Final answer:
Eye color is the least important adaptation for human survival when compared to language development, thumbs, and bipedalism, which have all been crucial for human evolution.
Step-by-step explanation:
Of the given options - developing language, eye color, having thumbs, and walking upright - eye color is the least important for human survival. The other adaptations have played significant roles in human evolution and survival. Developing language was critical for effective communication, which is necessary for the complex social structures and cultural advancements that are core aspects of humanity. Having thumbs, particularly opposable thumbs, is crucial for grip and manipulation, enabling tool use and fine motor skills. Walking upright, or bipedalism, has allowed humans to efficiently travel long distances, use tools, and interact with the environment in diverse ways.