By now, probably all of you have heard or read about the supposed discovery of fossilized bacteria in a meteorite that fell in 1864. Phil Plait the Bad Astronomer has a pretty thorough rundown of the situation, so go read what he says. I’ll wait.

OK, back now? I don’t have a lot to add to what he says: it could be life from beyond Earth, but there are so many possibilities (contamination by terrestrial organisms, non-biological structures that mimic life, etc.) that it’s going to be difficult to conclude anything definitive. However, I wanted to point to another article by biologist PZ Myers, who is a lot harsher and who says strongly that it can’t be life, no way no how. He points out (correctly) that the journal in which these results were published is highly questionable, and which has invested in very controversial ideas far out of the mainstream. (Plait also points this out.)

I try to be a fair-minded person as much as possible (with my share of blind spots, of course). I hate to be quick to dismiss something, even something fairly “out there”. Myers is most likely right: the chances of this being a real result are too small to take seriously, and the source is so lacking in credibility as to put it into the “crackpot” category. On the other hand, I prefer Plait’s tone, even though the conclusion is the same. Science doesn’t tend to deal in certainties, only probabilities; it’s safe to assign a low probability in a case like this, and it’s always good to say why the evidence in this case isn’t trustworthy.

We don’t know the origin of life on Earth. It’s possible that life may have originated elsewhere and arrived by comet—which of course doesn’t solve the problem or origin, just moves it off planet. (A common misconception of evolution is that it’s a theory of the origin of life, but that’s not true: evolution is a theory of the origin of species, of transformation between one type of organism to another over generations.) However, we don’t know the odds—we simply don’t have enough data to know whether life originated on Earth or elsewhere. We don’t even know the likelihood of life arising in the first place! Looking for trace evidence of life in a meteorite isn’t such a far-fetched idea as it might seem at first, since many chunks of rock on Earth are of Martian origin. But it seems premature at best to assign much credibility to this particular discovery.