is there some sort of cutoff point where scientists decided “everyone in Ireland 100,000 years ago will be considered 100% Irish”?
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We do all have 100% African DNA in that sense.
However, those people wandered about. As they did, they broke up into smaller groups. You can imagine a village getting large, then one day someone says “I was out hunting east of here and found a nice place 3 days from here with a nice lake, lots of berries, and good hunting, anyone want to head out there?”
A portion of the village breaks off and wanders away and starts a new village. The new villagers have children with others in the new village. Genetically, the new village is “enriched” in variations of genes carried by the relatively people that founded it. The smaller the founding group, the bigger the effect. It’s inbreeding to a degree.
Think of the tiniest break off group: a single man and woman. If the man has a genetic trait that only appeared in 1 in 500 villagers, when he and his wife leave / are separated, then the gene appears in 1 out of 2 of the people in the new group. If they have children and they mix with their children, you may have thousands of people generations later, and may 1 out of every 2 will have that genetic variant. You can use that to differentiate that group from the parent group, and if you tested a person and found that they had the gene, you could give a good guess whether they came from the break away village or not.
Ancestry tests do that, but look at hundreds of thousands of genetic differences. Computers look for clusters of genetic differences and build a hierarchy of genetic variations that correspond to points in human history where groups split and formed new groups with distinct genetic composition (the smaller, more isolated, and the longer separated the group was, the more pronounced). Sometimes the groups also merged (whereby a new group with a reduced variation occurred).
All you need to do afterwards, then, is look at modern groups and their genetic composition to label the ends of the tree and look at historical movements, and you can place names (places, ethnicities) to all the groups (with varying degrees of accuracy).
You can take an individual, look at the gene variants they have, and compute what groups they are most like and the probability of being from that group. Most importantly, every group can be traced back over time through a series of movements of people all the way to Africa.
I am part of an ancestry that traveled through North Africa, into the Arabian Peninsula, through southern Europe, and eventually to the British isles and Scotland and Ireland.
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