How can life which cannot live on the surface of a planet naturally reach the supermajority of the planet's caves?

Question

I want to create a setting focusing on a planet where nearly all complex macroscopic life lives underground, but I keep running into a problem: If a complex organism is adapted to cave life to such a degree it cannot survive the surface, then it logically cannot colonise caves not connected to the cave its species originated in, which would seem to mean that species on this planet will be limited to a minority of the planet's caves. However, I want some species to be ubiquitous, like on Earth.

I want the solution to meet the following criteria:

• It should be a natural phenomenon, where natural is defined as not having to do with the actions of sapient beings
• If most of the caves are interconnected, it should be explained how this is the case
• It should be applicable with plausibility to many species at once, rather than being some freak event that could only apply to a few species
• It should not involve the move underground having been sudden and/or recent
• It should be plausible within known science, even if quite unlikely
• It should not make it any more implausible than needed for the surface life to (nearly) all be simple and/or microscopic

Meeting the above criteria, how can a significant number of distantly-related species be found in the supermajority of a planet's caves without being able to survive on its surface?

Answer 1

Tunnel borers.

One of the forms of life you mention thrives on minerals that are found in rocks. Over time, it has developed the capability of dissolving most rocks with acid. To have significantly large cave systems, you need soft calcareous rock anyway.

The planet is geologically recent and the surface is crisscrossed with veins of minerals; several of those got hollowed out, connecting more and more cave systems. Once the way was open, larger forms of life endeavored to enlarge the passages searching for water and food sources.

Answer 2

Spores

Whether it's wind, water, or some of the (limited) surface life, spores given off by cave life are carried (eventually) to new caves. In fact, some borderline cave / surface creatures commonly pick up spores (mostly not on purpose), which are conveniently spread to other caves by those creatures.

But wait, you say - this might account for certain fungi to be common across many cave systems, but what about other life? Well, xenobiology being foreign, it's possible to have different animal / fungi / plants share DNA (or whatever) across species in a way unknown to Earth species. (Well, I mean there's viruses on Earth that can spread DNA bits in ways otherwise not possible...)

Ground Water

To a limited extent many cave systems are kind of sort of connected. We're talking about water-formed caves, right?

If life, and the information for life, could be transmitted in very small packets (like spores), it could go almost anywhere that water could go. And if we trickle down into the ground water from here, we could somehow crawl back up over there...

Conclusion

Life in your world is, in some ways, sort of cooperative, and sort of fungal. Sure, your cave-tribe of intelligent people (there's people, I assume) may reproduce like people do, but they can also spore-out and spore-in genetic diversity, and might even spontaneously "hatch out" of not-people-like growths. In this light, they may hunt other animals, eat other growths, but they will also cultivate and care for the world which can literally make them (or transmit the genes of distant tribes to them).

Answer 3

They go at night

Your life might not survive the surface, but they might live it for a certain time. When caves get overcrowded there is a part that will try to migrate. It is a death sentence for most that leave the cave, but some individuals or groups can survive until another cave. They could find a tiny crevice, fallen tree or the like to take shelter at times, making their migration more likely to succeed. All it takes in millions of years are a few groups hetting lucky, making them able to spread.

There is another way it could be done. Some creatures can give part of their offspring, like eggs or larvae, to natural methods. Think wind or rivers. Their offspring would go through the same chances as above, eventually spreading throughout the world.

Will every nook and cranny have these species? Probably not. But they will proliferate to much of the caves around the world.

Answer 4

The world was a Water World in recent geological history.

Relatively recently, by evolutionary standards, most of the planet was covered in shallow oceans/seas. Sudden and dramatic shift in climate led to Global Cooling, that pulled the water up into glaciers, leaving behind post-glacial ravines and caves.

Previously aquatic creatures had to evolve to become semi-aquatic, then effectively cave-dwellers, when the surface turned into a frozen hell.

Answer 5

Convergent evolution/selection

I'm not sure if this is a complete answer, or one that's satisfactory to you... but examination of real cave systems does show a lot of convergence. The vast majority of caves will have very specific conditions that are almost universal - low/no light, high probability of water, etc., which are generally not condusive to most species' survival.

Basically, pretty much every time a large animal of any kind is able to survive off of the available resources (remember, microbial life is pretty much the basis of your ecosystem), that animal is going to be a salamander. At least on Earth; depending on how long the species has been alive down there, it'll be in various stages of eyesight loss and depigmentation. But it's probably a salamander. I think a lot of caves also have crayfish, bugs, etc. in them below the salamanders in the food chain, but it really depends on the size of the cave & the available food resources.

What I would take away from this is that, in all likelihood, there will be a very limited selection of animals suited to adapt into cave niches, which allows you to realistically put the same kind of animal in every cave within that animal's range. It might not be the exact same breed of said animal, but according to Earth, it's likely to be the same lineage across the board. Naturally I would recommend something similar enough to the salamander for this, since there are going to be traits that make an animal more eligible for cave niches. That said, it won't be the exact same breed of animal the whole way around, different types of salamanders, in different caves, adapted in slightly different ways to their environment. Some are much further along the path than others, etc. If you want to have multiple creatures, this is tougher. My best bet would be some kind of symbiosis - like, maybe one species is linked to the reproduction of another, or is a parasite. But this does present the biggest ceiling for realism.

Drawbacks

• You are unlikely to have a high diversity of animals. Caves simply don't provide enough resources or niches for anything more than a few creatures... and few animals are well adapted to even start living in caves anyways.
• In general... cave ecosystems tend to be less interesting than we expect... like, no plants or bioluminescence or animals bigger than maybe your thumb, with some exceptions. It's going to be mostly microbes, algae, little guys and your occasional upper food chain animal.
• Might run up against your "only simple surface life" criteria a bit.

I would recommend Atlas Pro's video on the biogeography of caves, as it has a lot of the information I gave you here but probably better explained... also, if you haven't already, look at cave exploration videos! I think our perception of caves is a bit clouded by the fact that most public access to them is largely going to be the big, flashy ones... or fictional media.

Answer 6

Metabolic suspension

Perhaps the most famous "unkillable" critter is the tardigrade. It's not adapted to thrive in extreme environments, or even live in them - but it is capable of dessicating itself (drying itself out) in survival situations:

While in this state, their metabolism lowers to less than 0.01% of normal and their water content can drop to 1% of normal, and they can go without food or water for more than 30 years, only to later rehydrate, forage, and reproduce.

Now, tardigrades are microscopic, but life on your planet can take inspiration from their can-do attitude and evolve a way to survive exposure to the surface until it finds its way to more favorable conditions. Larger species with more complex organisms could roll up into a shell, another could spin a chrysalis, or erect some other protective structure and then just bide its time until more favorable conditions occur.

For smaller species such as insects, that could be as simple as wind or flood water blowing them from one cave to another. For larger fauna, it might mean a change in weather (such as a century storm or an eclipse) that allows them to transition back into a mobile mode and seek out new shelter.

Answer 7

You might get some inspiration from the shrimps that dwell on Uluru, an 863 metre high rock in the middle of the Gibson desert in Central Australia. The eggs tolerate drought, and can live (if you call it living) on top of the rock until it rains.

Temperature extremes in the park have been recorded at 46 °C (115 °F) during summer and −5 °C (23 °F) during winter.--Wikipedia

Can your critters do something similar? Survive as spores, and wait for winds to move them to a suitable place? Sure it is inefficient, and has a huge failure rate, but you only need a few successes.

Answer 8

The Surface is Periodically Habitable

Rarely, and for short periods of time, the surface can support life. During these times, the cave-dwellers migrate to the surface, and then when conditions degrade they retreat to the nearest cave.

Bonus points if rendering the surface habitable renders the sub-surface less habitable, since that would force the cave-dwellers out of their comfortable niches for a time.

Mechanisms

If the surface of your world is very dry, perhaps the ice caps periodically have outburst floods that coat large areas with water. Or perhaps changes in monsoon rain driven by slooow changes in axial tilt make the desert bloom for a few thousand years out of every 100,000, like the wet Sahara periods.

To get more exotic, perhaps the atmosphere is thin, and thus radiation is too high and air pressure is too low on the surface, but conditions are better underground. If that's the case, getting hit by a comet might increase atmospheric density enough to bring down the radiation and bring up the partial pressure of oxygen. Or collisions in space might produce a proto-ring system that is thick enough to lower radiation levels on the surface. Either of these effects would slowly fade away as solar wind did it's work.

Flood Basalt events inject huge amounts of CO2 and other gases into the Earth's atmosphere, and have been associated with several mass extinctions. It's possible that a similar event could change the surface of a world long enough to make it habitable, if conditions were right. Earth has had about a dozen major flood basalt events.

Timing

The key here is that no ecosystem is really stable over even short geological time scales. Look for something that could change your world to be briefly more Earth like before returning to the conditions that you want. Use major natural disasters in Earth's history for inspiration.

Then choose how often you'd like this to happen, and you're off to the races!