Are we aliens? This question has come up due to a new result from studying meteorites, and is getting a lot of web-chatter. I figure I'd better get on this sooner rather than later!
First, the science. Then the chatter. Finally, the caution flag. :-)
Some meteorites have been found to contain some relatively complicated organic compounds, including molecules that are components of amino acids, the building blocks of life. For example, the Murchison meteorite, which fell on Australia in 1969, has been found to contain purines and pyrimidines, which are crucial to a large number of biological molecules like DNA, RNA, and ATP (adenosine triphosophate, a chemical our cells use for fuel).
Now, you have to be careful. A meteorite might have had these molecules in it before it slammed into the Earth, or it may have absorbed them from the ground after impact. One way to tell the difference is to look at isotopes of the elements. An element like carbon usually has 6 protons and 6 neutrons in its nucleus, but an isotope is when the number of neutrons is different. Carbon 13, as an example, has 6 protons and 7 neutrons in the nucleus (the number of protons determines the chemical properties, so carbon 13 is still like carbon, though a tad heavier).
The ratio of isotopes in a sample can be different for objects in space versus on Earth. Various process can change the ratio, so that's a good way to find out if these molecules are native to a meteorite, or if it was contaminated after it fell.
OK, that's the primer. Now the good part: scientists studying the Murchison meteorite have determined that the purines and pyrimidines -- specifically, uracil and xanthine -- have a non-terrestrial origin. In other words, the molecules in this meteorite, so crucial for life, were actually formed in outer space and fell to Earth.
That is very, very cool.
So this means that the some of the basic building blocks of life formed out in space, and came to Earth via meteorites and, presumably, comets.
Note that "some of". That's important. Because...
This news is being picked up all over the blogosphere and news sites, of course. We're aliens! lots of people are saying. I'm quite sure the panspermia folks are having nerdgasms over this news as well.
Well, I hate to throw some cold H2O on all this, but I'm gonna. Just a little, a few drops. I think this is big news, and extremely awesome, but I want to make sure people don't take it farther than the evidence suggests.
The Caution Flag:
Let's take a step back and see what this new finding really tells us. First of all, I have little reason to doubt the results -- of course as a scientist I reserve some skepticism, but let's assume the team was on the money and the results are accurate.
(I don't have access to the journal paper, sadly, but I'll dig around and see if anyone else does) (I have the paper now; thanks to Preston Hart and Stan Gunn!).
So the Murchison meteorite has native chemicals in it that are the basis for life. Obviously, since they were found at all, that means they survived entry into our atmosphere and impact. This in turn means that in the distant past, when the Earth was bombarded by such rocks, these chemicals were scattered across the planet, where they could be incorporated into ever more complex molecules, which eventually became life. And, well, us.
Well, cool. And even, wow! It's been hypothesized for years, decades, that the basis of organic chemistry may have fallen to Earth from space. And now we know that it's true.
However, that does not mean that all of these chemicals came from space! Transport yourself back a few billion years. The Earth has just suffered a several hundred million year bombardment period, getting the crap pummeled out of it by impacts. But that finally ended, and the crust started to cool. The trickle of impacts that continued -- some of which contained the organic compounds -- fall to Earth. If they fall into a hostile environment, they'll get destroyed, of course? But if the environment is not hostile to them, then it's also possible these compounds could have formed right here, on Earth! It may be easier for them to form in space, where conditions may have been better, but that doesn't preclude them forming here.
So it's still possible that even though these compounds fell to Earth after the Earth settled down, it's also possible the majority of such compounds formed right here, in situ. The original molecules are long, long gone, so we cannot test them for isotopes. There may yet be some way of determining if life actually formed from space compounds or from terrestrial ones, but right now I don't think there is -- I haven't heard of one, at least.
So what these new results show is that life here might have formed from compounds that fell from space. It may even be likely. But it's not rock-solid fact. As the scientists say on the abstract of their paper (emphasis mine):
These new results demonstrate that organic compounds, which are components of the genetic code in modern biochemistry, were already present in the early solar system and may have played a key role in life's origin.
To be even more broad, and to stop any extrapolation here, this also does not mean that life itself formed in space and fell here. We're only talking building blocks here, not viruses, RNA, DNA, or bacteria. Just chemicals.
There is a group of folks who claim that every unexplained molecule must have fallen from space. Panspermia is a cool idea, and may even be right -- but the group at Cardiff (founded by Hoyle and Wickramasinghe) make outrageous claims about bacteria and viruses from space, with almost no real supporting evidence that I have seen.
So you have to be careful to not fall into a "God of the gaps" argument: because we can't explain something now, that doesn't mean it must be due to some supernatural (or extremely unlikely) cause. That's a bad path to follow, because eventually someone will fill that gap with evidence, and then your supernatural cause gets squeezed out. For example, creationists say that the eye could not have evolved, therefore God did it. But now we understand pretty well how the eye evolved, and the creationist's explanation looks silly. They found a gap in knowledge, proclaimed a supernatural explanation, and it turned out to be natural after all.
I put panspermia into that category for the moment. While there is some real science, and really interesting science, to pursue there, it's not a panacea for all things biologica-ex-nihilo.
So here are the big points:
1) Researchers have found that some molecules which are the basis for life on Earth can have an extraterrestrial origin.
2) These molecules survived their impact with Earth.
3) These alien molecules may have been crucial to the development of life on Earth.
4) These needed building blocks of life may have also been formed right here on Earth, so we can't fly off the handle.
5) Intact life from space -- bacteria, viruses, and such -- is still just an idea, with no credible supporting evidence.
But the really big point is that this is an amazing and wonderful discovery! It is entirely possible that life here -- or at least the necessary components of it -- began out there. This is one of those discoveries that makes you think, and sparks discussion, and also just happens to have some profound philosophical ramifications. I'm in favor of all three of those things!
So remember, when you go outside at night and look up, the sky is filled with far more than just the stars you see. Our future is up there, and our past. And, not incidentally, our present as well.