Final answer:
Early humans' key adaptation was the ability to think and express themselves with complex language.
Explanation:
Early humans developed the ability to think and express themselves with complex language as a key adaptation. This enabled Homo sapiens to navigate vast territories, communicate effectively, and collaborate on innovations like tools and agriculture. The evolution of symbolic spoken language allowed for the accumulation of shared knowledge and the development of human culture.
Learn more about Human evolution and language development here:
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