Final answer:
The essential ethical trait developed by humans to ensure the survival of kinship tribal groups is cooperation, as it facilitates shared knowledge, work distribution, and the establishment of behaviors critical for survival.
Step-by-step explanation:
When considering the ethical trait that was essential as humans live in kinship tribal groups to ensure the tribe's survival, we must examine the complex interplay of various human behaviors. In the context of social animals, empathy and expectations of reciprocity are key elements for forming and maintaining social orders. These aspects contribute to our understanding of human morality, which is heavily influenced by humans developed pro-social behaviors. However, when discussing survival and the success of a tribe, cooperation emerges as the most critical trait, as it allows for shared knowledge, work distribution, and the establishment of common habits and behaviors which enhance group survival.