Kohisan, Amerindian, and Dravidian (partially) people lacked diffusion, and the main reason for it was their relative isolation for very long time.
Explanation:The Khoisan people are native to the southern part of Africa. They lived in isolation in the savanna for thousands of years, and still do so. Because of the lack of contact with other people groups and continuing to practice their own way of life, there has been no diffusion among them.The Amerindian people were native to the Americas. While they have diversified greatly in this two large continents, they didn't had contact with the rest of the world. This contributed to development of culture based on the same roots, and as there was no outside influence there was no diffusion as well.The Dravidian people were native to South Asia. These people actually don't fall into the category of lacking diffusion as the other two. While for most of the time the Himalayas were isolating these people, at one point in ancient times there was a large migration of Aryan people from Central Asia. This actually resulted in large scale cultural diffusion where the two cultures intermingled and created a new culture which lasted and still lasts for several thousand years now.
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