cute stuff. We're trying to shuffle the atoms."

While Benner acknowledged that the other scientists' work could increase our understanding of life on Earth — its origins and limits — he maintained that his approach was the only one likely to expand our conception of the possibilities for extraterrestrial lifeforms.

"If you're going to look at alien life, you have to do it at our level because chemistry is the universal," Benner said. "I assure you that Craig Venter's genes are not going to be found anywhere else."

While NASA and the public are focused on the final frontier, looking for extraterrestrial life in our solar system and on Earth-like planets beyond it, Davies and Benner could be the first to find or create life as we don't know it right here on the Earth.