There is a general consensus among mainstream anthropologists that humans first emerged in Africa about two million years ago. Since then we have spread throughout the world, successfully adapting to widely differing conditions and to periodic cataclysmic changes in local and global climate. The many separate societies that emerged around the globe differed markedly from each other, and many of these differences persist to this day.
As well as the more obvious cultural differences that exist between peoples, such as language, dress and traditions, there are also significant variations in the way societies organise themselves, in their shared moral values, and in the ways they interact with their environment. It is debatable whether these differences are merely incidental artefacts arising from patterns of human migration or whether they represent an evolutionary trait that is key to our success as a species. By analogy with biodiversity, which is thought to be essential to the long-term survival of life on earth, it can be argued that cultural diversity may be vital for the long-term survival of humanity; and that the conservation of indigenous cultures may be as important to humankind as the conservation of species and ecosystems is to life in general.
This argument is rejected by many people, on several grounds. Firstly, like most evolutionary accounts of human nature, the importance of cultural diversity for survival may be an un-testable hypothesis, which can neither be proved nor disproved. Secondly, it can be argued that it is unethical deliberately to conserve "less developed" societies, because this will deny people within those societies the benefits of technological and medical advances enjoyed by those of us in the "developed" world. Finally, there are many people, particularly those with strong religious beliefs, who maintain that it is in the best interests of individuals and of humanity as a whole that we all adhere to the single model for society that they deem to be correct. For example fundamentalist missionary organisations such as the New Tribes Mission actively work to reduce cultural diversity by seeking out remote tribal societies, converting them to their own faith, and inducing them to conform to their own model of society.
Cultural diversity is a tricky thing to quantify, but a good indication is thought to be a count of the number of languages spoken in a region or in the world as a whole. By this measure there are signs that we ay be going through a period of precipitous decline in the world's cultural diversity. Ten percent of the world's languages are now spoken by 100 or fewer native speakers, while 49% of the world's population speak one of the ten most common languages. Overpopulation, globalisation and imperialism (of both the cultural and old-fashioned kind) are reasons that have been suggested to explain any such decline. There are several organisations that work towards protecting threatened societies and cultures, including Survival International, and UNESCO Global Alliance for Cultural Diversity.