What can evolve?

Perhaps we need a philosophy section...

Anyway, a common idea in science fiction is silicon life. We all know groups (columns) in the periodic table contain elements with similar chemical properties, as they are arranged by the number of electrons in their outermost shells. The group containing carbon, the basis for all known life, also contains silicon.

But the outermost electron shell is only a factor in what types of compounds an element can form. The atom's size is important, too, as is its electric charge balance. Thus carbon can form very large molecules but silicon cannot. How large can carbon-based molecules get? A typical strand of DNA in any cell in your body can be 3,000 meters long. Silicon doesn't come close to that.

More tellingly silicon is a far more common element on Earth than carbon. Yet it did not form large, organic molecules while carbon did. So whereas carbon forms things like DNA, RNA, proteins, lipids, which can be combined to form living things like people, cats, wheat and bacteria, silicon forms rocks, sand and quartz.

So the likelihood of silicon life is scant, to be charitable about it.

But this leads me to another prevalent SF idea: life made up of rock, or of magnetic fields or some kind of energy field.

Is this possible?

First let's look at how we think carbon life evolved.

we don't know this for certain, rather it's the reasonable supposition. Before there was biological evolution, there was chemical evolution. See, you can't simply gather carbon, hydrogen, oxygen, nitrogen, iron, calcium, phosphorous, iodine, copper, sodium and other elements and expect them to form a cell, or a DNA strand, or a DNA base pair, or a protein, or a lipid.

What they can form, rather easily under a variety of conditions, are amino acids. Amino acids are found in many places, on Earth and off. They've been found in meteorites, for instance. They've been detected in gas and dust clouds in space.

Once you have amino acids, they can combine together to form proteins, which are basically long amino-acid chains. Proteins can then form more complex structures, such as enzymes, lipids and DNA.

DNA lies at the threshold between the inanimate and animate reams. While not alive, it can reproduce by making copies of itself. That's the beginning of biological evolution, too.

Of course there's plenty we don't know about the whole process. Some things have been recreated experimentally. Urey and Miller produced amino acids in the lab by mixing gases believed to have dominated Earth's early atmosphere with water, then adding electrical sparks.

Now, if life evolved in this way, first chemically and then biologically, then we can see that long and big molecules are essential to the process. Rocks and minerals are very common, even those containing carbon (like diamonds, for example, or carbonates), but they remain rather simple lumps of small-molecule compounds.

Energy fields are not as common on Earth. To be sure there is an ever present gravitational field. There is also a magnetic field, generated by the Earth's spinning molten-iron core. And that's pretty much it. Lightning produces electromagnetic fields only briefly and sporadically. Lightning is formed by somewhat longer lasting electric fields in clouds and on the ground, but these too are sporadic and ephemeral.

The Sun, though, has a very strong gravitational field, a gigantic magnetic filed, and is constantly wracked by huge electromagnetic discharges. In addition it hosts ongoing nuclear fusion at its core, and contains large amounts of plasma (ionized gas). If energy can form the complex structures required for life, the Sun would be the place to look for them.

One last note. In the lab we've been able to create long molecules using silicon. These have produced things like synthetic oils, and one substance you may be familiar with: silly putty. But by far the larger uses for silicon lie in electronics and glass making. Both of these use simpler forms of silicates.

Somewhere someone is probably wondering if we're possible to exist and wondering if we do. There are so many possibilities and most of the universe is forever unreachable, so, so many things we will never know.