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The fourth of eight questions from an ‘Arrival’ fan: It’s the color of a cephalopod-like-descended humanoid question! Assume a cephalopod-like-descended air-breathing tool-using bipedal humanoid with a skeleton, two arms, and four digits on each hand with a prominent index finger. The humanoid is hairless, possesses skin that appears rubbery, has a round face with a slight curved pyramid top to its head, oval black eyes similar to ours but without lashes, small ears and small slit-like nasal apertures, and has a slit-like mouth up to three inches long. Individual variation appears to be much less than human in terms of variety, given all eyes are black, noses and ears are not prominent, there is no hair, genitalia are internal, and they don’t wear clothes (they are polikothermic); they do wear suits for functional purposes.

The humanoid’s ancestors transitioned from a coastal to a land habitat approximately 25 million years ago. Assume it evolved on a similar planet to Earth, but with axial tilt of 12 degrees, around 0.95 standard Earth gravity, and less arboreal cover, and in a similar solar system. The cephalopod remote ancestor resembled a cuttlefish-squid-octopus omnivorous generalist cross which originally had ten arms. It evolved into a humanoid while emerging onto coastal habitats and adapting to exploit them while evolving to escape predators in the sea. The cephalopod possessed and retained a first mover advantage on land; insects existed, which it hunted, but amphibians, reptiles, and mammals never evolved.

So, the skin of the humanoid is white, because it possesses chromatophores (white ones, technically leucophores) like amphibians and cephalopods, rather than melanocytes, like mammals. However, what advantages does a white leucophore skin have on an earth-like planet over melanocytes? Would it not restrict activity or range?

Most animals evolve colour to blend in with their background, and stripes or spots to aid that blending. You may not think so, but a leopard is very difficult to see if it is sitting still in the African grassland. Colour can be functional - some humans are white so as to make optimal use of sunlight in making vitamin D, but a hairy animal will not use photochemistry and will make its chemicals some other way. So, my answer is, describe the terrain your animal evolved from, and where it lived in that environment, and the question just about answers itself.


Okay, so the white alien in my universe is essentially a top-level generalist like us, so has evolved way past coloration for camouflage. Cephalopods don't have bones, therefore no need for vitamin D, but I think we have established that this particular alien evolved bones and therefore does require Vitamin D. From what you are saying, an entirely white skin is optimized for harvesting sunlight. It's not optimized for deflecting sunlight to avoid burning. This optimization for sunlight suggests the planet the aliens come from may be slightly further away than Earth in the Goldilocks Zone. So, with technology their range is essentially the whole planet, on which there would definitely be polar icecaps, with temperatures in the tropics not getting above much more than 30C, right? This would put their world at around 1.1-1.2 AU from an earth-equivalent sun, right?

It is most unlikely skin would reflect light. To totally reflect light you need an infinite refractive index in the skin, and skin, on average, has a refractive index of about 1.5. The major protection against sunburn that white people have is the scattering of UV light by the roughness in the skin (which is why smearing your skin with oil, etc, without some further sunblock agent is a really bad idea - coconut oil, for example, is a really good basting agent because its refractive index is usually very close to 1.5 so refractive index matching cancels the scattering. Actually, wet skin strongly increases the rate of sunburn.) 

The position of equivalent temperatures depends on the age of the star and the size of the star, but on the planet it also depends on the geometry of the land masses. This Antarctica had subtropical temperatures prior to the Eocene. If your star is the same size as the sun, I would stick to somewhere close to 1 A.U. and not get too stuck on details.

Thanks for the AU advice. The cephalopod-descended humanoid's skin is more of a poser. It's really very smooth and white, as squid skin has the ability to scatter light across the full visible spectrum, as well as infrared light, because of its use of chromatophores (leucophores). So, the image I have is that it's like a white semi-gloss paint. On a paint index, that gives it 41-69% light reflectivity. Thermal regulation is less of a problem for the species; it does not have pores - cephalopods don't sweat. it is, instead, polikothermic.

The color is important because it's kind of a civilizational marker for them. So, their craft are white on the outside, and their interior design is very white, too.

I don't really know much about physiology, but I'd been giving some thought since you wrote this on how would a cephalopod extend its arms if they had no bones. 

For many years I've been jocularly been telling people about an imagined alien cephalopod family sitting around a large TV and one asking the other to pass the remote with their left tentacle you see. 

Today it hit me during a Reboot Ideas live stream, featuring Sasha Sagan, Ann Druyan and David Pescovitz that a cephalopod tentacle could possibly extend like an arm if it also evolved  hydraulic pressure to extend their appendages like spiders but keeping a bit of tentacle free at the end.  With cephalopods having evolved as specialists to live in water, the latter seems possible eventually.

On spider hydraulic pressure 


Now, pass me a beer with your right tentacle, please

The problem is that hydraulic pressure is also a pretty major weakness in a physical body. Parts of the alien could burst if punctured. I'm just going with evolved skeletons in my cephalopod-descended alien right now. I don't see a way round internal skeletons for a generalist terrestrial bipedal humanoid with two arms and fingers, even if evolved from cephalopods. I can imagine exoskeletons and other options; they're just not as efficient for a generalist.

Without making a direct comment, I would suggest to you that life must start with RNA as the information transfer agent. The reason is, we know life evolved so while the transfer must be unambiguous, and only the four nucleobases we know of can do that (leaving out some extras like orotic acid, which was not used, probably because it would not be formed) but equally, to evolve, there has to be a way of getting the sequences changed. The transfer must be through hydrogen bonding otherwise thermodynamics kills it, and the solubility requirement to separate and duplicate can only be managed chemically by phosphate ester. So there is no option, and protein cannot do it. Just something to add to your "life form".

Yes, I agree with this. There are actually several interstellar civilizations in my universe, and what happened is just a lot of parallel evolution along RNA/DNA lines, with no (or limited) panspermia. However, there are not that many interstellar civilizations, and nothing intergalactic (except possibly one). What is perhaps most surprising is that there are amphibian, reptile, and cephalopod-descended interstellar species, as well as some mammalian. The most exotic is a forerunner civilization-created artificial superintelligence (ASI), the only one, which is suspected of regulating the development of AI by other civilizations; at least, no other civilization has created an ASI.

Life appears to have to start with RNA, because there is no obvious means of chemically making DNA abiogenically. I summarise the reasons in my book "Planetary Formation and Biogenesis", which also reviews over 600 scientific papers. You need the nucleobases we have because geometry restricts the pairing to one option only for each. You need the phosphate ester as a linking agent because you need links that are strong, but can be broken to get mutation/evolution, and you need ribose because so far the ONLY way to start phosphate ester formation, prior to enzyme formation. The ribozymes came first as catalysts because you can't get enzymes by random condensation - the probabilities are too low and once a peptide bond is formed, those amino acids are effectively locked away and cannot be recycled. The book, in its present form, missed one form of AMP/UMP condensation that has been demonstrated since I published it, although the possibility was implied. I didn't quite have the courage to go the whole way.

If you accept that analysis, to get the biogenic chemicals and then life there are three different temperatures that have to overlap in terms of stellar distance at different times. If all stars accreted in the same time from the same disk density, then life would be restricted to G and Heavy K stars. However, those restrictions are not going to be followed, and it is unclear what the consequences of that are. The variation in disk characteristics following the stellar primary accretion would mean that only a few per cent of those might qualify. Life is going to be rare in stellar systems, but there is an enormous number of them. Even if only 1 in 10,000 stars qualify, we know there are at least 10^22 stars so there will be plenty of life.


With you so far. What I am interested in for my book is ethnocentrism, i.e., the relationship between interstellar species (which have FTL Navarro-Alcubierre drives) and the diverse planets with life in their 'sphere of influence', including the relative few with civilizations, like Earth. In my universe, the galaxy out there is relatively peaceful, with philosophical arguments predominating over interstellar wars, which are rare. So, for example, what matters most is the fundamental propositions for sphere of influence. So, for example, one argument out there is that mammalian civilizations should mentor others. So, in that sense, I'm interested in how the astropolitics follows the astrobiology.

Good luck with the book. I have tried two fictional books on other planets, but I have always restricted myself to relatavistic travel, and the special restrictions that places on species. I have even had a war - which took almost a millennium for the centre of a civilization to realize it was attacked.


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