The survival of individuals with bad eyesight, or any other genetic condition, is not necessarily tied to evolution in the way you might imagine. Evolution is a gradual process that occurs over many generations, and the factors influencing survival and reproduction are complex and multifaceted.
Firstly, it's important to understand that "bad" eyesight, or myopia (nearsightedness), may not have been as significant a disadvantage for early human populations as it might be in modern times. The environment in which our ancient ancestors lived was quite different from today's modern society. They were hunter-gatherers living in small, close-knit groups, and survival depended on a wide range of skills and abilities, not solely on perfect eyesight.
Additionally, many genetic conditions, including myopia, can be influenced by a combination of genetic and environmental factors. Myopia can have both genetic and lifestyle-related triggers. In modern times, factors like excessive near work (e.g., reading, screen time) and limited exposure to natural light have been associated with higher myopia rates. It's uncertain how these factors may have affected ancient human populations, as their lifestyle and visual environment were different.
Evolution is not necessarily about eliminating "bad" genes but about the relative reproductive success of individuals and the passing on of genes to subsequent generations. Some genes or traits that might seem disadvantageous in one context can provide an advantage in others, and the overall genetic diversity of a population can be beneficial for its long-term survival.
Moreover, myopia is a complex trait influenced by multiple genetic factors, and its prevalence can fluctuate based on various factors. The genetics of myopia are not fully understood, and it's challenging to draw direct comparisons between ancient and modern populations in terms of eye health.
In conclusion, the survival of individuals with myopia in ancient times is a complex interplay of various factors, and it is difficult to make definitive statements about why certain genetic traits persisted. Evolution is a gradual and dynamic process that involves numerous factors beyond just a single genetic condition.