Carbon-based Lifeform Sci-Fi Trope

As a percentage of mass, the human body is 65% oxygen.
As a percentage of number of atoms, the human body is 62% hydrogen.

Therefore we should be either oxygen-based life forms, or hydrogen-based lifeforms in popular culture.

Checkmate, Sci-fi authors!

^^^^ Smartest guy in the room

Bzzzzzt! Wrong.

While water, which is where all those atoms are coming from, is vital for life/hydration, carbon is the element found in every organic molecule at the building-block level of life on earth.

See organic chemistry.

In other words, there’s a ton more carbon by mass than hydrogen in DNA.

Regardless of actual weights, your point about the basics of the buildings blocks is what matters. Carbon is the “scaffolding” on which everything else is attached and interacting. No other atom is capable of forming long, flexible chains with a virtually unlimited length.

For a very simplistic example of why carbon is so advantageous: it can support 4 bonds with other atoms. Nitrogen only does 3, oxygen only 2, and hydrogen only 1. With four bonds available, carbon can even support 2 double bonds.

And before anyone talks about silicon, silicon forms much weaker bonds than carbon. So the long chains and rings and complex structures formed from carbon compounds are much rarer and much more fragile with silicon compounds.

Silicon also would require a very strange atmosphere; if a silicon creature breathed oxygen, it would exhale silicon dioxide – i.e., sand.

In David Brin’s *Uplift *series, the major divide amongst living creatures is oxygen-breathing vs hydrogen-breathing, presumably both carbon-based. (There is also an order of mechanical life.)

If that’s really the case, then I’m a beer-based lifeform.

This analysis reveals a near-total ignorance of biochemistry.

I usually answer to “giant ugly bag of mostly water.”

Just for the record, how many guys were in the room when you posted this? I’m thinking fewer than two.

Isaac Asimov pointed out (surely others have also, but he’s where I first met the idea) that carbon bonds are so strong, they permit a kind of flexing or twisting of the molecule. In a similar silicon molecule, the bonds would just break apart, but carbon can form weird-shaped molecules. Much more versatile than it seems at first glance.

“If we hit that bullseye, the rest of the dominoes should fall like a house of cards. Checkmate.”

My argument has always been that abiogenesis basically is founded on the idea that:

[ random chemical reactions + time ] -> [ self-replicating reactions + random chemical reactions + time ] -> [ reactive self-replicating reactions + evolution + time ] -> [ life ]

So really, life is predicated on a strong mixture of chemicals and a lot of input energy, which would imply that there’s a greater probability of life appearing on Venus than on Mars.

The last time I raised this point, someone pointed out that you need a solvent, like water, in order to have a lot of good reactions.

So I did some research and discovered that the “gaseous” climate of Venus is basically about as thick as water and largely made of super-critical carbon-dioxide, which is a solvent. So you’d probably still end up carbon based critters, but probably with super-hot, thick carbon-dioxide coursing through their veins.

So far, the only argument I can think of to look for life on Mars instead of Venus that makes any sense is that Venus is too harsh an environment to explore. But otherwise, I think our odds are much better.

Nah, you don’t need all that much energy input to form biological molecules, not that much at all, and, more to the point, they are delicate, and the earliest ones were probably more delicate than the highly evolved ones we have now. In an environment with too much energy kicking around, i.e., too much heat, those long-chain molecules you need for life are never going to form, let alone survive long enough to reproduce.

I had a chemistry teacher once who had done a Ph.D. in germanium chemistry. Germanium is chemically similar to carbon and silicon in a way: it forms four bonds, and you can also get it to form chain molecules. However, to get it to form a stable chain even only three Ge atoms long (Ge[sub]3[/sub]H[sub]8[/sub],the germanium analogue of propane) and to do any further chemistry with this trigermane chain, everything had to be kept at super-low temperatures (I forget exactly how low - I am thinking liquid nitrogen sort of range but that may be wrong). At sufficiently low temperatures you could do some interesting chemistry with it, but let it get any warmer and the molecule would just fall apart.

At Venusian temperatures, carbon chains would fall apart too. Indeed, they do at only slightly elevated temperatures on Earth too. We call it cooking or (with just a little more heat) burning.

Life on Venus is very improbable. On Mars it is a real possibility, given that there now seems to be some water there. Even somewhere like the sub-surface oceans of Europa or the methane pools of Titan are possibilities. Both are way, way colder than Mars, but there is still energy around that might be sufficient to get life going.

That’s quite interesting actually. I wonder then if germanium-based life might be more feasible than silicon-based. Makes me wonder about tin, even. Though carbon being non-metallic and just so damn versatile in allotropes and chemical compositions (being one of the softest and hardest elements depending on circumstance) over the semi-metalics being more fragile and less versatile. Then tin being a metal might make for some damn interesting biology if possible.

Spoken like someone who isn’t standing on a planet nicely mantled and crusted over with just such complexsilicon compounds:dubious:

What Silicon doesn’t do, is make complex Si-H bonds like Carbon does.