The color of human skin is primarily influenced by the amount of melanin in the skin cells. Melanin is a pigment that helps protect the skin from the harmful effects of ultraviolet (UV) radiation from the sun. In regions with higher UV radiation, humans developed more melanin to shield their skin from sun damage. As a result, people living closer to the equator, where there is intense sunlight, tend to have darker skin with more melanin.
If a group of humans were to live underground for thousands of years, away from direct sunlight and UV radiation, it's likely that their skin would gradually lose some of its melanin content. This is because there would be no evolutionary pressure to maintain high levels of melanin, as the primary purpose of the pigment is to protect against UV radiation.
Over time, the skin of these underground-living humans might become lighter in color due to decreased melanin levels. However, it's important to note that the exact changes would depend on various factors such as genetics, diet, and the overall living conditions underground. Additionally, humans are highly adaptable, and any changes in skin color would occur over an extremely long period of time, spanning thousands of years or more.
It's also worth mentioning that skin color is a complex trait influenced by multiple genes, so the process of skin color adaptation would not be as straightforward as just becoming uniformly lighter. Different groups within the underground population might develop various skin tones based on their unique genetic makeup and environmental conditions.
Overall, this scenario is speculative, as humans living entirely underground for thousands of years is a highly unlikely and unprecedented situation. In reality, human evolution and adaptation to different environments are complex processes shaped by numerous factors.