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The Man In A Suit - Alien Evolution

Boron appears to be a good choice, capable of similar or greater diversity, but is still someone unstable outside of a narrow range of conditions. It's also far less prevalent than Carbon, so would require an anomalous concentration in order to become the basis for a life form.
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Though we generally agree, I would say that the low amounts of Boron in the Solar system shouldn't be a limitation here. Boron could be more common in other star systems or galaxies.

McHrozni
 
GraculusTheGreenBird said:
I was thinking of a very low gravity environment. Where there would be less sense of up and down. So dont think of a tripod, but a tube with arms sticking out at maybe 0, 120, 240 degrees around the body, which would be used to anchor/propel oneself at any convenient point which might be in any direction. Here 3 would be better than 2 as it would have better range of accessible directions, but I wonder if 4 be necessarily be any better, precisely because of the extra cost of evolving the extra one. And if so, they could well be one set of limbs at 0,90, 120, 270, rather than two sets in the conventional way we think of arms and legs.
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As mentioned by others, such low gravity makes atmosphere problematic. That said, I don't see any reason why a 3-limbed animal couldn't evolve, but I also don't see many reasons that would favor it. In your particular example you would actually favor more limbs, not less. Four forming a tetrahedron seems notably better, imho.

It all depends on that particular environment, but its pretty easy to see there might be optimum configurations that wouldn't depend on exactly 4 limbs.
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Considering what we see on Earth it's sensible to conclude that an even number of limbs is favorable to an odd number. I suspect Delvo could explain better why, I'd say that body formation is less complex if you're doing two very similar halves.

McHrozni
 
Furthermore, any species that has mastered interstellar flight probably has mastered genetic engineering as well. Maybe the "human" looking aliens created a centaur-like animal which eventually took over the planet. Or these "human" looking aliens might have decided that bones might be more of a hindrance in zero-G and they created from scratch an intelligent life form without bones.


ETA: Perhaps someone more knowledgeable than I can explain if it is possible for an alien species to not have its birth canal located between its legs. If so, then it might be easier to accommodate the large heads.
 
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Ladewig said:
ETA: Perhaps someone more knowledgeable than I can explain if it is possible for an alien species to not have its birth canal located between its legs. If so, then it might be easier to accommodate the large heads.
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On Earth, all species that don't have legs. What limits the brain size of a whale?
 
McHrozni said:
Considering what we see on Earth it's sensible to conclude that an even number of limbs is favorable to an odd number.
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With bilateral symmetry, it not only makes sense to balance forces to make "forward" the most natural, easy, neutral direction of travel and make both right and left turns equally easy instead of favoring turns in one direction over the other, but it's also practically biologically impossible to avoid. A bilaterally symmetrical body doesn't really know how to grow just one of something instead of a pair. It might work for structures right down the middle of the body (like the organs of your digestive system, before they get pushed around), where there can be just one of something and it's still symmetrical, but limbs on the midline wouldn't work very well for reasons I expect we can all visualize without explanation, and anything that isn't along the midline is out in "everything in pairs" territory.

Without bilateral symmetry, it's a different story. The next symmetrical pattern to consider would be meridional, which is an easy word to remember if you remember that the Earth's lines of longitude can also be called "meridians". Pentaradial symmetry is one version, best known in starfish. But meridional symmetry could come in other numbers of meridians instead of always five. Jellyfish and their relatives are tetraradial, although it can be hard to see in some species; it's easiest to see in box jellyfish, with their rounded square bodies, four groups of tentacles at the corners, and four groups of eyes. I can't say whether octopus tentacles follow the bilateral symmetry that the rest of the body has (four tentacles on each side) or break bilateral symmetry and got the way they are by octoradial symmetry around the mouth. Flowers can be radially divided into three, five, or six meridians, although some species such as roses hide that by having more than one petal per slice (just as some starfish have 10 or 15 arms instead of 5). Don't get me started on stems & twigs. There could be radially symmetrical bodies, not divided into any number of meridians, but those wouldn't have limbs; as soon as there are limbs on a radial body, there's some particular number of them, which turns it meridional (or not symmetrical at all, in theory, but I don't believe there are examples of that).

However, none of those meridionally symmetrical critters are known for moving around on their own power, and as soon as there is much of a call for that, there's clear physical incentive to move toward bilateral symmetry. This has already happened in two groups of echinoderms (relatives of starfish) independently. They're pentaradially symmetrical as adults. You can see this even without the starfish arms in sand dollars (sea cookies) because of their pentagonal bodies, and in sea urchins and sea cucumbers because of the 5 lines running from one end to the other; think of starfish and sand dollars as what happens when you squash a sea cucumber/urchin end-to-end until it splats out to the sides.
  • But sea cucumbers, instead of being oriented with the mouth down like other echinoderms, lie on the side, and move with the mouth in front. Now, out of those five meridional lines running between mouth and anus, three touch the ground and two don't, which is like a starfish or sand dollar going up on edge with three arms/corners touching the ground and two above... but in sea cucumbers, the three lines on the ground are where you'll find tube feet, like the bottom of all five arms on a starfish, and the two lines above are where you'll find thickened, hardened, or spikey skin, like the top of all five arms on a starfish. That gives sea cucmbers a primitive version of bilateral symmetry from a pentaradial starting point, with different front & back, different top & bottom, and reflected right & left.
  • Sand dollars, meanwhile, tend to move with one corner in front all of the time, even while the body stays flattened in starfish-like orientation with the mouth in the middle of the bottom surface. Now the other four corners aren't evenly distributed around the body anymore; they've migrated around toward the front, with the anus migrating down into the widened space this creates between the two in the back. That gives them a primitive version of bilateral symmetry from a pentaradial starting point, with different front & back, different top & bottom, and reflected right & left, just in a completely different way from how sea cucumbers got there.

Ladewig said:
Perhaps someone more knowledgeable than I can explain if it is possible for an alien species to not have its birth canal located between its legs. If so, then it might be easier to accommodate the large heads.
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Birth canals aren't something we can make comparisons with on Earth because not enough lineages have internal gestation and birth canals at all. But if we expand it to egg-laying organs, of which birth canals are just a derivative anyway, then we can. And egg-laying organs nowhere near the legs are pretty common. Some insect species are known for pretty long ovipositors sticking out in back, and their legs don't come from anywhere near the back end of the body. For an example more closely related to us, some fish also have ovipositors on the bottom surface up closer to the pectoral fins than the pelvic fins, which is like you having an external reproductive organ somewhere between your navel and the bottom of your breastbone, but it's where it really is in some relatives of yours. That may even be the ancestral condition for humans, since your reproductive glands started up higher in your torso and then had to migrate down before you were born to get where they are now (a longer trip if you're male than if you're female). So where it ends up in one lineage or another seems to be not particularly constrained, but just incidental to that lineage's past.

What happened to the human vagina is incidental to a history that might sound strange but isn't hard to see by comparison with other living tetrapods. The ones that aren't mammals, along with monotreme mammals, have a cloaca, which just means an anus which is used not only for solid wastes but also for liquid wastes and reproduction; the kidneys and reproductive openings simply empty into the area right before the anus's opening to the outside, instead directly to the outside themselves. (Remember what I said in a previous post in here about the predictability of using the mouth for both eating and breathing because there's a tendency to use one hole for multiple uses instead of multiple separate holes? This is another example.)

In mammals other than monotremes, the urethral opening and egg-laying opening have migrated out of the anus. But they didn't get far because by that time there was a pubic bone in the way. That's the part of the pelvic girdle which sticks up and forward from near the sockets for the femurs, ending just barely under the skin right above the external reproductive organs. Whoever put it there when there was a cloaca just wasn't planning ahead for having the cloaca's three functions move away from each other later. Babies wouldn't need to escape through the space under that bone if we either hadn't ever had a stage in our history with a cloaca, or hadn't gotten a circle of bones fused around it. Some other lineages either don't have a cloaca or have one that isn't surrounded by fused bones, so the particular arrangement we've got there didn't need to happen.
 
Delvo said:
With bilateral symmetry, it not only makes sense to balance forces to make "forward" the most natural, easy, neutral direction of travel and make both right and left turns equally easy instead of favoring turns in one direction over the other, but it's also practically biologically impossible to avoid. ... (etc)
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Thanks :)

McHrozni
 
SezMe said:
On Earth, all species that don't have legs. What limits the brain size of a whale?
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The question was about species smart enough to develop interstellar travel.

Delvo said:
Birth canals aren't something we can make comparisons with on Earth because not enough lineages have internal gestation and birth canals at all. But if we expand it to egg-laying organs, of which birth canals are just a derivative anyway, then we can. And egg-laying organs nowhere near the legs are pretty common. Some insect species are known for pretty long ovipositors sticking out in back, and their legs don't come from anywhere near the back end of the body. For an example more closely related to us, some fish also have ovipositors on the bottom surface up closer to the pectoral fins than the pelvic fins, which is like you having an external reproductive organ somewhere between your navel and the bottom of your breastbone, but it's where it really is in some relatives of yours. That may even be the ancestral condition for humans, since your reproductive glands started up higher in your torso and then had to migrate down before you were born to get where they are now (a longer trip if you're male than if you're female). So where it ends up in one lineage or another seems to be not particularly constrained, but just incidental to that lineage's past.

What happened to the human vagina is incidental to a history that might sound strange but isn't hard to see by comparison with other living tetrapods. The ones that aren't mammals, along with monotreme mammals, have a cloaca, which just means an anus which is used not only for solid wastes but also for liquid wastes and reproduction; the kidneys and reproductive openings simply empty into the area right before the anus's opening to the outside, instead directly to the outside themselves. (Remember what I said in a previous post in here about the predictability of using the mouth for both eating and breathing because there's a tendency to use one hole for multiple uses instead of multiple separate holes? This is another example.)

In mammals other than monotremes, the urethral opening and egg-laying opening have migrated out of the anus. But they didn't get far because by that time there was a pubic bone in the way. That's the part of the pelvic girdle which sticks up and forward from near the sockets for the femurs, ending just barely under the skin right above the external reproductive organs. Whoever put it there when there was a cloaca just wasn't planning ahead for having the cloaca's three functions move away from each other later. Babies wouldn't need to escape through the space under that bone if we either hadn't ever had a stage in our history with a cloaca, or hadn't gotten a circle of bones fused around it. Some other lineages either don't have a cloaca or have one that isn't surrounded by fused bones, so the particular arrangement we've got there didn't need to happen.
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Thank you.
 
These discussions that focus on whether aliens will have bilateral symetry, be vertebrate/invertebrate, have hands, have birth canals between their legs, etc always seem so extremely parochial to me. We don't even know if something as basic as the plant/animal dychotomy will be apparent in aliens. And even that trait of Earth life was decided billions of years beyond other traits that might merely be frozen accidents that only happen on Earth.
 
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Ladewig said:
The question was about species smart enough to develop interstellar travel.
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No, the question was "... if it is possible for an alien species to not have its birth canal located between its legs." It's a straightforward question and I gave a straightforward answer. Your post contained nothing about intelligence nor space travel.
 
SezMe said:
No, the question was "... if it is possible for an alien species to not have its birth canal located between its legs." It's a straightforward question and I gave a straightforward answer. Your post contained nothing about intelligence nor space travel.
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I'm confused as to why ladewig thinks it would matter to your answer, but just for clarity in the conversation, his original post was about aliens who've mastered interstellar travel and genetic engineering.
 
SezMe said:
I'm apparently not adding to the conversation so I'll drop out.
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Just to be doubly clear, your original answer was useful and I'm not sure why ladewig chose to act as if forgetting that his post was about interstellar aliens matters.
 
SezMe said:
No, the question was "... if it is possible for an alien species to not have its birth canal located between its legs." It's a straightforward question and I gave a straightforward answer. Your post contained nothing about intelligence nor space travel.
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ETA: I have deleted much of this post.

Here is the point I was trying to make. I can see how the error was mine because I did not lay out the reason for my question.

I was under the (possibly false) impression that the size of human babies' head was a great factor in both the development of advanced intelligence and the shape of modern humans. IF it were possible to give birth to offspring with gigantic heads and have a birth canal in a completely different place, then such advanced aliens might look very much less like humans.

In the end, I agree with RecoveringYuppy. If someone shows up, his/her/its shape is very, very, very unlikely to be similar enough to humans that adding a suit would make it indistinguishable from modern humans.
 
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luchog said:
Speaking of science fiction, this pretty much describes about half of Stanislaw Lem's oeuvre. Particularly Solaris and His Master's Voice.

Even if the lifeforms were substantially human-like, chances are there would be so few mutual points of reference that communication would be extremely difficult, particularly since there's no guarantee that the environment that produces such a lifeform would generate the sort of evolutionary pathways and divisions we see in Earth animals. Finding a common frame of reference may end up being an insurmountable barrier.
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This is certainly very true. "Humanoid in basic difference" is of course a far cry from "biologically the same", and once you get into cultural considerations the range of variety within human cultures tells a pretty clear tale.

I think, however, you could establish the very first common frames of reference by using principals found throughout the galaxy. In a similar way, when early explorers first encountered native peoples around the world there was often little or no frame of reference from a cultural point of view, but we still had the basics in common; land, water, sky, etc.

By that same principal, we and any advanced lifeform have things in common; elemental particles, orbital patterns, stars, pulsars, etc.
 
RecoveringYuppy said:
These discussions that focus on whether aliens will have bilateral symetry, be vertebrate/invertebrate, have hands, have birth canals between their legs, etc always seem so extremely parochial to me. We don't even know if something as basic as the plant/animal dychotomy will be apparent in aliens.
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These things all build on each other. At the very basis of these discussions is our current understanding of scientific principals as basic as gravity, energy, and the behaviour of atomic particles. Ultimately, everything we are is dictated by the fundamental behaviour of matter and energy.

Never mind our understanding of anatomy or evolution, life forms that varied dramatically from those we know (to the degree that we may not even recognise them as life) would quite likely force us to reassess basic physics and chemistry.
 
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