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Some planets may very well be true water worlds - totally covered in water. Though some, like Gliese 581 C (a "super-earth" 50% larger than the Earth 20.5 light-years away) may have dense water - maybe a thin layer of liquid water on top of compressed water. What sort of life would arise there? Think about it - no dry land, or extremely rare dry land that is easily flooded by storms or tides. Life transitioning to land would not happen, at least not large roaming life which needs territory to grow and thrive on (digression: could high level stationary sentient life ever evolve, I wonder?).
On Earth, life developed limbs and walked out of the seas - large tracts of land allowed for evolution to proceed in that direction. On a planet covered with one giant ocean, that direction would be blocked, unless the poles were cold enough to keep up the continued production of ice floes - then there could be an evolutionary path for living on ice floes part or full-time. Otherwise, any evolutionary development of limbs would go in the direction of underwater ambulation. But how useful is underwater ambulation? As useful as fins on land? And so, over millenia, higher sentient life would evolve totally adapted to the oceans. I don't think they would have just fins - hard to build and use tools without some means of griping and manipulating the physical environment.
That is not to say intelligent life needs a means to physically manipulate the environment, but that the ability to manipulate the environment does allow for greater evolution and progress of the brain, or at least to make it much easier for it to happen.
Maybe on some water world planet, a species of sentient life has evolved that can not manipulate the environment, but they have survived for millenia and so have slowly evolved to be able to do high order abstract thinking - their art, culture, science, and theology would all be based on communication - their only tool left to them; they would manipulate the mental environment. Art would be, for those that communicate via sound, vocal music and oral literature. Science would be based largely on observation and mental experiments (of the kind that Einstein made famous, but that the Greeks did to an extent as well) since they could do little experimenting (some, probably, but not much). They would not less likely to physically explore space, as they would not be able to leave their planet (water is extremely heavy, especially compared to air, lifting a craft full of water out into space would be extremely difficult -albeit not impossible - to do. In addition, making space suits for exploration, especially that of dry surfaces, would be extremely problematic as well). Would they be more likely to become telepathic then? To astrally project themselves? Or is that too "newagey?"
For a large planet with several times greater gravity, a thick ocean may have "normal" water at the top, but definitely, for a thick ocean, water that would become denser quickly the deeper one went - water become plastic, or even solid. We have a slight inkling of that here on Earth - mountain climbers know the air gets thinner as they climb, it is a danger if ignored. For our water world aliens, something similar may be in play.
Depending upon their evolutionary track, if they evolved as deep sea creatures, rising up to the surface may be dangerous - especially if they can not work with tools to create devices to help them breath or deal with the pressure change. Even sea creatures on Earth have ranges - some that live closer to the surface can dive rather deep, but they don't live in the depths. Other deep living creatures tend to stay in the depths, only coming near or to the surface when they are sick or dying (like giant squid) - near the surface is not a friendly environment for them to linger in.
Another thought - would, after millenia, creatures evolve to be like our flying fish? Would the air be conquered there as it has long been here as well by flying creatures? They would have to be creatures that feel at home surrounded by oceans, who do not need land to survive. Probably most likely flying fish like creatures, though maybe one some planets, the flying fish make the evolutionary steps toward fish that fly more than they are fish, and develop lungs and end up spending their lives either floating on the surface (when resting, for instance) and flying.
There are some sea birds on Earth that can live far out at sea, and may spend much of their life out at sea. Such birds tend to glide or soar more than the powered flight. This is because they can take advantage of the wind deflected by waves, and or by ground effect which reduces drag.
Because of convergent evolution (where unrelated species tend to develop similar characteristics due to their sharing similar environments, and due to the fact that the same physical laws apply to all species in the same environment) we can make educated guesses that creatures on other planets will tend to try to be efficient in adapting to their environments just like Earth life. If deflected wind and the ground effect is still in effect on this alien water world, then flying creatures there will glide more than they would use powered flight since the latter uses more energy.
Would these fully pelagic sea bird like creatures be the ones to most likely to become tool makers? They would have, possibly, develop feet (webbed most likely), which could have an opposable digit to help them grip prey (like modern Earth birds) and which could eventually evolve to manipulate tools (as a previous blog entry has noted, some birds, crows most notably, are known to create and use tools, and may be as smart, or even smarter, than a chimpanzee).
Or would the creatures be more like flying fish, or half-bird/half-fish - able to live under water (maybe to nest and breed) as well as live on the water surface and fly over it (to more easily hunt for food - flying through air is faster than flying through water - less dense, less drag).
Though one drawback (to sentient beings) is that on a water planet it would be hard to work with metals - to melt, smelt, and other wise work with metal to create structures and devices that would allow them to eventually explore the stars. Mining ore would be more problematic as well. Water is heavier than air, more dense - it takes more energy to move through it, and probably more difficult to shore up tunnels (not only would they have the weight of the stone above, but the pressure of the water on top bearing down). Light has a harder time penetrating water than it does gaseous atmospheres (sound, however, could travel great distances under water). Working with electricity would be harder. Building telescopes to view the heavens would be harder (though maybe on a planet with rare land, a species that could tolerate the air for short periods could build telescopes on such land - or the avian species, which would have a higher ability to tolerate the air).
On a larger planet, covered with water, how would that affect territorial issues? Would such creatures tend to be more nomadic - especially if they never develop the ability to farm, there is no need to settle down and build cities. Are cities necessary for advancement? Do cities help speed up advancement of civilization, and if they do help (which it does seem like they did for humans), are they the only way? Could nomadic species find their own way to help quickly spur on advancements in civilization? Without natural barriers like mountains, oceans, ice fields, deserts, and large rivers are to humans, would their be less isolation between groups and thus a more "we are one" sort of sentiment develop? Or would groups still develop, some adapting to more colder regions, for instance? (but would this still, in the end, create fewer insulated, insular groups than is the case on Earth?) Would this create a species that would be less xenophobic?
What theology would exist for such creatures? For those on planets with rare land - would the inhospitable land be their version of hell? Or for deep dwelling sentient species, which can not bear being near the lower pressure regions in the upper regions of the sea, would the upper, more lighted, and more dynamic regions be more like hell and heaven more like the darker, heavier, and calmer pressured regions? Would they think all planets are ocean, and that heaven would be a calm ocean? They probably would have some tectonic activity - underwater volcanoes - as well as deep, dark trenches that could play roles in primitive theologies. And for the water bird sentient species, how would their primitive ancestors first think of the world, theologically (I suppose they would love the O.T. verses that describe God as a mother hen).
1. Would a water world be less susceptible to extinction level events from meteor impacts (no dust to throw up into the air to create a long lasting year round winter that kills off the plant life, no cracking open part of the crust and letting out lava, etc)?
2. There is an article in DVICE.com about the Focus 21 France, a hovercraft prototype that would use the ground effect to achieve helicopter speeds. It would have to fly close to the water to take advantage of the ground effect (a height equal to twice the wingspan or less). I thought that maybe that is how water world sentient species would fly, at least for their earlier flights. Sort of like sub-orbital or low earth orbits for us.
"Astronomers Find First Earth-like Planet in Habitable Zone." ESO. 25 April 2007. 9 December 2007. <http://www.eso.org/public/outreach/press-rel/pr-2007/pr-22-07.html>
White, Charlie. "Focus 21 France uses ground effect to zip above the waves" DVICE.com. 3 December 2007. 9 December 2007. <http://dvice.com/archives/2007/12/focus_21_france.php>