In a future world, where colonists live in underground caves on Saturn’s moon Iapetus, humans may reside underground for generations, developing as a species on this planet in almost total darkness. In such a scenario, how might evolutionary pressures lead to physical changes in future human cave dwellers? Life on Earth has managed to adapt to even the harshest of environments, and organisms adapted to the dark provide stark examples of how gradual changes spanning multiple generations can lead to physical changes in a population.
Cave fish (also known as Mexican tetras) are a popular illustration of selection effects in a population. These cave fish have lived in total darkness for many generations and have lost any capabilities for vision that their ancestors may have exhibited. Cave fish also have lost nearly all skin pigment, compared to other fish, and seem to have adapted their bodies to the dark cave environment. Because vision, as well as dark skin pigment, provides no advantage to the survival of cave fish in a dark environment, any cave fish that are born with lighter skin or deficient eyesight will be no worse off than any other cave fish. This means that eyesight, and skin pigment, do not provide survival advantages to cave fish living in the dark, which provides one explanation for why these traits slowly drifted out of the population over time. Perhaps the loss of eyesight also provides other advantages to cave fish, such as less susceptibility to disease, which may also promote blindness among cave fish populations.
Organisms known as extremophiles have adapted to live in conditions otherwise thought inhospitable to life, and extremophiles known as endoliths thrive in dark regions underground and in the pores and cracks of rocks. Many of these endoliths feed on inorganic material, such as iron, sulfur, and potassium, making use of minerals not typically thought of as food. While dark regions are devoid of most sunlight needed for photosynthesis, some endoliths have even managed to develop an extremely efficient form of photosynthesis--such organisms can thrive on less than ten single photons per day that leak through cracks in the surface rock. Endoliths tend to have very long reproductive life cycles, perhaps on the order of a hundred years, as they must conserve much of their energy to repair damage from cosmic rays while simultaneously living in a low-energy environment. Still, endoliths fill an ecological niche on Earth that would otherwise remain uninhabited.
In modern human civilization, being born with good eyesight provides less and less of an advantage as technology progresses. Prior to the invention of glasses, such individuals would be at a severe survival disadvantage, whether this be the inability to hunt, or find a job, or secure a mate. Today, most people with subpar eyesight are fitted with corrective lenses that allow them to function as if they had typical eyesight. We are not quite in the same boat as the cave fish, as humans live in a visual world, but a population of future human colonists will likely include many people dependent on corrective lenses.
What does this suggest for future humans that reside in caves on Iapetus? Evolutionary selection effects operate on populations over generations of time, so it will require a sustained underground human colony with relatively little interaction from outsiders in order to begin to see dark-adapted selection effects. Nevertheless, after generations of living underground, humans may find that they can relinquish their corrective lenses, as vision provides no advantages underground anyway, and many more generations may ultimately lead to poor eyesight--or even blindness--propagating through the underground human population. Perhaps, in this distant future scenario, blind cave dwellers might develop heightened senses to help them navigate their dark environment. Then again, humans may use technology that alleviates some of these pressures of living underground: artificial lighting, for example, might keep humans dependent on eyesight, although the lack of ultraviolet radiation from such lighting could still lead to changes in skin pigment. Even with technology, any population that lives in isolation for a long enough time will start to change, and humans are no exception.