Scientists Urge a Search for Life Not as We Know It

A panel of scientists convened by the country’s leading scientific advisory group says the hunt for extraterrestrial life should be greatly expanded to include what they call “weird life”: organisms that lack DNA or other molecules found in life as we know it.

“The committee’s investigation makes clear that life is possible in forms different from those on Earth,” the scientists conclude in their report, “The Limits of Organic Life in Planetary Systems,” published yesterday by the National Research Council.

Other experts hailed the report as an important rethinking of the search for life.

“It’s going to help us a lot to make sure we go exploring with our eyes wide open,” said Michael A. Meyer, lead scientist for NASA’s Mars exploration program.

Starfish, sequoias, salamanders and the rest of Earth’s residents may seem very diverse, but they are surprisingly similar on the molecular scale. All species that scientists have studied need liquid water to survive, for example. Further, they all rely on DNA to carry genetic information, and they all use that information to build proteins from the same set of building blocks, known as amino acids.

NASA has long looked to life on Earth to guide its search for life in other worlds. Planets and moons that have hints of liquid water have been ranked high on the list of potential sites for life-detection missions.

But there is good reason to suspect that other kinds of chemistry could support life as well, the authors of the new report argue. Weird life could differ from life as we know it in small or big ways.

For example, while DNA uses phosphorus in its backbone, it might be possible to build a backbone out of arsenic instead. And life might exist in liquids other than water, perhaps ammonia or methane.

The report, which is posted on the Web site of the National Academies, www.nationalacademies.org, even explores the possibility of life based on silicon, not carbon, though Dr. Meyer, who had no role in the work, thinks that astrobiologists should limit their search to carbon-based life forms.

“When we look in the universe,” he said, “the only compounds we see with more than six atoms are all carbon chemistry. So there’s a hint that looking for carbon chemistry may be a better bet. There we have some idea of what to look for.”

The report calls for NASA and the National Science Foundation both to support research into weird life. Chemists need to investigate “the chemical possibilities for what forms life might take,” said one member of the committee, Steven A. Benner, a distinguished fellow at the Foundation for Applied Molecular Evolution, in Gainesville, Fla.

Scientists should also search Earth for weird life, the authors maintain. “There’s much about Earth life we don’t understand,” said the panel’s chairman, John A. Baross of the University of Washington.

Dr. Benner said there was “good evidence that the life we know on Earth was preceded by a weird form of life.” Early Earth life may have been based on RNA, a single-stranded form of DNA. Though DNA-based life may have outcompeted earlier forms on the surface of the planet, RNA life may still exist in refuges. One potential hiding place is deep below the ocean floor.

“It’s an incredibly primordial world down there,” Dr. Baross said. “If you’re going to look for remnants of an RNA world, those are the environments you want to go to.”

To find weird life, however, scientists will have to build new kinds of detectors. “There’s no question that the surveys of life on the planet we’ve done so far would have missed it,” Dr. Benner said.

The scientists also said the possibility of weird life should prompt NASA to reorder its future missions. They singled out Saturn’s moon Titan as particularly promising. The Huygens probe that visited Titan in 2005 found evidence of liquid methane raining down on its surface, as well as a mix of water and ammonia seeping up from its interior. Both could conceivably support life, although not necessarily life as we know it.

“Nothing,” the report concluded, “would be more tragic in the American exploration of space than to encounter alien life and fail to recognize it.”