Speaking as someone with virtually no background in biology or genetics, here’s what I understand what I have read. Some of this may be erroneous and all of it is simple minded, but hopefully, it makes some sense.
The most accepted theory of human migrations is called “The Recent Out of Africa Theory”. Scientists determined from mitochondrial DNA that non-African populations are descended from one group of modern humans that left Africa about 50,000 yeas ago and that there were no more than about 550 females. This group represented the sole ancestors of all populations of non-sub-Saharan African modern humans.
Sometime after this one-time out of Africa exodus, one group left Southwest Asia and hiked into Central Asia. Their descendents are the various branches of the Caucasian and Mongoloid races. Another group settled in Southast Asia continued out into the Pacific. These became the Malayan, Melanesian, Polynesian , and Aboriginal Australian races.
Mutations are pieces of genetic material of an identifiable molecular structure that spontaneously form on a specific site in the DNA of some individual. They are thereafter passed on to his or her descendents . Mutations may, but usually don’t, have any effect on the physiology of the person posessing them. But since they are inherited they are useful determining the ancestral background of his descendents.
Another useful characteristic of mutations is that they accumulate in the DNA of an inbred population (such as, say, an isolated tribe) at a somewhat steady rate. Thus, geneticists believe, the number of mutations can be used estimating the time interval that has passed between specimens taken from a DNA samples taken from say, a particular group in different stages of a migration.
The same type of data and the underlying theory that shows that Japanese and Koreans share a more recent common ancestor than either has with an Australian also is the EXACT SAME type of information used to determine that humans and chimps shared a more recent common ancestor than either has with a gorilla.