How much of life on Earth is universal truism?

General Questions
#1

Some discussion or other about the idea of silicon based life forms started me thinking:

What other forms are there that life could have taken.
I know that silicon life has been pretty much dismissed because of things like silicon dioxide being a solid whereas carbon dioxide is a gas and therefore easier to move about, but that strikes me as a rahter limited view; I’m not asking if you could re-engineer existing life with different elements, I’m asking if it’s concievable that other ways could exist.

So:
Water? (I know water has some interesting and unique properties, but are there other suitable chemistries? (maybe involving molten metals or some such)?
Carbon?
Carbohydrates/fats?
Proteins?
RNA/DNA? (I’m assuming no Panspermia)
Cell-like stuctures?
Chlorophyll?(are there other, perhaps better ways to trap radiated energy?
Vascular plants?

In essence, I’m asking how much commonality should we expect as default between ourselves and lifeforms from other planets (if we ever get to see 'em)? and is it possible that life might arise that is entirely different from our own?

#2

I think this kind of question is limited to speculation until we “get out there” and see what the other possibilities are. For one thing, we don’t even know how life started on Earth, never mind in other planetary environments. But I suppose biologists/chemists could have some idea of what is more or less likely. But even the improbable is still possible.

But if we’re open to speculation, I’ll throw one more idea out there…liquid hydrocarbons instead of water as the primary solvent/medium…the hydrophilic/hydrophobic ends of a cell membrane could be reversed (compared to cell-life on Earth in a water environment). Other cell structures could be similar. Liquid hydrocarbon seas may exist (e.g., on Saturn’s moon Titan which). Plenty of organic matter to work with in such an environment.

#3

Thanks Phobos, yes, it is speculation that I’m after, but based on what we know about chemistry.

For example I can say fairly surely that there aren’t likely to be many life forms out there with a body chemistry based on gold, because it is so inert, but are Carbon/Hydrogen/Oxygen the only viable options?

#4

Another thing to keep in mind is what’s available to be used by life.

Here are the abundances of elements in our solar system, relative to silicon having an abundance of 10[sup]6[/sup] atoms, in order of decreasing abundance.

H 2.79 10[sup]10[/sup]
He 2.72 10[sup]9[/sup] (inert)
0 2.39 10[sup]7[/sup]
C 1.01 10[sup]7[/sup]
N 3.13 10[sup]6[/sup]
Ne 3.44 10[sup]6[/sup] (inert)
Mg 1.07 10[sup]6[/sup]
Si 10[sup]6[/sup]
Fe 9.00 10[sup]5[/sup]
S 5.15 10[sup]5[/sup]
Ar 1.01 10[sup]5[/sup] (inert)
Al 8.49 10[sup]4[/sup]
Ca 6.11 10[sup]4[/sup]
Na 5.75 10[sup]4[/sup]
Ni 4.93 10[sup]4[/sup]
Cr 1.35 10[sup]4[/sup]
P 1.04 10[sup]4[/sup]
. . .
Au 1.87 10[sup]-1[/sup]
. . .
(Data from http://www.geo.cornell.edu/geology/classes/geochemdata/cosmoabund.html )

So, is it suprising that the elements necessary life (as we know it) are hydrogen, oxygen, carbon and nitrogen? If you want silicon-based life, you not only have to explain the chemistry, you have to explain why the life would be based on an element 10,000 times less abundant than carbon. I’m not saying that it’s impossible, of course, just that assuming carbon-based life is the most likely option isn’t mere hubris or carbon-o-centrism. :slight_smile:

Water is made of hydrogen and oxygen, the two most abundant non-inert elements. While there are many enviornments where liquid water is not available, these environments are typically cold (as is Titan), and cold environments typically mean slower chemical reactions–another strike against life. Again, I’m not saying it’s impossible, just less likely than LAWKI.

#5
#6

Gah! ignore me! - I was trying to extract Podkayne’s post for reference in another thread and posted it here again by mistake.

#7

Silicon is quite common in the universe, being formed in giant stars; it also commonly forms the crust planets and moons so you might expect silicon to be available as an alternative to carbon in some places.

It might be possible for carbon-silicon organic compounds to allow life to develop at slightly higher temperatures than it does on Earth; bear in mind simple life can exist at 100+ celsius around thermal vents,
I wouldn’t rule out a wax/silicone/silane/protein basis for extremophile carbon based lifeforms…

while on colder worlds ammonia has been proposed for a solvent to replace water, and of course the carbon-based organic proteins and lipids associated with this life chemistry would be quite different to the ones we are made of;

someone else suggested methanol, which is liquid at a wide range of temperatures; but how methanol would occur without a biogenic origin I don’t know.
But then our atmosphere is one fifth biogenic so I wouldn’t rule it out.

Water is a very good option, being a polar solvent and everything; but it may not be the only option.

SF worldbuilding at
http://www.orionsarm.com/main.html

#8

If silicon is such a relative rarity, perhaps we will improve the chances for life on other planets if we mine and libertate the massive quantities stored in Pam Anderson’s boobs.