# How would evolution unfold if the only land was underwater caves [closed]

Let's say that Earth forms having no land, only underwater cave systems, on every level of the ocean. How would evolution unfold? How would fish evolve to live in small to large lightless caves? What would be the dominant species, and what types of technology would be created?

## closed as too broad by Aify, Hohmannfan, MichaelK, Frostfyre, John DallmanSep 5 '16 at 18:05

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• A note: you say "lightless caves", but there is very little light once you go deeper underwater than a few hundred feet, so deep-dwellers already live in lightless environments. Also, are these caves fully submerged, or do you mean that the entrances are entirely submerged while portions are in air, like a cavern with entrances at its bottom being partially flooded? – Palarran Sep 4 '16 at 21:58
• Some parts of the caves would be in the air, but mostly underwater. I said lightless because the caves would exist in the continental shelf and sea pen, but I suppose that goes without saying. – Jun Hayakawa Sep 4 '16 at 22:03
• is there water on the surface? because most (if not all) cave and deep-water ecosystems rely on outside sources of food (guano from bats/birds, dead matter falling to the depths, deep sea vents). And remember, all life's energy comes from the sun (and sometimes heat vents) so I feel like if you want life that's anywhere near our own you'd need some sun-exposed water for cyanobacteria (photosynthesizing bacteria) to evolve. Without this we'd have to start our evolutionary branch without the oxygen catastrophe and life will evolve from anaerobic bacteria. – XenoDwarf Sep 5 '16 at 0:37
• Welcome to the site, Jun. While an interesting question, there doesn't appear to be any objective way to judge which answer to this question is "best." There is simply no telling which organisms would arise through evolution. Additionally, asking about evolution, dominant species, and technology at the same time makes for a very broad question. – Frostfyre Sep 5 '16 at 15:05

You can take a look at how other species have evolved to live in near-total darkness under the sea, and how they communicate in water. You'd see bioluminescence, the ability to withstand very cold temperatures, and the ability to use heat (not just sunlight) for photo-synthesis. Sight would be almost entirely replaced by very advanced olfactory systems, pressure and magnetism sensing nodules, excellent hearing. You would even have some animals and plants that could survive being frozen.

"Verbal" communication inside a cave would tendto be higher-pitched and consonant-heavy (whales use low frequencies and vowel sounds to communicate across vast distances, but dolphins 'chatter' in high pitches and lots of clicking type consonant sounds, since their conversation partners are usually nearby and the sound doesn't need to travel as far.)

If the caves are fairly near the surface, then life won't need to evolve to withstand great pressure, so you'd see shapes much like typical marine life. (Think hard, compact bodies like fish, crustaceans, etc.,) If the caves are very far beneath the sea, life will need to evolve to cope with the pressure.

In any depth, animals would almost certainly have some form of echolocation to create a 'picture' of what the cave looks like. The picture could 'look' quite detailed, since echoes would bounce off the cave and give higher resolution to the surroundings. You might have amphibious species that use echolocation above-water, but rely more on smell underwater.

Dominant species would have particularly well-developed senses of hearing, both above and below water. I'm assuming that the caves are connected; a dominant species would be able to travel between the caves and so would be small enough to fit in the tunnels. There would be no whales, or large sharks. The dominant species would be some sort of blind small dolphin. The largest animals would be native to the larger caves, and would probably be large turtles or tortoises.

What might be interesting could be swarms of piranha-like fish that hunt in packs. They could go almost anywhere in a cave system and would be able to corner even large prey in smaller alcoves.

The technology would mainly be used by crustaceans, who are the only species that have evolved dexterity in their 'fingers'. They'd probably make spears to hunt with, and they would learn how to pack cave sand to create barriers against the piranha. Dolphins' technology would be mostly language and communication, which would be highly evolved. Some of the dolphins would also probably figure out how to weave nets using seaweed or other fibers, so they could catch fish and also create spaces that are protected from the piranha.

• dolphins evolved from relatively large (probably dolphin sized honestly) predatory dog-like creatures that decided to take a dip, so I'm not sure how they ended up in a cave. – XenoDwarf Sep 5 '16 at 0:27
• Well in the cave world, dolphins evolve from predatory fish (like piranha) that decided to surface and breathe the air in the cave. – Daniel M Sep 5 '16 at 0:38
• ah, so by 'dolphin' you mean 'dolphin like'. I could definitely see this, convergent evolution means it's likely, although I think the tail will still be vertical (fish tail) - just a little unimportant note. – XenoDwarf Sep 5 '16 at 0:49
• Yes that is basically what I mean. That would be one scary looking dolphin-like creature. Why do you think the tail would be vertical? Space constraints? – Daniel M Sep 5 '16 at 0:52
• simply that it evolved from a fish, there doesn't seem to be an evolutionary advantage to tail orientation (sharks have had the same tails since before the dinosaurs) except for speed, and I don't know if you'd want to be going too fast in a cave. – XenoDwarf Sep 5 '16 at 0:56

Why exactly is land an issue? Evolution is driven by plants as primary producers. Phytoplankton evolve to form aquatic plants, whose bodies and roots serve as habitats for primary consumers and their predators. Eventually, these would form large floating islands like water hyacinths. These would eventually form substrates for larger plants, and thus bigger consumers. The substrates formed by dead plants and plant waste would make water shallower in low flow areas, leading to siltation and eventual land reclamation.

• but they're underground, how can the phytoplankton and plants make energy? – XenoDwarf Sep 5 '16 at 23:00
• @XenoDwarf: When the question says ...no land, only underwater cave systems..., I assume it means there is water on the surface – nzaman Sep 6 '16 at 6:09
• It is quite unclear, no matter though, it seems this question is on hold and Jun seems to be inactive – XenoDwarf Sep 6 '16 at 23:02