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Who is the "they" in the title? This is a post about the "they" who might be traveling the Universe in technologies of various sorts and might come here to Earth. It is not about any "theys" who don't come in physical technologies. It's not about "theys" who have been artificially constructed by other "theys" elsewhere or in the past. It's about "theys" who have evolved on planets over millennia and have, therefore, long histories. This post is one of several which will try to explore what science has to say about advanced life in the universe.
We start with chemistry. You can find all manner of science fantasy commentary about life based on every conceivable thing. It is all entertaining. It also has essentially no grounds for being taken seriously. The picture on the right says "millions of possibilities". On a number of things about our universe that is true. There may well be millions of stars having inhabitable planets. There may be millions of civilizations more advanced technically than we are. But there are not millions of biochemical systems. In fact, at a gross level of comparison, there may be only one. That is the CHON-P elemental system. Why? Isn't this a failure of creativity? Perhaps. But it is not a "parochial" bias as some would wish to claim. Consider these statements: a). a living organism must be at least a little complex. It must be able to maintain its existence in the face of damage [self-maintenance].
It must be able to pass on the information which gives the recipes for making itself [reproduction]. It must be able to alter its information without necessarily terminating itself [potential to "mutate" and evolve]. Simple atoms and molecules don't have the wherewithall to do this.
. a living organism therefore must have a complex physical system which can do these "living" tasks and have developed from the non-living environment preceding it. c). It must be able to control a microenvironment wherein simple pieces can effectively be built into more complex ones. d). the available pieces for use in this "life" endeavor are atoms/molecules/chemicals. What chemicals have the qualities necessary to pull this off?Science fantasy writers have "engineered" a variety of odd life forms for their books. All, even of them, realize that you have to string atoms together in structures of significant length and shape to achieve the chemical powers necessary to create the behaviors of Life. Because of that, they, if they're paying any attention at all, know that they have only two atoms which can combine in such long structures: Carbon and Silicon. It's not imaginative, but it IS the law. So, silicon creatures arise! But they won't...not really. I take this news as sadly as anyone. Diversity is more fun. But it ain't gonna happen. Why not? Note these few facts, among many: 1). the carbon-carbon bond is twice as strong as the silicon-silicon bond.
Well, who cares? Mother Nature does. These chemicals have to come together as very small molecules in chancey inefficient ways back in the days when there was no life at all and the molecules were just blundering about occasionally bashing into one another and then making a bond. Then they had to "hang tough" until the next lucky "bump-into" occurred in just the right way and a yet bigger molecule was formed. And on, and on. You need some stability to survive. Carbon is twice as stable with its bonding. 2).Generally speaking you want interactions to take place so that you [the forming molecule] can grow more complicated. In the Universe, and in the Solar System, and in the atmospheres of primitive Earths, Carbon is ten times more abundant than silicon. When Carbon connects to the most abundant atom [hydrogen] it forms Methane and happily goes about in the atmosphere and even readily dissolves in water. Silicon with hydrogen is unstable in water.
Carbon with oxygen makes Carbon Dioxide, a useful chemical and dissolving in water. Silicon with oxygen makes, basically, sand. 3). One can go on and on with these chemical facts. The story is simply: carbon is so superior to silicon in almost any measure that even on the surface of the planet, where silicon outnumbers carbon by 600 to one, life paid no attention to silicon at all. Life, for these and many other basic science reasons, is going to be made of long and diverse chains of carbon atoms with other atoms which react well with carbon [and are reasonably plentiful] attached to those chains. In this sense, all advanced chemically-evolved life forms will have the same base.
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