Some of the earliest human populations lived in Southern Africa, and evidence from sites there has inspired key debates on human origins and on the emergence of modern humans. The sub-continent has one of the world's richest heritages of rock art, and specialists have developed innovative theories about its meaning and significance that have influenced the understanding of rock art everywhere. Passionate arguments about the hunter-gatherer way of life have centred on Southern African cases, and the relationship between archaeological and anthropological data is also central to understanding the past of Southern Africa's pastoralist and farmer communities. The pre-colonial states of the region provide some of the best documented cases of the influence of external trade on the development of African polities.