Khoisan people are often cited as the oldest branch of humanity. How is it that they have no Neanderthal DNA?
Khoisan people carry original Genetics close to identical to the first Homo Sapiens.
Over the course of human migrations most people living today have lost genetic diversity due to migration bottle necks.
A population bottleneck or genetic bottleneck is a sharp reduction in the size of a population due to environmental events such as famines, earthquakes, floods, fires, disease, and droughts or human activities such as specicide and human population planning. Such events can reduce the variation in the gene pool of a population; thereafter, a smaller population, with a smaller genetic diversity, remains to pass on genes to future generations of offspring through sexual reproduction. Genetic diversity remains lower, increasing only when gene flow from another population occurs or very slowly increasing with time as random mutations occur.[self-published source] This results in a reduction in the robustness of the population and in its ability to adapt to and survive selecting environmental changes, such as climate change or a shift in available resources. Alternatively, if survivors of the bottleneck are the individuals with the greatest genetic fitness, the frequency of the fitter genes within the gene pool is increased, while the pool itself is reduced.Population bottleneck followed by recovery or extinction
The genetic drift caused by a population bottleneck can change the proportional random distribution of alleles and even lead to loss of alleles. The chances of inbreeding and genetic homogeneity can increase, possibly leading to inbreeding depression. Smaller population size can also cause deleterious mutations to accumulate.
Scientists have witnessed population bottlenecks in American bison, greater prairie chickens, northern elephant seals, golden hamsters, and cheetahs. The New Zealand black robins experienced a bottleneck of five individuals, all descendants of a single female. Geneticists have found evidence for past bottlenecks in pandas, golden snub-nosed monkeys, and humans.
The first great bottle neck occurred with people leaving Africa. The first modern Humans to leave Africa were East African. As a result all non African people living today are the descendants of ancient East Africas that left Africa over 70,000 years ago:
Leaving African created a large genetic bottleneck, and a series of smaller bottlenecks with every great land migration:
How is it that they have no Neanderthal DNA?
Neanderthals were limited to polar and temperate climates in Europe and Asia. Humans were originally only a tropical species from Africa. Once people left Africa they encountered and mixed with Neanderthals. However, early Humans traveled back and forth between Northern Africa, Eastern Europe, and West Asia. The Sahara desert is only fairly recently a huge desert. It was once a forest with lakes.
As a result modern Africans can have Neanderthal Ancestry, however specifically the Khoisan have not had contact with Neanderthals.