Lets say the whole human race lived in caves for millions of years, with lots of water and no light. What features would they gain/lose?

Question

In a million years or so, descendants of humans (intelligence comparable to wolves) are driven into caves due to competition with land and resources from descendants from alligators and crocodiles. They stay in the deep parts of the caves, a few hundred feet down, there are some fish who feed on plankton and some fungi as well, but the caves are mostly barren, with water being common. Lets say the humans decide to stay in these caves (or the species gets stuck there) for a few million years (it can be 10 million years to 50, i want long term evolutionary effects) what would happen to them, over millions of years?

I know they would probably lose eyes, but i am also interested in the idea of echolocation (they could develop bigger ears to sense slight sounds) and them developing long fingers to sense their surroundings with, that also function as fishing spears, with maybe a bit of webbing to help swim. What do you think? Feel free to write what you think as well as give commentary on my theories.

Extra thing: The food web and oxygen/carbon/nitrogen cycle does not collapse because there is a species of bacteria in the water that feasts on the bat poop and washed up corpses of animals, and the bacteria also take in carbon and expel oxygen, which allows other organisms to breathe.

Answer 1

To start with, we'd have to consider what current human features would be considered advantageous to a race suddenly plunged into that cave. By advantageous, I mean that it will allow an individual with those features to find more food, survive longer, and procreate more often. Those are the features which will be 'selected' and honed by evolution.

I would guess that being small (as mentioned by Giacomo) will be an advantage. Also, I would imagine that fat would be favoured over muscle (insulation more important than strength), so our humans will be small, soft and probably very, very pale.

I feel that humans are too dependent on sight for it to be evolved away, since good night vision will be advantageous at the beginning. Your future cave-human might have exceedingly good night vision, in fact, and perhaps a highly-developed sense of smell. I'm not sure about echolocation, but his hearing is likely to be far better than ours, too.

Given that humans are quite smart, they may attempt to survive by agriculture and austerity instead of hunting off a very limited resource pool. This means they may not develop features that help movement through water (i.e. fins, lung capacity, etc), but instead features which enable them to go through long periods of deprivation (i.e. the ability to hibernate).

In summary, your future cave humans may be small, deformed (to us) and soft lumps of white flesh (I'm thinking an average height of less than 1m), with huge pupils, nostrils, and ears. They will be physically weak and slow, but they may compensate for that with even more intellect than we have today.

Answer 2

You describe a very hostile environment. In place where there is no light and heat life can't develop at the same speed than on the surface. Cave are very slow ecosystem, introducing a big social mammalian would be disastrous.

A group of human would be to heavy for the ecosystem to bear. It is already to heavy on the surface today so in a cave, they would consume all life in the cave in a year.

Humans can't be social in those condition. They would be like lone carnivore with a big territory like tigers, they would never be in community. That mean a very small population so the mutations are less likely to happen.

Unless you find a magical source of energy, and I mean glucose not mushrooms, your cavemen will die from malnutrition. I don't know any way to synthesize glucose without photosynthesis.

Even with a very large cave. An area produces a limited quantity of resources that animals use to live. Each area is different. For example a tiger in India need a territory of 30 km2 because it is warm, wet and sunny there is a lot of vegetation and animals. In Siberia tigers need a 1000 km2 to live. Because it is cold and dry nothing grow so no animals to hunt.

Your cave can't produce the amount of resources susceptible to provide food for a group of humans. There is very few exclusive cave creature most of them are cold blooded and humans are certainly not one of them. Except Gollum ...

Answer 3

As purely conjecture, I would say that they would evolve to hold their breath for much longer, in particular the mammalian dive reflex would be vastly enhanced. The best free divers generally have this reflex pretty strong, and can hold their breath for up to 22 minutes.

They would also likely become smaller, as there is limited food in cave ecosystems and many tunnels are quite small.

As for eyes, they would likely stick around. It's not too unreasonable to assume that some bio-luminescent organisms are present, such as glow-worms. They would likely lose colour vision and pupil dilation, as neither are needed in a cave.

Long spike fingers seems a bit unnecessary. Most likely they would develop longer, sharper fingernails and use them like claws. We can already catch fish with our bare hands anyway.