Hypothesis
The existence of a Y-chromosomal Adam was determined by applying the theories of molecular evolution to the Y chromosome. Unlike the autosomes, the human Y chromosome does not recombine with the X chromosome but is transferred intact from father to son. Mutations periodically occur within the Y chromosome and these mutations are passed on to males in subsequent generations. These mutations can be used as markers to identify shared patrilineal relationships. Y chromosomes that share a specific mutation are referred to as haplogroups. Y chromosomes within a specific haplogroup share a common patrilineal ancestor who was the first to carry the defining mutation. A family tree of Y chromosomes can be constructed, with the mutations serving as branching points along lineages. Y-chromosomal Adam is positioned at the root of the family tree as the Y chromosomes of all living males are descended from his Y chromosome.
Researchers can reconstruct ancestral Y chromosome DNA sequences by reversing mutated DNA segments to their original condition. The most likely original or ancestral state of a DNA sequence is determined by comparing human DNA sequences with those of a closely related species, usually non-human primates such as chimpanzees and gorillas. By reversing known mutations in a Y-chromosome lineage, a hypothetical ancestral sequence for the MRCA, Y-chromosomal Adam, can be inferred.
Determining Y-chromosomal Adam's DNA sequence, and the time when he lived, involves identifying the human Y-chromosome lineages that are most divergent from each other—the lineages that share the least unique mutations with each other when compared to a non-human primate sequence in a phylogenetic tree. The common ancestor of the most divergent lineages is therefore the common ancestor of all lineages.
The existence of Y-chromosomal Adam was confirmed by a worldwide sample of Y chromosomes that included individuals from all continents. A number of Y-chromosome lineages, or haplogroups, from Africa were found to be the most divergent from each other, and non-African lineages were determined to be subsets of a few lineages found in Africa. This suggested Africa was the most likely home of Y-chromosomal Adam.
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