Imagine if nature had evolved its own version of the modern blimp, or the old fashioned zeppelin. Picture a creature built like a huge hydrogen balloon, a thin tissue of living flesh and tenuous skeleton supporting vast empty sacs filled with lifting gas. A lighter-than-air animal much, much larger than the biggest whale or dinosaur that has ever lived in our world. An enormous living island, large enough to support a tenuous topsoil and bedecked in hanging vines and sprouting trees from its back.

This island drifts on the winds up in the sea of clouds, living for thousands of years without ever once touching the ground far far below. The plants and small animals that thrive on its back have lived there for countless generations. When the winds chance to bring two of these huge creatures together, animals clamber from one to the other, and birds fly the huge distances between the living islands. At first they were unwitting passengers, marooned in the sky when the islands first rose up to the clouds, or else temporary visitors, flying birds stopping to rest on long migrations. Over time however they have all evolved to fit this curious niche, forming a new ecosystem high above.

Beneath the island trail long, gossamer thin roots and tendrils, drinking water from the clouds to quench the thirst of it and its many passengers. It survives by living off the produce of the plants on its back, or perhaps a more direct symbiotic relationship; siphoning nutrients from their roots in exchange for water. All is not entirely peaceful however, and it keeps a wary eye on the skies and clouds for its larger carnivorous cousins, menacing shadows beneath the cloud tops.

How did these living islands evolve? What would drive them to the skies? How plausible is this ecosystem in the sky? Is the air thicker here and so floating easier perhaps? Could they survive off the plants on their own backs? Or something else?

Note that there are other questions about flying this or flying that and how might it work. This question is different, in that the lifetform can be anything and the focus is on the evolution and how such a thing came to exist.

  • $\begingroup$ Struggling with coming up with what gas this creature could be filled with and not be a giant sized island explosion in the air if struck by lightning. Helium isn't that feasible as it's as inert as you can get and most others are either too heavy or too flammable. $\endgroup$ – Twelfth Aug 11 '16 at 21:09
  • $\begingroup$ @Twelfth Yeah my thoughts were definitely hydrogen. Avoiding lightning is a fascinating problem. Must be possible though. Biological capacitors? I think I'm right in saying that planes are simply conductive all over and Zepplins managed it somehow. $\endgroup$ – Drgabble Aug 11 '16 at 21:14
  • $\begingroup$ Zepplins and lightning books.google.co.uk/… $\endgroup$ – Drgabble Aug 11 '16 at 21:16
  • $\begingroup$ Hey good link, kinda bypasses that...though this creature will need a way to ensure hydrogen doesn't escape and you are still risking a rather tragic death to hydrogen combustion (over multi-thousand year life span its hard to beleive there wouldn't be atleast one puncture event). Neon would work..but it's heavy and might not provide enough lift. Methane is a good by-product of life and would give the lift, but still flammable. The creature could just be heating the air within itself (hot air, like me, rises), which gives the need for it to fuel itself. $\endgroup$ – Twelfth Aug 11 '16 at 21:33
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    $\begingroup$ Tragic death? Or is that how it reproduces? $\endgroup$ – JDługosz Aug 11 '16 at 21:41


Just like the sponge was the most primitive animal, simply a large-scale organization of the single-celled life forms, a biofilm could develop structure to hold gas trapped while being extremely primitive without specialized organs.

So, the flying islands, a further evolution of floating islands, could be one of the earliest multi-cellular forms and lead the colonization of life beyond the sea!

Any waste gas serves to make floating islands. Eventually hydrogen gas was used with even lighter structures to make flying forms.

Think of a coral reef. The early mounds (floating and flying) can provide a habitat for others. This started before internal organs were even a thing, so life evolved to make use of large-scale symbiosis from the get-go.

A billion years later, the islands (both floating and airborn varieties) form a major division of life, being essentially integrated ecosystems of complex forms. Many species across major kingdoms (plant, animal, fungi equivilents) have their own lifecycles within the larger body, and they are all playing a role in the functioning of the macro-creature.

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    $\begingroup$ Good answer, I hadn't even thought of non moving animals +1 $\endgroup$ – TrEs-2b Aug 11 '16 at 20:34
  • $\begingroup$ This sounds suspiciously like a Portuguese man o' war. $\endgroup$ – Kys Aug 12 '16 at 16:47
  • $\begingroup$ @kys, Cnidaria is cool for SF because it shows a different way to get a larger scale of complexity, and I’ve drawn upon that in other answers. That’s not what I was thinking about here though. $\endgroup$ – JDługosz Aug 12 '16 at 20:09

My first thought was something like a whale evolved up, but that doesn't make much sense; in the ocean, once you're big you typically keep doubling down on size.

Rather, I'd think something like a small lizard would be the base creature. It started growing larger as a defense mechanism, and storing hydrogen in sacks on its back to keep itself light. Eventually, it largely abandoned the ground, floating lazily through the air, pushing itself from branch-to-branch.

During this time, it underwent radical structural changes. It's front legs evolved into webbed, stubby wings, while it's back legs and tail elongated to reach below. The hydrogen sacks became more and more pronounced, until it no longer needed to touch the ground at all. It's tail split several times, giving it a mass of tentacles that reach down, catching onto treetops or bringing up food and water. It's mouth dropped lower than it's front legs, and grew wider, straining the bugs out of the air like a whale strains krill.

Still, it grew larger and larger. Some of its hydrogen sacks became too large to support with its circulatory system, so it stopped supporting them. Once a hydrogen sack gets old enough, it's sealed off and abandoned. It remains within the mass of the floating island, but is no longer a living part of it. The top-most layers routinely rupture, venting hydrogen into the air, and leaving a decomposing flesh bubble, which eventually turns into a kind of soil. In this soil, plants began to grow.

It's larger than a blue whale now. Small plants and animals live on top. The roots of the plants reach all the way down, through the cruft on its back, and hook directly into its circulatory system. The plants get water from the creature, and in return pump oxygen directly into its blood stream. It no longer needs lungs, and develops a distributed heart network to pump blood throughout its body.

It's massive now, five, ten, twenty times the size of the largest blue whale. Much of it, however, is old-mass, not living mass. It lives high in the air, just below the tree line. It's back is covered in alpine firs, aspen groves, small mammals, birds, and insects. The plants provide all it's energy needs. It provides the plants their mineral and water needs. Its mouth has changed, looking more like a circular set of gills than a real mouth. With this, it still strains insects out of the sky.

It floats a careful line: if it stays too close to the ground, the plants on its back will grow too large and sink it out of the sky. If it floats too high, the plants will die in the cold and it will suffocate. Storms are its greatest enemy. Storms... and them...

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    $\begingroup$ the idea of having roots in its dead flesh is very cringe... love it, very creative! $\endgroup$ – Drgabble Aug 11 '16 at 21:06

Well, the lifting gas is definitely hydrogen. Helium just doesn't show up in compounds that living creatures can access, because it doesn't form compounds to any useful extent. Hydrogen is relatively easy for living creatures to produce, and also much easier to confine with a thin membrane. Helium is monatomic, and the atom is so small that it tends to leak through barriers that will confine almost any other gas.

However, lightning is a deadly danger to these living hydrogen balloons. They have to avoid it, because a lightning strike is likely to send them up in flames. But they lack the ability to move quickly, simply because they can't afford the weight for it and don't have the structural strength. I don't see how to get round that. Lightning-conductor tentacles are an appealing idea, but making them conductive enough, and getting them a good contact with the ground, is a problem, made much worse by the lack of weight budget.

How does it evolve? One way would be from a seed dispersal mechanism. Start with sycamore seeds, which have "wings" to help the wind blow them a long way. Have some hydrogen between the layers of the wings to make the seed lighter and blow further, and after a while you have hydrogen balloons as a seed dispersal mechanism.

Then the climate changes, so that there's a great deal of fog around, and the seeds start germinating in mid-air and trailing roots to pick up nutrients, and we're on the way to the desired result.

  • $\begingroup$ The question is “how did it evolve”? $\endgroup$ – JDługosz Aug 11 '16 at 21:43

Obviously the lifting gas must be hydrogen assuming the organism exists in an atmosphere similar to our own. I agree with JDługosz that a collection of unicellular organisms could create such a structure, trapping in the hydrogen and growing large enough to become buoyant. I’d like to propose some hypotheses on why such an entity would form to sequester hydrogen gas and its evolutionary process.

The Hydrogen Hypothesis is a theory you might learn about in a college-level biology textbook regarding the origin of Eukaryotic life. It posits that one single-celled organism (a bacterium) produced hydrogen as a waste product, which was consumed by another single-celled organism (a methanogen). This led to a symbiotic relationship in which the methanogens became dependent on the hydrogen production of the bacterium, eventually enclosing the bacterium fully. According to the hypothesis this was the first step towards the formation of Eukaryotes and their organelles. You can find a very simple video explanation of the concept here.

But what if it didn’t happen quite that way? What if instead of a single methanogen engulfing a bacterium, a group of methanogens formed a balloon around a colony of hydrogen producing bacteria? This arrangement would be equally efficient, trapping all of the hydrogen produced by the bacteria for the use of the methanogens. The result would be microscopic buoyant balls filled with a mix of sea water, hydrogen, and potentially methane that would float at the surface of the ocean. As both the hydrogen producing bacteria and the methane producing methanogen individuals replicate the balloon would expand in size until perhaps it pinches off and splits into two. The two symbiotic species might form concentric spheres with the methanogens outside and the bacteria directly next to them inside. Now perhaps another symbiotic relationship could develop between photosynthetic autotrophs and our balloon. With the balloon floating on the surface of the water it might algae perched atop it to have an advantage over their submerged peers, better access to sunlight or maybe better access to atmospheric gases.

The end result is a large, flat hydrogen bladder sitting on the surface of the sea. If it grows large enough and the interior becomes sufficiently concentrated with hydrogen it might even take flight. Of course, we are still nowhere near a floating island. The various components of the balloons will need to adapt to being in the atmosphere instead of the ocean and a more rigid structure will have to form before any significant weight can be placed on top of the island. Moreover, a fundamental flaw in the design is that if the balloon is top heavy a strong wind could knock it over, upside down, and it would never be able to recover. It would be much more stable if the plants and animals were clinging to the bottom.

Anyways, I hope this sort of evolutionary process is what you were looking for. This model explains how these floating islands might have evolved from mere single-celled organisms as well as why they are filled with hydrogen gas. If you have any questions or would like me to elaborate I’d be happy to try.


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