All modern humans descend in the male line from a particular man, nicknamed Genetic Adam, who lived in Africa about 142 thousand years ago (~7,100 generations). All living men have inherited his y chromosome (yDNA), along with the mutations that have accumulated in our individual family lines.
Unlike autosomal DNA, yDNA does not recombine. It passes intact from a man to his sons. (Women do not have y chromosomes.) And, the mutation rate for yDNA is very low, perhaps one mutation at a given spot every 14,000 years. Therefore, a family’s yDNA changes very slowly over many generations. Every man whose y chromosome contains a particular mutation shares a common paternal ancestry extending back thousands of years with every other man who has the same mutation.
These characteristics make it useful to both anthropologists and genealogists.
Anthropologists use information about the y chromosome to study the remote origins of the human race, and to trace population movements in the time before recorded history. By comparing accumulated mutations, anthropologists can identify different population groups and place individual men within the overall framework of the human family. The geographic patterns formed by the different groups can show where the groups originated, where they traveled, and where members are living today.
Genealogists use information about the y chromosome to match men who belong to the same male line. Very distant cousins, if they share the same paternal line, will have similar y chromosomes. Mutations accumulate slowly enough that men descended in the male line from a common ancestor who lived, say, about 1200 or 1300 CE — about the time Europeans were adopting surnames — will have nearly identical y chromosomes. Men with the same surname who do not belong to the same male line will have very different y chromosomes.
In short, y chromosome test results can be used:
- To understand the remote origins of the human race.
- To trace population movements in the time before recorded history.
- To identify male lines that share a common ancestor.
- To show that two men with the same surname have a common ancestor, who must have lived after the time when surnames were adopted.
- To show that two men with different surnames have a common ancestor, who might have lived before the time when surnames were adopted.
- To show that men with the same surname do not belong to the same male line, and therefore do not have a common ancestor.