Comment by fermienrico | Hacker News Reader

Comment by fermienrico | Hacker News Reader

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I think there is more to life than just flesh and senses. Fundamentally, what makes living organisms "living" is the DNA (in primordial soup, it was RNA). Self-replicating systems lead to evolution almost magically... spontaneous processes (that minimize energy and are driven by thermodynamic imbalance) tend to create "competition" as they use up resources. The ones that make barriers or membranes, can protect the resources and end up replicating in larger numbers or faster.

We know that Carbon is essential in creating these self replicating entities. Its abundance is also known in the universe. So, it is not so unrealistic to presume that aliens won't be something that we can see on earth. I am curious of the differences of the forms of life that can originate though.

Imagine if we rerun 2 billion years of evolution again with a different initial conditions. Surely, we'd have different forms. Contrarily, eyes have evolved independently in many organisms over a dozen times. So, there is some commonality there. Just like the eye, there are many similar biological processes that evolve independently (arteries/veins, neuron, heart for circulation/extraction, ceramic structural components/bones, etc.)

I used to think about the same question that you are also wondering but the deeper you go down this rabbit hole, you are realize the importance of why things in living beings are the way they are. They're solutions (however imperfect) to an evolutionary problem.

> Contrarily, eyes have evolved independently in many organisms over a dozen times.

I'd argue that this is because we all have the sun in common. On a planet that orbits a star with a different spectrum or a planet that has a different atmospheric makeup, "vision" might look very different to what we're used to.

Why would DNA be essential? Why limit "life" to just carbon-based lifeforms?

It could be another self-replicating entity. It doesn't have to be DNA. But self-replication is the __basis__ for evolution of life. Sure, it could be made from other elements...but why wonder if other elements can create self-replicating entities when you can measure the amount of Carbon in the galaxy...right from here on earth! Carbon is more abundant than say Strontium. Carbon is created inside a star, Strontium is created through Supernovae explosions.

Also, Carbon can form 4 covalent bonds more easily than Si (same family). Si can form 4 covalent bonds but they're weaker than Carbon due to the distance from the nucleus. I am sure someone more familiar with Chemistry can chime in, but Carbon is a special element in the Universe.

Indeed carbon is special in this regards it's also produced in star fusion and space is filled with organic material because carbon, oxygen and hydrogen are abundant.

Also the main reason why silicon is worse is because of catenation, which carbon is much better suited for: https://en.wikipedia.org/wiki/Catenation

Thanks for adding to the discussion. I learned something new.

You were right it's because of the covalent bonds formed, however the biggest difference seems to be not in how these are formed with other elements but how carbon is essentially optimal at bonding with itself creating huge chains.

This property is why there are so many different and interesting forms of elemental carbon.

These chains are the scaffolding for all biochemistry both because they are very stable and also because they have very interesting properties.

Chains can be either open or closed depending on various factors such as charge, temperature and other forces that play on molecular levels allows you to play with that geometry.

Having strong chain bonds not only builds stable chains but it also build chains that can change their state while remaining intact and biochemistry heavily relies on the ability to change the geometry of complex molecules without breaking them apart.

Because of physics.

Life may also come in the form of a group of "artificial" robots that can mine and manufacture copies of themselves, with AI that can learn and teach each other.

Such "beings" (and indeed they should be indistinguishable from "natural" beings given a few hundred years) wouldn't need to evolve from scratch, so they don't necessarily have to be mostly built from basic elements like carbon.

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