Y-chromosomal Adam

n human genetics, Y-chromosomal Adam (Y-MRCA) is the most recent common ancestor (MRCA) from whom all living men are descended patrilineally (tracing back along the paternal lines of their family tree only). Y-chromosomal Adam probably lived between 90,000 and 60,000 years ago in Africa and is the male counterpart of Mitochondrial Eve, although he lived much later than she did, possibly 50,000 to 80,000 years later.^[1]

Y-MRCA date

By analyzing the Y chromosome DNA from males in all regions of the world, geneticist Spencer Wells has concluded that all humans alive today are patrilineally descended from a single man who lived in Africa around 60,000 years ago.^[2]

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Possibly there was a genetic isolation and remixing of early ancestral groups within Africa, with one group having been more isolated and therefore having a higher predominance of an ancient Y-chromosome haplotype extant in their culture.[3] Wells says his evidence based on DNA in the Y-chromosome indicates that the exodus began between 60,000 and 50,000 years ago. In his view, the early travelers followed the southern coastline of Asia, crossed about 250 kilometers [155 miles] of sea, and colonized Australia by around 50,000 years ago. The Aborigines of Australia, Wells says, are the descendants of the first wave of migration out of Africa.[2]

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The article also says that, "many archaeologists disagree, saying the fossil record shows that a first wave of migration occurred around 100,000 years ago". However, microbiologists tend to disagree with this conclusion. According to the article, Wells agrees that there may have been early human forays into the Middle East, but argues that the Levant of 100,000 to 150,000 years ago was essentially an extension of northeastern Africa and was probably part of the original range of early Homo sapiens. These early settlers were replaced by Neanderthals in the region about 80,000 years ago.

By 1995, in scholarly literature, first "Adam" or rather first paternal ancestor date was estimated at 270,000 years ago.^[4][5] Later new set of markers was chosen and the age was adjusted to mostly cited value.^[6] The dates calculated by Whitfield on new markers was 37,000–49,000 years ago ^[7] adjusted from 51,000–411,000 years ago^[7] in 1994 by Hammer.

The dates are calculated from present day distribution with the following assumptions for unique mutation events (UMEs):

1. "no strange Y lineages were introduced into this population by means of migration
2. each UME occurred for the first time within our population
3. each UME occurred only once
4. detected all UMEs present in the male subjects studied"^[6]

Nomenclature

Y-chromosomal Adam is named after the Biblical Adam. This may lead to a misconception that he was the only living male of his times; in fact he co-existed with plenty of men around.^[8] However, all his male contemporaries failed to produce a direct unbroken male line to the present day.

Time frame

Y-chromosomal Adam probably lived between 60,000 and 90,000 years ago, judging from molecular clock and genetic marker studies. While their descendants certainly became close intimates, Y-chromosomal Adam and mitochondrial Eve are separated by tens of thousands of years.

The more recent age of the Y-MRCA compared to the mt-MRCA corresponds to a larger statistical dispersion of the probability distribution for a Paleolithic man to have living descendants compared to that of a Paleolithic woman. While fertile women had more or less equally distributed chances of giving birth to a certain number of fertile descendants, chances for fertile men varied more widely, with some fathering no children and others fathering many, with multiple women. (This difference in variance was first pointed out, in the number of descendants of male versus female fruit flies, by Bateman, 1948.)

Population genetics states that in the past, our Y-chromosomal Adam was not the common ancestor of the entire population, and in the future one of his descendants may take over. The Y-MRCA of all humans alive today is different from the one for humans alive at some point in the remote past or future: as male lines die out, a more recent individual becomes the new Y-MRCA. Patrilineal lines are less likely to die out in times of rapid population growth, such as the present, than during a population bottleneck.