The iguanas of the Galapagos Islands have evolved many unique characteristics due to their isolation from mainland iguanas. Because they can't swim long distances, biologists believe that the first Galapagos iguanas arrived on natural rafts made from vegetation.
The same thing may have happened across the ocean of space. Some researchers speculate that life on Mars — if there is any — may be composed of "island species" that were carried away from Earth on interplanetary meteorites.
Or perhaps both planets were seeded by life from an even more distant "mainland."
"Earth may not be the center of the DNA-based universe," says Gary Ruvkun, professor of genetics at Harvard Medical School and Massachusetts General Hospital.
The idea isn't too crazy. We already have evidence that some biologically important molecules, such as the ingredients for amino acids, are delivered by comets. And we know that around 3.5 billion years ago, meteorite impacts often kicked up rocks from the Earth's surface and launched them into space.
Microbes and/or bio-molecules may have hitched a ride on one these cosmic "rafts."
To test this theory – called panspermia – Ruvkun and his colleagues have started a project called the Search for Extraterrestrial Genomes (SETG, as a play on SETI). They are putting together an instrument that could go to Mars and search soil or ice samples for the presence of DNA. If the device finds any, it could then analyze the genetic code to see if the "Martians" are related to us.
A SETG prototype will have its first field test this year with funding from NASA's Astrobiology Science and Technology Instrument Development program.
Some argue that it is too soon to be thinking about DNA on Mars.
"If there were other signs of life, more specifically biomass, I would applaud DNA analysis," says Norman Pace of the University of Colorado, Boulder. "Without even trace target biomass, talking about DNA sequences seems premature to me."
But Ruvkun argues that his team's technique can detect a single DNA molecule in a sample, whereas other tests for biology – such as identifying chemical constituents with mass spectrometry – are not nearly as sensitive.
"It's very hard to detect a single molecule with chemical analysis," concurs Harry Noller from the University of California at Santa Cruz. "But you can uniquely amplify DNA," so that its signal clearly stands out.
In order to amplify small traces of potential alien DNA, Ruvkun and his collaborators have looked for a stretch of DNA that would likely be conserved in both Martians and Earthlings.
They believe this common thread should be in the 16S ribosomal RNA gene, which is vital to the protein-making process in cells. This gene has regions of its sequence that have barely changed over billions of years of evolution.
In fact, short segments in the 16S ribosomal RNA sequence are exactly identical in more than 100,000 species that have so far had their ribosome genes analyzed.
"There's no way to have a mutation here and live to tell about it," says Noller, who is not part of the SETG team.
As a consequence, any Martians that share our genetic heritage will presumably carry the 16S ribosomal RNA gene with the same conserved pieces that all of us Earthlings have.
A primer for life
The proposed strategy is for the SETG instrument to receive a Martian sample and add small extracts from the 16S ribosomal RNA gene as "primers" for DNA replication. If the sample contains DNA and if some part of that DNA's genetic code matches the primer's, then a suite of chemical reactions will produce a million or so copies of the sample's DNA.
The amplified DNA can be detected with special markers and part of its code can be sequenced in order to identify what sort of life-form is the owner of this DNA.
If the sample were contaminated by Earthling DNA, then the SETG researchers should be able to recognize signatures in the sequenced code that will pinpoint whether the contamination comes from a human or a bacteria or something else familiar to us.
But if nothing on Earth matches the observed sequence, Ruvkun and his colleagues will claim to have found our long-lost Martian cousins.
Ruvkun and his colleagues have built a prototype of their DNA analyzer and are in the midst of calibrating it. The team will travel to Argentina's Copahue Volcano, which is considered to be one of the most Mars-like environments on Earth. There, they will test whether the prototype can sequence the DNA of some of the hearty microbes that live in the acidic runoff from the volcano.