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In my opinion, the evolutionary pressure to develop a strong skeleton is size and the desire for speed. For the herbivore, it is to escape being eaten or to make them difficult to be killed. The gazelle tries the former, the elephant the latter. Both need strong skeletons.

I see no reason for five digits. Therapods had only three. The opposable thumb, or some equivalent, would seem desirable for tool use. As for skeletons, I would not bother. I have written some SF ebooks, and none of my aliens have their skeletons discussed. I think that is unnecessary, unless it is plot dependent, like them all collapsing into a heap if CO2 levels on their planet are not constrained, or someone develops an acid bomb. As for the middle finger length, my guess is it aids grasping, which is necessary while arboreal, but that is only a guess.

So, this is the second of eight questions from an ‘Arrival’ fan: It’s the senses 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 it evolved on a similar planet to Earth, but with axial tilt of 12 degrees and less arboreal cover, and in a similar solar system. The cephalopod remote ancestor resembled a cuttlefish-squid-octopus omnivorous cross which originally had eight arms. It evolved into a humanoid while emerging onto coastal habitats and adapting to exploit them while evolving 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. What I’m interested in is how acute the cephalopod-descended humanoid’s sense of smell, hearing, and eyesight would be compared to that of a human’s, together with how prominent its nose, ears, and eyes would be?

Your proposed organism would not be the sole species for 25 million years because as the planet would have a number of niches, the line would split into a "tree" with species separating out to take advantage of the niches. So the answer to your question would depend on what niches they chose. The acuteness of specific senses would depend on how much they relied on them for whatever niche they adopted. In evolution, if something is not important, it gets sacrificed for that which is.

Yes, I understand that. Pretty much all fauna on the surface of the planet is cephalopod or crustacean/insectoid descended. Flora is more diverse. However, amphibians, reptiles, and mammals never evolved there. But, sure, what turned into the dominant species has its own ancestral tree of subspecies that competed against each other, sure, mainly for coastal resources originally against other marine species (e.g., crabs), fishing, some farming, e.g., of insects. There's just way less diversity than on Earth. What I'm thinking of is what a cephalopod would bring with it to a human-like head in terms of senses, which I see as optimally efficient for many reasons. So, sight is important, so on a human-like head (not round but more like oval with a softened pyramidal top), I'm thinking oval human-like black eyes, with no eyebrows or eyelashes. Now, Terran cephalopods can hear, but not well, mainly surf, waves, and wind sounds, using statoliths instead of cochleas, and only at low frequencies of up to 500Hz, although this would have become gradually important on land, and so their heaving would have improved. So, I'm thinking small, inset ears. Cephalopods can also smell, using small pits located beneath the eyes, so I'm thinking separate small noses with slits could develop roughly where our noses are, between the eyes and mouth. So, overall, the most prominent feature compared to a human would probably be sight, i.e., human-size oval black eyes.

You seem to have it under control. My question that you should address is, if you are going to have a skeleton, how do tentacles work? Once you have rigid bones with joints, the tentacles no longer work like that, although I suppose you could have bones in the legs but not in the others, in which case you might as well retain all the tentacles, the ends of which would correspond to fingers. Just a suggestion.

So in that 25 million year ago era when the eight-armed squid/octopus/cuttlefish hoofing it around the coastline evolved towards exploiting more land resources and avoiding the predators in the depths, what was a relatively primitive backbone did begin to evolve into a skeleton. Every large fauna we know on earth has some kind of internal skeleton, and so does this. So, the species evolved a backbone, with a skull, with a maximally efficient generalist design of two legs with knees and two arms with elbows, to allow it to look up and around, run, climb, etc. Tentacles have many advantages: https://www.irishtimes.com/news/health/octopus-tentacles-inspire-ne.... However, I am picturing tentacles-like appendages that evolved into arms and then sprouted hands with skeletal structure that developed high-precision manual dexterity and grasping for what is now a land-based species but which never really went through an arboreal phase. So, there are hands and fingers with bones, but the joints are not as pronounced. Also, the skin is almost rubbery in appearance, so from a human perspective this makes hand and toe joints harder to see, too. But, they are there.

The third of eight questions from an ‘Arrival’ fan: It’s the build 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 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 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.

What I am interested in is their height on planets with different gravities. So, I guess their height is about the same as ours, e.g., 5’10”. Would you please speculate on the humanoid’s build relative to a human in .5, and 1.5 Earth standard gravity?

I am not sure you can be sure, after all, consider the variation in hight of life forms on Earth. However, as a general rule, the stronger the gravity, the thicker the animal will be because it needs thicker bones and stronger muscles. Higher gravity probably favours shorter, but food availability and environment is probably stronger drivers. My opinion only.

Thanks for the reply! So, in my universe, they're a lot wirier than we are, i.e., their arms and legs are proportionate to ours, but just thinner. I'm figuring that home planet gravity could be 0.9-0.95 standard gravity at their height.

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.

Ian,

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?

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