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Comment by dieselgate | Hacker News Reader

This was a good write up as it contained criteria and points for "defining life", it was nice to have a refresher on the meaning of "entropic" life as well.

I've often thought if complex extraterrestrial beings were discovered they may not "look" much different than anything found on earth, since the spectrum is so vast on this planet (bacteria, plants, mammals, cephalopods, etc.) - considering the universal confines of physics and locomotion. Isn't there something special about crab/spider anatomy as well and it being very "evolutionarily stable" or something? Cannot recall any papers or information on specifics here at the moment.

Meanwhile, I always laugh watching old Twilight Zone episodes and how they have regular humans playing aliens - they don't even bother to differentiate them as a species (likely because of available technology) but think this can work well in a way.

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There was a really cool episode on Mindscape with Arik Kershenbaum: https://m.youtube.com/watch?v=QFLrpIslOOk

He talks about why certain things simply make sense. Like how wings evolved six times, why communication via sound is simple and communication via some sort of telepathy is not, and why it makes sense for a large animal to be symmetric. From that he reasons why alien life probably wouldn't look all that different.

Communication through sound is just vibrating your inner organs at a pattern of frequencies, causing the air to vibrate at the same frequency, and resulting in someone else's ear drums vibrating at the same frequency. This causes thoughts to spontaneously appear in your partner's head. How is this different from telepathy?

Everything? You're describing two different channels, not two different intentions.

The takeaway is not how speaking is like telepathy, but that telepathy wouldn't be that different than speaking. It would presumably have some different characteristics, like speed-of-light propagation, but it would still be just a sense organ.

"Dammit, Joe's thinking at me again. I'm going to ignore him while I watch TV."

There's a lot of convergent evolution towards the crab body plan, a phenomenon known as Carcinisation. (https://en.wikipedia.org/wiki/Carcinisation)

Who is to say that is a universal phenomenon? May have something to do with our level of gravity, life cycle of the blastocyst, metabolic requirements from respiration, other fauna competition, etc. Which is not to say there is a better alternative on the table, but I would hesitate to say that we should expect crabs on other planets.

Edit: Kicking myself over the missed opportunity to say "space crabs" or a Zoidberg reference

The fact that there are a large number of crab-like land and water dwelling organisms, and in a range of sizes and regions, suggests there's a pretty big range of environments that crabs thrive in. That's still far from universal, but it's also not very limited either.

Earth has been through similar changes too. Life started in water, a "low gravity" environment, then moved onto land. Similarly, the atmosphere was initially very oxygen saturated, causing animals to be very big. Declining oxygen rates in the atmosphere caused animals to converge to a smaller size. But, we don't see extreme differences when you compare animals from millions of years ago to today despite that the ecological parameters have changed significantly.

But those are all coming from the same progenitor life form. Initially selected components are relatively fixed from that point onwards. For example, mitochondria having their own genome and replication machinery feels like a poor solution, but it is "technical debt" that can never be corrected. A different set of circumstances could have favored all organisms having skeletons instead of exoskeletons, a singular orifice for ingestion and excretion, etc.

The existence even of mitochondria is insane though. A cell happened to eat another cell and instead of just digesting it, the prey cell fused with the predator and became mitochondria. Bananas. Similar story with plant cells.

So, monkeys had to type for only a half a million years after all.

The spectrum on this planet really isn't all that vast. Sure, to you looking at an organism's morphology it looks vastly different, but so do dog breeds from eachother yet they are all dogs. You have a lot in common with all forms of life in terms of how your cell biology works, because we all share a common ancestor.

Life on another planet might originate from an ancestor that functions dramatically different than the last common ancestor for all life on Earth, and that might have huge implications for what a potential multicellular organism with this evolutionary trajectory would look like (if such life even utilized multicellularity, or cells at all)

The same is true of trees! They've independently evolved many times [1]. Any solar-powered biome will probably have trees.

[1] https://eukaryotewritesblog.com/2021/05/02/theres-no-such-th...

> Isn't there something special about crab/spider anatomy as well ...

Carcinization?

https://xkcd.com/2314/

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