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November 23, 2007

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"This applies to the sex lives of your neighbors, and to alien biology."

And then you spend the rest of the post on alien biology! Talk about a marketing failure....

A very interesting idea. As the article says, it would be very hard to find microbes that used a different molecule than DNA for their hereditary information, for example, since most of the techniques used to find non-culturable microorganisms these days are DNA-specific.

I think it would be difficult, though, if such an "alien" microbe was found, to really prove that it had an independent origin. Make it as different from known cells as you wish: you would never know for sure that it wasn't just a very deeply branching offshoot of the first, perhaps unique, origin of life on earth. In other words, the question stated in the article, "Is life rare in the universe?", might not be definitively answered, even if a completely alien cell were found on earth. But that question clearly isn't the only motivation for looking.

Perhaps the more important conclusion goes the other way: since as far as we can tell, all life on earth has a single origin, this is evidence that life doesn't just develop automatically on every suitable planet; otherwise we might well expect to find several distinct origins, as even Darwin suggested.

Unknown, that's a very good point.

But I think that in the end it will never be a clear-cut matter to make sound inferences about the unlikeliness of origin events on earth, and by extension the rarity of life on other planets, based on what we find now.

If we only ever find one "tree" of life, that is consistent with both a single origin on earth, and multiple origins (and either extinction or coalescing to explain why only one tree exists today).

If we find what looks like multiple trees of life, that is consistent with a single origin (and subsequent diversification that destroys our ability to link them together) or multiple origins.

But we might learn more in the future (especially from people trying to create life de novo) that would change this, so I shouldn't be too pessimistic.

Well, there might have been multiple times life originated, but I would guess whatever form had already been established would tend to very quickly wipe the 'new' form out. Or alternately, the new form would quickly supplant the other. For all we know 'new' life could be starting all the time, even in the present conditions, but the DNA/RNA monopoly quickly destroys it (that is, eats it).

This is a bit of a murky subject, because there doesn't seem to be any hard and fast definition of what is and isn't life. Unless I'm totally out of date, viruses aren't considered to be alive, even though they're undoubtedly far more complicated organisms (structures? chemicals?) than whatever our form of life originally came from.

"For all we know 'new' life could be starting all the time, even in the present conditions, but the DNA/RNA monopoly quickly destroys it (that is, eats it)."

That's a central point. To survive an alien lifeform would have to fit into some ecological niche where it has such an advantage it can't be outcompeted. That might be hard when the dominant life is filling most of the other niches -- it would have to live in the presence of their waste products etc.

So the best candidates for alternative life might be in places where our kind of life can't get a foothold. In the magma beneath the earth's crust, say. Find a way to harness an energy source and use the energy to reproduce structures that harness that energy to make more structures....

It might not look like life to us. It might live on a very different timescale. But in a place where DNA life can't survive and has very little influence, that wouldn't matter to the local life. No way we could outcompete them there.

I wonder if such a discovery would suddenly trigger warm feelings in us all towards fruit flies, avocadoes, leeches, E. Coli....

Can I coin the phrase 'Lifeform Bias', being the bias one feels towards beings that share a biological genesis?

D-N-A! D-N-A!

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