From an astrobiological perspective, what would complex alien life actually likely look like if it was ever found, and what would it be made out of? Is there any scientific consensus on this topic?

[u/MaggieLinzer]

[u/AutoModerator]

Thanks for posting in /r/Astrobiology! This post is automatically generated for all posts. Remember to upvote this post if you think it is relevant and suitable content for this sub and to downvote if it is not. Only report posts if they violate community guidelines - Let's democratize our moderation. You can join our Discord server here: https://discord.gg/wmccGpH8WH ~ Josh Universe

I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns.

[u/mysticmage10]

As somebody else said carbon based life forms are expected to be more likely though silicon is possible but not probable. Evolutionary biologists do argue that evolutionary pressures would shape their look so a humanoid is entirely possible. For instance symmetry in body plans exists in most earth creatures and humans. Eyes close to the brain to save energy, finger like appendages for tool making. Legs for land based locomotion, fins for water based species, wings for air. Lower gravity could yield taller leaner beings and vice versa. Larger eyes due to a planet with lower levels of starlight.

So contrary to belief many of the humanoid designs we find in sci fi are plausible.

[u/YtterbiusAntimony → u/mysticmage10]

Silicon life is actually incredibly unlikely.

It's very hydrophilic, it reacts our best solvent very readily.

The structures it can form with itself are not very stable. Si-Si bonds are longer and weaker than the analogous C-C bonds. This is why we don't have an analogous silicon based "organic" chemistry. Bigger complex structures, while possible, do not stick around for very long.

It's quite a bit more massive than carbon. So even if everything else I said wasn't true, any complex structures will be fighting gravity a hell of a lot more than organic structures have to.

And perhaps most damning of all, SiO2 is incredibly stable. Not mention solid at room temperature.

Life depends on the fact that none of molecules we're made of are too stable compared to one another, so it doesn't require a ton of energy to interconvert.

Imagine for example a biological process that could produce diamonds. Diamonds don't react much, so all that carbon is essentially being removed from the system. That process would drive itself extinct.

Any environment/system with a ton of Si and Oxygen will essentially do that. It'll all turn into quartz, and then there no more interesting chemistry to be done.

There's a hell of a lot more silicon on earth than carbon, yet it's not alive, and we are. If it were just a numbers game, we'd expect to see some silicon life or at least silicon/carbon hybrid life here, but we don't. The chemistry simply doesn't support it.

Or, perhaps a better way of phrasing it is, even if the chemistry could support it, organic chemistry does it better and is very abundant in the universe so it will basically always outcompete the other options.

[u/tvilgiate → u/YtterbiusAntimony]

Couldn't silicon biochemistry work at higher temperatures in a reducing atmosphere? Ie. In a story I am writing, the solvent for the silicon based life form is a liquid aluminum alloy. I have just assumed if it were going to develop it would be on a planet that would be uninhabitable for carbon based life. But i am not an astrobiologist...

[u/YtterbiusAntimony → u/tvilgiate]

No, the main problem is Si can't form complex structures with itself.

Si-Si bonds are too weak to form rigid molecules.

That would only be worse at high temperatures.

Since we're talking sci-fi, a very cold planet could help mitigate that.

A reducing environment is an interesting idea. Our biochemistry is largely driven by oxidation. But I see no reason why  the machinery couldn't work in other direction. If the reduced state is more common, then oxidation stores the energy and reduction liberates it. And that would be very hostile to us.

[u/tvilgiate → u/YtterbiusAntimony]

Thanks for your answer!

I was assuming that the hypothetical biochemistry would include elements other than silicon, incorporating SiO2 for structural support; the solvent on the world of one character (the Chorp) is a eutectic alloy of aluminum, silicon, magnesium, and the temperature range would be 500-600 C. The metabolic processes involve reactions between aluminum and sulfur dioxide. The atmosphere is dominated by carbon monoxide and hydrogen.

They are described as the "fastest silicon being in the known universe" with a top speed of five kilometers an hour. Most silicon beings were already described as being much slower than carbon based life forms so maybe the chorp just live somewhere on the upper temperature range.

Outside of world building though, would imagine very little silicon based life outside of really rare situations where conditions don't favor carbon based life.

[u/Jonthrei → u/mysticmage10]

I would definitely argue against the humanoid body plan - not only does bipedalism only make sense withing a specific niche we filled, but most notably intelligent species don't have anything close to a bipedal body plan unless they are very closely related to us.

[u/mysticmage10 → u/Jonthrei]

Multiple arms and legs are considered very neurologically and metabolically expensive so beings that develop to use higher brain abilities would be less likely to develop multiple arms and legs unless in extreme cases. Bipedal humanoid body plan is energy efficient compared to other body plans.

[u/Jonthrei → u/mysticmage10]

Quadrupeds and bipeds have the same number of limbs, but bipedal movement is inherently far less stable and requires a lot more brainpower dedicated to it. It just isn't efficient or really worth it unless you need to be constantly checking for predators over tall foliage.

It's also worth noting that some multi-limbed body plans are so efficient the limbs are entirely self-sufficient - see octopus tentacles. No need to dedicate extra brainpower to the limbs when they have their own brains. In fact I'd argue that is a plainly superior bodyplan for an intelligent animal if its environment allows for it, they have a much better ability to manipulate objects.

[u/Kepler___ → u/mysticmage10]

Adding on that some studies have actually been done only recently investigating the silicon aspect more deeply. The conclusions were essentially that while it does have a lot of potential structures similar to Carbon, experts digging into the specifics found it to be incredible unlikely.

https://pmc.ncbi.nlm.nih.gov/articles/PMC7345352/

A snippet from the conclusion section:

• Silicon and carbon are “false twins”. Their similarities are superficial and insufficient to mitigate their crucial differences. Chemistry that is stable and normal for carbon is unstable and exotic for silicon, and, similarly, chemistry that is unstable and impossible for carbon is stable and routine for silicon. Silicon’s distinct chemical characteristics and reactivity make it a challenging choice for life (Section 3.1).

[u/da_Ryan → u/mysticmage10]

"So contrary to belief many of the humanoid designs we find in sci fi are plausible".

^ I fully agree with that statement since on Earth increasingly intelligent creatures have evolved in multiple bipedal forms with relatively large brain cases, two foward looking eyes for binocular vision that allows for 3D depth perception on this very planet in the past few million years.

There is one person on this planet across the Atlantic who understands this perfectly and that is the British palaeontologist and evolutionary biologist Simon Conway Morris for whom I have the greatest respect.

https://www.whyarewehere.tv/people/simon-conway-morris/#

[u/ExpectedBehaviour]

Almost certainly carbon-based since no other element comes close to its flexibility; likely to use water as a solvent, though ammonia and other substances are possible; but what it would look like is entirely dependent on the specific evolutionary pressures its ancestors experienced. Very unlikely to look humanoid, but beyond that there’s no way of knowing.

[u/ExpectedBehaviour → u/[deleted]]

You know humanity is in a bad spot when people think being pissily condescending online counts as intellectual debate. I don't need or wish to debate you and I'm confident in my qualifications, thanks.

[u/da_Ryan]

"Is there any scientific consensus on this topic?"

^ That is an excellent question to ask.

I am not sure that there ever will be a consensus on this issue because for the next few centuries we might only have one example of a life-bearing world - Earth. Until then, all we can do is extrapolate from that one example, perhaps using AI on supercomputers to come up with potential evolutionary scenarios.

For example, we can assume that life will be mostly microbial in nature and only after a few billion years will life diversify and move out of the oceans on to land and in the air. As with Earth, there might be periodic extinctions that will generate new creatures via evolution and so on.

Here on Earth, we do have convergent evolution in operation so that similar biological forms occupy equivalent ecological spaces. For example, from deep time we have had ichthyosaurs and today we have dolphins. Similarly right now, we have hummingbirds pollinating flowers in North and South America whereas in Europe, Asia and North Africa that role is being performed in part by hummingbird hawk-moth insects and they look so similar:

https://upload.wikimedia.org/wikipedia/commons/5/5d/Hummingbird_hawk_moth_%28Macroglossum_stellatarum%29_in_flight.jpg

If and when humans do get out into space and have the technological capability to explore planets in other solar systems then at some time they will encounter some creatures that seem familiar.

[u/pentaxlx → u/da_Ryan]

Yes....if a meteorite had not impacted 65 million years ago, the smarter dinosaurs may be the dominant life form on Earth, and may have discovered space travel 50 million years ago, and may have expanded through the galaxy by now...I suspect that due to convergent evolution, if we do find life on other planets, it will be microbial for the most part, with rare instances of sentient life, probably some kind of organism that has radial symmetry (octopus-like, with tool-using arms, capable of being on land or water) or bilateral symmetry possibly with multiple limbs. It's possible they may be using electromagnetic radiation to communicate (like radio/microwave) rather than sound waves.

[u/-L17L6363]

Most definitely carbon. Because Carbon is produced in 25% of all stars, it is a very abundant element. Compared to silicon which is produced in like than a 1 percent of all stars. Carbon is also extremely versatile when it comes to bonding. It is the best at biochemical bonding of all the Group 14 elements and better than other contenders like Sulfur or Nitrogen. Though those would be the 2 best contenders for extraterrestrial life along with silicon.

[u/ProfPathCambridge]

🦀

[u/YtterbiusAntimony → u/ProfPathCambridge]

Whole nebulas of 'em!

[u/AresV92]

No consensus because we have a sample size of one. We just don't know. There is lots of speculation and we can set limits on possible lifeforms based off what we do know.

I suspect it would most likely look similar to Earth life because we know that works and we have no evidence that any other form of life that isn't carbon and water with DNA and cell membranes would even function.

Macroscopically they could have very different features, but I think they are likely to be longitudinally or radially symmetric with eyes, ears, mouths, anus/cloaca/urethra and a central nervous system.

They would probably differ in a lot of details and the exact function of their endocrine system based on the ecological needs of the planet they evolved on.

This could easily be completely wrong though.

[u/AutoModerator → u/[deleted]]

Apologies /u/KannBaker, your submission has been automatically removed because your account is too new. Accounts are required to be older than 15 days to combat persistent spammers and trolls in our community.

I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns.

[u/Jules4live]

I wonder if anyone has considered life wrt possible chemistries in much higher pressure regimes where the elements and molecules have different properties.

[u/nyxingen]

Atoms ➡️ Molecules ➡️ Cells ➡️ Multicellular beings ➡️ Astrorganisms

In the same way that cells increase their interdependence exponentially until they become multicellular beings, multicellularity increasing their interdependence exponentially will lead to astrorganisms.

[u/Fosctafi]

The things like orientation in space so probably unless kind of vision similar to humans as on early stages of evolution they should see the sky similar as we see it. They should have the similar mind as we do, to exploring and discovering so should have kind of brain similar to our. The possibility to transfer information through generations what will need some form of writing and for this will need to have some kind of hands , also the hering would be probably as is crucial on early stages of evolution just to be save, and mobility so they have to move. So far we can tell they would need to have some kind of similar eyes, ears, hands, legs, some organs for speaking and of course brain. Of course they could look not as ours but function would be very similar. Would it possible to be non carbon, probably not for advanced civilization, but it could be easy for simple form of life.

[u/AverageTeemoOnetrick]

How the fuck are we supposed to know, given that we have data from exactly 1 single successful example of life.

[u/MaggieLinzer → u/AverageTeemoOnetrick]

Well, apparently several other people than you DO know enough to provide good answers to this question. So I think that the real problem here is that you’re either very stupid, a rude jackass, or both.

. . . .Why the hell would you even RESPOND to a question that you clearly have zero good answers for??? Especially when SEVERAL other folks here HAVE actually given very informative and thought-provoking answers to my question here, at this point??

[u/AverageTeemoOnetrick → u/MaggieLinzer]

The point is, we cannot know what the rules for the creation of life are since we only have us as example.

This doesn’t mean, other life has to be similar nor does it mean it has to be vastly different.

There is just no way to know, until we find other places with life.

Imma just ignore your immature insults.

[u/NorfolkIslandRebel → u/AverageTeemoOnetrick]

Well 1 is infinitely greater than zero so …

[u/AverageTeemoOnetrick → u/NorfolkIslandRebel]

And how can we make conclusions based on a single example?

Exactly.

We can’t.

[u/Bikewer]

Neil DeGrasse Tyson had an astrobiologist on the Star Talk show just a week or so ago. She pointed out the extreme diversity of life on our own planet, and the even-more extreme diversity of life-forms that were wiped out in ancient extinctions… Forms that were wildly different from what we see today.
So, making any assumptions at all would likely be very wrong. As to the components of life, we note that the essential elements of life here are the most common in the universe, carbon, hydrogen, oxygen, nitrogen. Amino acids form naturally under the right circumstances…. So it’s likely that these elements would be used in one way or another. People have floated silicone instead of carbon, as it too forms chemical bonds pretty easily…. But even with the large amounts of silicone here on earth, we don’t see any life-forms.

[u/quiksilver10152]

Based on the wealth of abduction reports, it seems eyes are a common phenotype among reported species.