Who is Y-Chromosome Adam?
"Y Chromosome Adam" is the name given to humanity's most recent common ancestor (MRCA) on our direct paternal lines (that is, our father's father's father's father, etc.). These paternal lines can be traced by studying mutations on the relatively stable, non-recombining Y-chromosome passed from father to son.
Because like "mitochondrial Eve", Y-Adam is named after the biblical character, there are some common misconceptions about who this genetic Adam was. Y-Adam was not the only man on the planet, and he never met mitochondrial Eve – most calculations place these two of our ancestors thousands of years apart.
Y-Adam was simply a man who had unbroken lines of male descendants, to the point where all lines of direct male descent today lead to him. Y-Adam and mtDNA-Eve "were not the source of all human genetic material on the planet today," explains the Genetic Genealogist blog. "Instead, the terms refer to the founders of all the mtDNA and Y-DNA respectively."
It's also important to note that this genetic Adam and Eve are the founders of the mtDNA and Y-DNA seen today on the planet. The title of Most Recent Common Ancestor isn't permanent; different Y-DNA lines die out, either because a man has no descendants or has only female descendants (who of course have no Y chromosomes).
Eventually one of Y-Adam's descendants will be the sole source of unbroken paternal lines on earth and will become the new Y-MRCA. The former Adam, like his direct male ancestors, will still be one of our common ancestors, just not the most recent.
Our Dad: The Traditional Picture and New Findings
The search for "Y Chromosome Adam" is still too early and complex for ironclad answers, but in the early 21st century, the popular idea was that Y-Adam lived in Africa about 60,000 to 90,000 years ago. In 2000, Stanford researchers claimed that Y-Adam's closest descendants live amongst the Sudanese, Ethiopian and Khoisan in Africa, and estimated that he lived about 89,000 years ago. In 2003, geneticist Spencer Wells released his book The Journey of Man: A Genetic Odyssey, which claimed Y-Adam was African but lived about 60,000 years ago.
In May of 2011, however, a paper published in The American Journal of Human Genetics suggested a somewhat different picture. In "A Revised Root for the Human Y Chromosomal Phylogenetic Tree: The Origin of Patrilineal Diversity in Africa," Italian scientist Fulvio Cruciani et al. claim that Y-Adam lived about 140,000 years ago and that his closest descendants are found in central/northwest Africa.
The study's surprising results have raised some controversy. "I'm kind of left scratching my head," wrote Razib Khan, whose Gene Expression Blog runs on Discover magazine's website. Khan points out on his blog that the study's authors sequenced more of the Y chromosome and tested the Y DNA of over 2,000 Africans. Khan also observes that the attempted dating on Y-chromosome lineages has been "a total mess".
What does it matter if our common paternal line Adam lived 90,000 or 140,000 years ago, or if he lived south of the Sahara or somewhere around the northwest of the continent? Every step toward discovering the truth of our origins has brought controversy, from the idea of sharing a common ancestor with apes to whether we homo sapiens mixed with Neandarthals. Even the general consensus that modern humans emerged from Africa is challenged by some in the field of paleoanthropology.
Thinking about the importance of fathers in our own lives, the relentless drive to identify our common Y-DNA father as accurately as possible should be expected. This might be why the Italian scientists behind the study offer some caution along with their surprising results: "Although this point requires further attention," they write, "we think that it offers a new prospect from which to view the initial development of our species in Africa."