The specter of aliens among us has been a staple of sci-fi books and movies since… well, ever since there have been sci-fi books and movies. But what if we are all aliens?

That's another proposition – i.e., that life on earth originated somewhere out in the vastness of deep space and was transported here – that has received a lot of play in both the popular media and scientific circles over the years.

Since we can't turn back the clock a few billion years to watch the first life develop, we'll likely never know for certain what actually happened. Much of the commentary therefore has been playful, of the "Oh, isn’t that a far-out idea?" variety. At the same time, there have been steadfast denials by those who believe in creationism, because an extraterrestrial origin for mankind would ruin just about everything.

But there's been little in the way of hard science to inform our thinking.

There was a flurry of publicity last spring when media sources proclaimed that researchers had found fossilized bacteria in a meteorite. That turned out to be untrue. Sure, it's theoretically possible for an anaerobic bacterium to hitch a ride on a rock that's cruising through the cosmos. But we haven't found even that fossil yet, let alone something alive.

For all that we know, life arose here – and possibly, only here – through some mysterious convergence of ingredients and forces. Chemical reactions, electrical discharges, impact shocks, cosmic radiation… all may have played a part.

Or not.

The fuss over the false meteorite report muddied the scientific waters for a while, and perhaps that's why there was little publicity back in August when NASA researchers announced that they truly had discovered at least the precursors of life in a meteorite.

This isn't entirely new. "People have been discovering components of DNA in meteorites since the 1960's [sic], but researchers were unsure whether they were really created in space or if instead they came from contamination by terrestrial life," says Dr. Michael Callahan of NASA's Goddard Space Flight Center, in Greenbelt, Md.

Callahan, lead author of a paper on the discovery that was published in the Proceedings of the National Academy of Sciences goes on to say that, "For the first time, we have three lines of evidence that together give us confidence these DNA building blocks actually were created in space."

The Goddard team ground up samples of twelve carbon-rich meteorites from Antarctica and Australia, ran them through a liquid chromatograph, and further analyzed the samples with a mass spectrometer, which helps determine the chemical structure of compounds. They found adenine and guanine, two of the four basic molecules (AKA "nucleobases") from which DNA is built. And they're pretty confident those molecules came from beyond our planet.

Why? They give three reasons.

First, the team discovered for the first time, in a meteorite, trace amounts of three molecules related to nucleobases: purine, 2,6-diaminopurine, and 6,8-diaminopurine. The latter two are almost never found in nature. These compounds have the same core molecule as nucleobases, but with a structure added or removed.

"You would not expect to see these nucleobase analogs if contamination from terrestrial life was the source, because they're not used in biology," says Callahan. "However, if asteroids are behaving like chemical 'factories' cranking out prebiotic material, you would expect them to produce many variants of nucleobases, not just the biological ones, due to the wide variety of ingredients and conditions in each asteroid."

Second, the team analyzed adjacent ice samples from Antarctica with the same methods used on the meteorites. The amounts of the two nucleobases, plus hypoxanthine and xanthine, found in the ice were much lower – parts per trillion – than in the meteorites, where they were generally present at several parts per billion. More significantly, none of the nucleobase analogs were detected in the ice samples. With the Australian rock, the team analyzed a soil sample collected near the fall site. As with the ice samples, the soil had none of the nucleobase analog molecules present in the meteorite.

Finally, the team determined that these nucleobases – both the biological and non-biological ones – could be produced in a completely non-biological reaction. "In the lab, an identical suite of nucleobases and nucleobase analogs were generated in non-biological chemical reactions containing hydrogen cyanide, ammonia, and water. This provides a plausible mechanism for their synthesis in the asteroid parent bodies, and supports the notion that they are extraterrestrial," Callahan states.

Add the three together, and you have a persuasive case that in fact chemical reactions in deep space can result in the building blocks of DNA. That doesn't prove that ours came from extraterrestrial sources, of course… only that it might have.

But the possibility has just become a whole lot more intriguing. For those wanting more, see this video presentation of Dr. Callahan's research.