Otdfbt

In my country, we are split down the center by an intensely large mountain range. Us desert people live on the eastern half of the country, while on the other side of the mountain range the forest people live. Although we are separate for most of the year, every winter we are forced by brutal winter storms into the mountain caves.

200 years ago, a third group of people, the ones who built out the caves and made the central trade tunnel from east to west, suddenly disappeared. Just as they started to fade from memory, there has been an increase in reports of attacks on caravans journeying through the tunnels. It has been suggested that the attacks have been more brutal then the standard highway pirate fare, only a single survivor had managed to escape but without water he died in the tunnels.

It is becoming clear that the attacks were carried out by the lost people, but we have little to no idea of what they even look like. How would 10 generations of living underground with no light, sourcing all foods from things grown and found underground, and a societal focus on digging tunnels change the human body?

Edit: To specify I am not expecting much evolutionary change. What I am expecting is that living in those conditions will result in a body that does not look like a surface dweller. Most obvious would be the pale skin. Not an evolutionary trait, but a product of the environment that changes the population's body. The lost people have developed massive fungi that reproduce quickly and have a high caloric value, so starvation is not an issue for the lost people still associated with the main pack. And technology is equal to that of 14th century Europe, with significantly more metallurgy knowledge. The best answer will focus on physical changes based on living in a cave and less on the evolution of the species.

You want a fast change on the evolutionary timescale? Here is one:

Think of blind people. Those who were born normal, and lost their eyesight later in life. Soon, their other senses take over to make-up for the loss. Some blind people may even develop echolocation abilities as shown here:

https://abcnews.go.com/Health/MindMoodNews/blind-man-echolocation/story?id=13684073

Now, imagine how developed is that echolocation compared to bats. Well, not much, but an evolutionary starting point does exist. If the talent becomes integral part of mate selection, evolution will do its part. Other senses, such as smell and vibrations through the tunnel walls (Infrasonic hearing included) are helpful. Something like a cross between a man, a bat and a mole.

Food: That's where your plot may fail. Use a cave echology. Chemosynthesis or other non-light energy sources found in caves may give you a good thing to hold on. You have a few:

Chemosynthesis -- the cave dwellers may have found a cave that is fed by some sulfides from sulfurous springs. The movile cave in Romania, cut-off from the outside world for millions of years has its own energy source from sulfurous springs that feed a complete ecosystem. The cave men must find a way to harvest what grows there without being poisoned. They may be more resistants as a result, though not necessarily being able to dwell the spring on a permanent basis.
http://www.bbc.com/earth/story/20150904-the-bizarre-beasts-living-in-romanias-poison-cave

Radioactivity like near a Uranium ore deposit, your fungus absorbs radioactive elements to use instead of photosynthesis. It can be a mushroom, if you want, since this fungus seems to do that:
https://en.m.wikipedia.org/wiki/Radiotrophic_fungus

Water Ah! Most important. Aquifers!

Other changes -- The need to dig-out and enhance the tunnels may favor the short and stocky physique.

Nothing they did not see after year 1, except some eyesight issues you will see after generation 1 that will disappear in generation 11.

The only changes you will see is the same ones people saw a year after living underground, pale skin from lack of sunlight and vitamin D deficiency for the same reason. Malnutrition and tiny population because isolated caves have abysmal production (surviving 10 generations is unrealistic). Eyesight will be the big issue, generation 2-10 will never use their sight, they will not even know what it is, and will have to learn to see when they return to the surface. They will also not have any depth perception, that develops early and only with sight, but as someone without it that will not have much effect.

10 generations is not enough time for any noticeable physical evolutionary changes no matter how strong the pressure, especially in something social like humans. The bottleneck caused by who was trapped will have a much bigger effect.

Cultural changes will be large, they will not know many things, and there will likely be strong shifts in language due to lack of sight, basically you have a blind culture that can suddenly see (poorly at first).

10 generations is a long time... I think they would have started to fade into memory about 8 generations ago lol but anyway:

Species that only live inside caves are known as troglobites. Troglobites have a few distinct adaptations. They may have developed an ability to use food more efficiently by slowing their metabolism. They maybe have developed anophthalmia or a loss of eyesight or an efficiency in low-light. They may also have devloped depigmentation of the skin. Some creatures may develop extra chemical and tactile sensory organs to make up for loss of visual stimuli. Keep in mind they may not actually be troglobites they could just be troglophile's. Troglophile's have the ability to live both in and outside of caves.

So for your cave dwelling humans I am imagining a semi-loss of hearing, eyesight and skin pigmentation. They are most likely getting shorter and more stocky. They could be developing hunched backs and hardened skin. They are perhaps even developing extra sensory organs such as antennae.

EDIT: Based on others insights I would have to agree that 10 generations is not going to be long enough a period of time to see any significant evolutionary changes such as antennae.

Everyone so far has focused in on the idea that evolutionary changes can't happen in the span of 10 generations, but there is sufficient scientific evidence to suggest the epigenetic traits can change within one individual's lifetime.
https://www.whatisepigenetics.com/exposure-to-cold-temperatures-can-change-our-gene-expression-and-fat-cells/

So I'd say you could have just about any changes you wanted - they naturally produce more Vitamin A and have sharp low-light vision - but maybe get easily stunned by bright lights.

They grow to shorter heights, their skin is tougher - the nails of their hands and feet have grown thicker and stronger.

The ability to sense temperature, pressure, and moisture has been enhanced.

You name it, you can probably justify it.

You could look for studies describing developmental differences for human populations living at high altitudes. Some of those characteristics are likely to be present here due to the lower oxygen levels.