Mitch Sogin, who heads the NAI's Marine Biological Laboratory team, is a molecular evolutionist. He explores the history of life on Earth by studying DNA.

Sogin spoke at NAI 2005 about the contributions of molecular biology to the field of astrobiology. In this first of a three-part series, Sogin explains why microbial life is the focus of his work.

I have to confess that when this session was organized, I was very worried about talking about evolution in the solar system. I don't even understand evolution on Earth. And so, at first I thought it would be maximal speculation based on minimal data.

But I realized it was an opportunity to talk to both biologists and planetary scientists and physical scientists to try to bring us together in an understanding of what molecular evolutionists do and what molecular ecology is all about.

The title of my talk is "Did/Does Life Exist Elsewhere in the Solar System?" The answer is we simply don't know.

The discipline of astrobiology is an exercise that involves exploration. Some of us seek information about the distribution of water and organic material in the solar system. And others of us are interested in the environmental and physiological limits of terrestrial life.

These two disciplines converge so that what we're seeing today are heightened expectations that there is life elsewhere in the solar system and beyond.

When the NASA Astrobiology Institute (NAI) first formed, there was a lot of excitement at that time because we were just learning about the cracks on the surface of Europa. That was interpreted - and still is - as likely to represent a subsurface liquid water ocean on Europa, maintained by the tidal heating that occurs.

That raises the possibility of hydrothermal vent type communities. John Baross would argue that if you have hydrothermal vents, you have life, given enough time.

More recently we've seen an avalanche of information about the idea that Mars has a wet history, at least a past wet history, and the possibility that there is liquid water on Mars today.

There are spectacular images from Mars Express which indicate the possibility of frequent ice deposition and removal on Mars. We see these spectacular photographs of water ice on the surface of Mars.

More recently, there's been quite a bit of discussion - it's shown up often in the press - about methane on Mars, and the possibility that it has a biological origin. The alternative is that it's geological.

And then there are the results of the Omega spectrometer studies, which show sulfate deposits and other indications that there was at one time standing water on Mars.

And then there's new information from Titan about what the atmosphere there is like.

There's a reciprocal relationship between exploration of microbial diversity and the extremes of life in terrestrial environments, and where we might to go to look for life elsewhere in the universe.

As we learn more about terrestrial environments, possibilities continue to expand about what kind of life we might find elsewhere and where we might find it.

In our program - mainly I'm referring to the Marine Biological Laboratory at Woods Hole, where I work, but I think it's true throughout the NAI - we take a microbial-centric view of astrobiology when we think about looking for life.

That's because the microbial world accounted for all known life forms for nearly 50 to 90 percent of our Earth's history. (The exact percentage depends on how you interpret evolutionary history.)

I would argue that microbes are likely to be the only kind life we're going to find elsewhere in our solar system. Perhaps microbial life is the only kind of life that's going to occur beyond our solar system.

In fact, I would make the argument that multicellular life, complex life, is impossible without the microbial world to underlie the fundamental processes, the biogeochemistry that drives our systems.

Many people to argue that 3.9 billion years was the earliest time that we had a sustainable habitable environment on Earth, and that therefore life can't be any older than that.

I don't agree with that. I think life actually could be older than that period when heavy bombardment was coming to an end, simply because life could have gotten started multiple times.

Or you could imagine that particles sheltering life might have been shot up into the higher atmospheres, and rained back down on the planet when things cooled down. Now that's not a likely scenario, but it's possible. Certainly the molecular clock would argue that life might be 3.8 or 3.9 billion years old.

I'd like to point out that microbes - the bacteria, the archaea, the eukaryotes - they're everywhere, or almost everywhere. We find all kinds of microbes in hot environments, sometimes as hot as 121 C (250 F).

The only microbes we don't find in hot environments are the eukaryotes, although we do find eukaryotes everywhere else.

In extreme cold: microbes can exist at minus 15 to minus 30 Celsius (5 to minus 22 Fahrenheit), inside of solid ice.

In high pH (very alkaline) environments; in low pH (very acidic) environments, such as the Rio Tinto. These are the kinds of sites that astrobiologists in the NAI are studying today.

Hypersaline environments; desiccated environments like the Antarctic and the Atacama Desert; high-radiation environments; deep-sea and deep subsurface environments; and inside of rocks.

Any environment where you can imagine you might have a usable source of chemical energy, one can find life.

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