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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.
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