As sun disappears, there is no more normal vitamin d source, thus it’s harder for organism to absorb calcium and phosphorus, which is the main bone structure part for bones to be strong. So when human bones gets weaker, skeleton changes its structure due to gravity that drags all human weight down, This affects legs most, they will bend in period of time, which will go even further because there is nothing that could stop doing it. As our top part of the body is holding by spine, it will bend too, dragging chest towards.
But this is not the only case that affects human body. Constant darkness cause damp weather conditions due to no warm source to dry water (in this case sun).
“It's typical for joint pain to start even before the first raindrops fall, says David Borenstein, MD, FACP, FACR, a rheumatologist and clinical professor of medicine at George Washington University Medical Center and past president of the American College of Rheumatology. If you really listened carefully to Grandma or someone who had arthritis, they actually told you it was going to rain," he says. "They said, ‘It's going to rain today,' and more likely than not, they were usually correct.
How to explain?
There's no full agreement among scientists that weather causes pain, or if a specific mechanism is at fault, Jamison says. But there are plausible theories.
One leading theory points to changes in air pressure. Although many people say that their pain worsens with damp, rainy weather, research has shown that it's not the cold, wind, rain, or snow, Borenstein says. "The thing that affects people most is barometric pressure." Barometric pressure is the weight of the atmosphere that surrounds us. If you imagine the tissues surrounding the joints to be like a balloon, high barometric pressure that pushes against the body from the outside will keep tissues from expanding.”(www.webmd.com, 2014).
It might be that body would adapt to low pressure conditions, but this would take long time and cause constant pain, which then could lead to physical illness like anger or depression. We can see this issue today in space with astronauts. They have limited time of staying in space (around 6 months) due to bones getting less dense. Nasa researchers says:
“They found that astronauts using the advanced system came home with more lean muscle and less fat, and kept more of their whole body and regional bone mineral density. Those same astronauts also consumed sufficient calories and vitamin D, among other nutrients. These factors are known to support bone health and likely played a contributing role. "After 51 years of human spaceflight, these data mark the first significant progress in protecting bone through diet and exercise," said Dr. Scott M. Smith, NASA nutritionist at the agency's Johnson Space Center in Houston and lead author of the publication. Since the 1990s, resistance exercise has been thought to be a key method of protecting astronauts' bones. Normal, healthy bone constantly breaks down and renews itself, a process called remodeling. As long as these processes are in balance, bone mass and density stay the same. Earlier studies of Russian Mir space station residents found an increased rate of breakdown, but little change in the rate of regrowth, resulting in an overall loss in bone density. In the new study, astronauts who used the ARED device still had increased bone breakdown, but their bone renewal tended to increase, likely resulting in a better balance in whole bone mineral density. "The increase in both bone breakdown and formation suggests that the bone is being remodeled, but a key question remains as to whether this remodeled bone is as strong as the bone before flight," said Dr. Jean Sibonga, NASA bone discipline lead at Johnson and coauthor of the study. Bone density loss in astronauts on long-duration missions has been a major medical concern. In the past, astronauts have lost an average of 1 to 2 percent per month. By comparison, an elderly person loses about 1 to 2 percent per year.””(www.nasa.gov, 2013)
There is major difference between space and earth gravity and pressure level, but it is almost related, space shows us in short term what could happen in long term on the ground, because symptoms are the same.
Overall, considering that all symptoms are very possible, we can predict that human kind can stop evolve due to low pressure and lack of vitamins in body. But this is only one way, there might be a technical solution to help keep bone and muscle structure healthy by using artificial vitamins that would be created in laboratories.