Wednesday, July 11, 2007

Truly Alien Life On and Off the Earth

The National Academies has published a report from a panel of chemists, biologists, geologists, and astronomers on the "Limits of Organic Life in Planetary Systems". The press release sums it up: we shouldn't assume that life off the Earth will look like life on the earth.

The tacit assumption that alien life would utilize the same biochemical architecture as life on Earth does means that scientists have artificially limited the scope of their thinking as to where extraterrestrial life might be found, the report says.The assumption that life requires water, for example, has limited thinking about likely habitats on Mars to those places where liquid water is thought to be present or have once flowed, such as the deep subsurface.

However, according to the committee, liquids such as ammonia or formamide could also work as biosolvents -- liquids that dissolve substances within an organism -- albeit through a different biochemistry. The recent evidence that liquid water-ammonia mixtures may exist in the interior of Saturn's moon Titan suggests that increased priority be given to a follow-on mission to probe Titan, a locale the committee considers the solar system's most likely home for weird life.

"It is critical to know what to look for in the search for life in the solar system," said Baross. "The search so far has focused on Earth-like life because that's all we know, but life that may have originated elsewhere could be unrecognizable compared with life here. Advances throughout the last decade in biology and biochemistry show that the basic requirements for life might not be as concrete as we thought."

Besides the possibility of alternative biosolvents, studies show that variations on some of the other basic tenets for life also might be able to support weird life. DNA on Earth works through the pairing of four chemical compounds called nucleotides, but experiments in synthetic biology have created structures with six or more nucleotides that can also encode genetic information and, potentially, support Darwinian evolution. Additionally, studies in chemistry show that an organism could utilize energy from alternative sources, such as through a reaction of sodium hydroxide and hydrochloric acid, meaning that such an organism could have an entirely non-carbon-based metabolism.

The introduction to the report acknowledges that science fiction is a source of public information on the topic:
The natural tendency toward terracentricity requires that we make an effort to broaden our ideas of where life is possible and what forms it might take. Furthermore, basic principles of chemistry warn us against terraceentricity. It is easy to conceive of chemical reactions that might support life involving noncarbon compounds, occurring in solvents other than water, or involving oxidation-reduction reactions without dioxygen. [. . .] It is easy to conceive of alien life in environments quite different from the surface of a rocky planet. The public ahas become aware of those ideas through science fiction and nonfiction, such as Peter Ward's Life as We Do Not Know It.
Meanwhile, the report encourages astrobiologists to study the odd organisms that live under extreme conditions here on Earth (or Terra if you like). As ScienceNOW reports:
The report urges scientists to adopt a threefold approach to finding extraterrestrial life: research in the lab, in the field, and in space. Chemists need to create life in the lab with building blocks not used in Earthly organisms. Research already indicates that the four nucleotides that make up our DNA aren't the only possibility for genetics--a 12-letter alphabet makes a perfectly fine genetic code. Field studies of extreme environments, such as the martianlike Atacama Desert in Chile or the Arctic waters, might turn up organisms with a biochemistry vastly different from our own. Combining such lab and fieldwork, space missions should be better equipped to find strange life.
Read the full report (free online).

By coincidence (or design?) Discover published an article about the "Aliens Among Us" a couple of weeks ago:
The common assumption is that DNA triumphed because “our form of life is seemingly so superior that we would have eaten” all other life-forms, says Steven Benner of the Foundation for Applied Molecular Evolution in Gainesville, Florida. “That’s the sum total of the argument. But that’s just anthropocentric. These sorts of ‘we’re at the center of the universe’ arguments have always failed.” When Davies first started quizzing other scientists about alternative life a few years ago, he remembers their eyes widening as they asked, “Why hadn’t we thought of this?”

Benner believes there may be some organisms hiding on Earth today that are based not on DNA and proteins but on a more primitive type of biochemistry. A number of researchers now theorize that DNA-based life evolved from an RNA-based predecessor. RNA is an unusual molecule that can both store genetic information and act like an enzyme, cutting apart other molecules or putting them together. Benner is convinced that 4 billion years ago, Earth was home to simple RNA-based organisms that could find food, grow, reproduce, and even evolve. Over time, some of these developed the ability to build proteins and switched to double-stranded DNA to carry their genes.

The remnants of the RNA world - or even stranger organisms - may still survive today. We just have to figure out how and where to look for them.

Image: NASA Europa explorer.

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