Life from outer space?

[Podkayne]

*Originally posted by Kaz *
After these last 6 billion years (or however long we want to say the world was around for)

Life has existed on Earth for somewhere between 4 to 4.5 billion years.

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Therefore, if it involved in us, and worked, why would it evolve differently somwhere else?

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A few thoughts.

Life on another planet would obviously be very different evolved under very different conditions. There’s no reason to believe that Earth-like planets are rare, but we also don’t know enough to say that Earth-like planets are the only, or even the best, place for life to evolve. Consider Europa, for example. There may be liquid water there, a requirement for life as we know it. But the chemical balance of that ocean would be quite different from Earth’s early ocean. The surface of Saturn’s moon, Titan, might be covered with an “ocean” of hydrocarbons and frigid temperatures. Given an organic-rich liquid environment, could life emerge? If so, I’d wager against it being DNA-based! And that’s just looking at one solar system out of tens of billions.

Is DNA-based life the inevitable result of an environment like the early Earth? Or was it a fluke, a one in a quintillion chance, that just happened to succeed? (Drifting into the realm of the anthropic principle, here. . .)

Then again, DNA-based life was fantastically successful on Earth, so it would not be suprising at all to find it in other places.

And come to think of it, it really would make sense for there to be a general physical law that says something to this effect. For example, many of the same ideas or same types of products can be found in totally different cultures, before they had outside influence. This may be really pushing the boundries of analygies, but it makes sense to me…

I don’t know of a law, but the processes to which you refer is called convergent evolution. It’s seen in biology as well, where animals that fill similar niches have solved their problems in the same way, to the point where they look almost identical even though they evolved completely independently and one, for example, is a marsupial and the other is a placental.

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MEBuckner wrote:

So, is the genetic code arbitrary or not? I don’t really know. [ … ] On the other hand, this Duke University biology class notes page says that are “[n]o thermodynamic reasons to believe that the common genetic code is any better than a large number of other possible genetic codes”.

Any biochemists or evolutionary biologists care to weigh in on this?

There are a few species of bacteria – and some non-mammalian critters’ mitochondria – whose DNA does not use the same genetic code as everybody else’s. The changes are slight: one of the codes normally used for one of the lesser-used amino acids instead represents a “stop”, for example.

[MEBuckner]

Well, can we say those organisms use a “different genetic code”, or can we say they have a “(relatively) minor variant of the common genetic code”? It doesn’t sound to me like those particular life-forms likely evolved completely independently of other Terran life.