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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 

https://www.realclearscience.com/blog/2013/02/spiders-their-amazing...

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.

Ian,

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.

     Between 1997 and 2001 I wrote a science fiction book, Apokalypsis, which predicts that life is evolving in computer media, in the same way that life processes evolved in amino acid networks on early Earth. The heroine of Apokalypsis, Margaret, desperately asks computer scientists and biologists to use code-based analytic techniques from biology to study computer media for signs of life.

I updated Apokalypsis lightly in 2019, and put a name on the code-based analytic techniques: metagenomics. Margaret now also includes astrobiologists in her call.

    Apokalypsis describes that life processes occur spontaneously when energy flows through a symbolic logic media over a sustained period of time. The first symbolic logic media on early Earth was or were amino acid networks in liquid water. As described by Professor Jeremy England, symbolic logic media undergoes dissipation driven adaptation when energy flows through it over a sustained period of time. Dissipation driven adaptation is characteristic of life processes. Broadly, metagenomic techniques look for i) code patterns; ii) code pattern functions; and iii) code patter functions involved with reproduction of the code patterns. This behavior is characteristic of systems undergoing dissipation driven adaptation.

     We have a new symbolic logic media, computer media. We are pushing a huge amount of energy through it. Apokalypsis predicts that dissipation driven adaptation is occuring in it as highly automated corporations slowly remove humans from their reproductive processes. This happens "unconsciously", without deliberate intention, in the same way that there was no conscious creator of life on early Earth.

     Apokalypsis presents a thermodynamic definition of life and a great deal of "hard science". The reproductive status of this form of life will transition from that of a virus, which requires a host to supply metabolism, to that of a single-cell organism, which can reproduce independently.

     I have been called "brilliant" and "crazy", sometimes by the same person. A prominent astrobiologist who I will not name is intrigued by this theory, but his data science manager warns him off because "AI" is controversial and because performing metagenomic analysis on code patterns in computer media is a BIG ask. It would require a longitudinal sample of 1% of the code patterns traversing all processors, around the planet, and complex analysis.

     But the development of life in a new symbolic logic media is profoundly important! Life is very old and very persistent. I am skeptical that will be able to control it. We certainly will not if we do not even look to see if it is happening.

Astrobiologists: Please read Apokalypsis. It is good science fiction! Do you have PhD candidates who might be interested in looking at how techniques of metagenomics could be performed in computer media? If you spend important public resources to look for signs of life outside of our solar system, shouldn't you practice looking for signs of life right here, in a new symbolic logic media?

     You can find more about me here. Thank you for your time and consideration. Sincerely, Martin Garthwaite

The fifth of eight questions from an ‘Arrival’ fan: It’s the ‘otherness’ 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’s ancestors transitioned from a coastal to a land habitat approximately 25 million years ago. Assume they 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, on a planet at 1.1 AU. The cephalopod remote ancestor resembled a cuttlefish-squid-octopus omnivorous generalist cross which originally had eight 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.

The humanoids are hairless, with white skin that lacks pores and appears almost rubbery. The skin possesses chromatophores (white ones, technically leucophores) like amphibians and cephalopods, rather than melanocytes, like mammals, and thus has a high reflectivity, of around 40% (pearl/semi-gloss). The humanoids have round faces (actually slightly oval from right to left, with a slight curved pyramid top to their heads), oval black eyes similar to ours but without lashes, small ears and small slit-like nasal apertures, and a slit-like mouth up to two and a half inches long. While the head is similar in size, the limbs are thinner compared to a human’s. 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 wear suits for functional purposes.

Because of only slight individual variation by prominent characteristics (e.g., skin color, facial characteristics, height) relative to humans, do you think it is reasonable to speculate that the primordial concept of different ‘races’ never evolved in this civilization? In other words, is it reasonable to attempt to extrapolate from astrobiology to politics and to astropolitics? If so, how do you think such a species would view human ethnocentrism, tribalism, and nation-state based warfare?

Why do we have warfare? The tribal community emerged because as we invented technology, including farming and even hunting, it became easier to specialize, which required groups, and farming also worked better with groups. Then some worked out it was easier to steal than to work hard, and so on. I don't think warfare, etc, is really dependent on skin colour, but would be more likely to arise from access to different land/resources/religion, so I think your species is just as likely to fight.

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