In human genetics, Y-chromosomal Adam (Y-MRCA) is the theoretical most recent common ancestor (MRCA) from whom all living people are descended patrilineally (tracing back only along the paternal lines of their family tree). Many studies report that Y-chromosomal Adam lived as early as around 142,000 years ago [1] and possibly as recently as 60,000 years ago.[2] All living humans are also descended matrilineally from Mitochondrial Eve who is thought to have lived earlier about 190,000 - 200,000 years ago. Y-chromosomal Adam and Mitochondrial Eve need not have lived at the same time.
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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 chromosome 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.
The title "Y-chromosome Adam" is not permanently fixed on a single individual. Because knowledge of human Y-chromosomes is still incomplete, Y-chromosome Adam's DNA sequence, his position in the family tree, the time when lived, and his place of origin, are all subject to future revisions. In addition, demographic changes during the course of human evolution would have frequently caused the title of Y-chromosome Adam to change hands.[3] The following events would change the individual designated Y chromosome Adam:
Y-chromosomal Adam had at least two sons and two of his sons have unbroken lineages that have survived to the present day. Initial sequencing of the human y-chromosome suggested that two most basal y-chromosome lineages were Haplogroup A and Haplogroup BT. Haplogroup A is found at low frequencies in parts of Africa, but is common among certain hunter-gatherer groups. Haplogroup BT lineages represent the majority of African y-chromosome lineages and virtually all non-African lineages.[4] Y-chromosomal Adam was represented as the root of these two lineages. Haplogroup A and Haplogroup BT represented the lineages of the two sons of Y-chromosomal Adam.
However, a recent paper[1] places this event around 142,000 years ago. Cruciani et al. 2011, determined that the deepest split in the Y-chromosome tree is found between two previously reported subclades of Haplogroup A, rather than between Haplogroup A and Haplogroup BT. Subclades A1b and A1a-T, now descend directly from the root of the tree and now represent the lineages of Y-chromosomal Adam's two sons. The rearrangement of the Y-chromosome family tree implies that lineages classified as Haplogroup A do not necessarily form a monophyletic clade.[5] Haplogroup A therefore refers to a collection of lineages that do not possess the markers that define Haplogroup BT, though haplogroup A includes the most distantly related y-chromosomes.
The M91 and P97 mutations distinguish Haplogroup A from Haplogroup BT . Within Haplogroup A chromosomes, the M91 marker consists of a stretch of 8 T nucleobase units. In Haplogroup BT and chimpanzee chromosomes, this marker consists of 9 T nucleobase units. This pattern suggested that the 9T stretch of Haplogroup BT was the ancestral version and that Haplogroup A was formed by the deletion of one nucleobase. Haplogroups A1b and A1a were considered subclades of haplogroup A as they both possessed the M91 with 8Ts.[4][5]
But according to Cruciani et al. 2011, the region surrounding the M91 marker is a mutational hotspot prone to recurrent mutations. It is therefore possible that the 8T stretch of Haplogroup A may be the ancestral state of M91 and the 9T of Haplogroup BT may be the derived state that arose by an insertion of 1T. This would explain why subclades A1b and A1a-T, the deepest branches of Haplogroup A, both possess the same version of M91 with 8Ts. Furthermore Cruciani et al. 2011 determined that the P97 marker, which is also used to identify haplogroup A, possessed the ancestral state in haplogroup A but the derived state in Haplogroup BT.[5]
Initial studies implicated East Africa and Southern Africa as the likely sources of human y-chromosome diversity. This was because the basal lineages, Haplogroup A and Haplogroup B achieve their highest frequencies in these regions. But according to Cruciani et al. 2011, the most basal lineages have been detected in West, Northwest and Central Africa. In a sample of 2204 African y-chromosomes, 8 chromosomes belonged to either haplogroup A1b or A1a. Haplogroup A1a was identified in two Moroccan Berbers, one Fulbe and one Tuareg from Niger. Haplogroup A1b was identified in three Bakola pygmies from Southern Cameroon and one Algerian Berber. Cruciani et al. 2011 suggest a Y-chromosome Adam living somewhere in Central-Northwest Africa, fits well with the data. However they suggest that this interpretation is still very preliminary due to the incomplete sampling of African Y-chromosomes, and incomplete knowledge of past demographic events.[5]
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, even though he co-existed with plenty of men around,[6] including his own father who was not the "most recent". However, all his other male contemporaries failed to produce a direct unbroken male line to the present day.
The time when Y-chromosome Adam lived is determined by applying a molecular clock to human y-chromosomes. In contrast to Mitochondrial DNA, which has a short sequence of 16000 base pairs, and mutates frequently, the y-chromosome is significantly longer at 60 million base pairs, and has a lower mutation rate. These features of the y-chromosome have slowed down the identification of its polymorphisms and as a consequence, reduced the accuracy of y-chromosome mutation rate estimates.[7] Initial studies, such as Thomson et al. 2000[7] proposed that Y-chromosome Adam lived about 59,000 years ago. This date suggested that Y chromosome Adam lived tens of thousands of years after his female counterpart Mitochondrial Eve, who lived 150,000-200,000 years ago.[8] This date also meant that Y-chromosome Adam lived at a time very close to, and possibly after, the out of Africa migration which is believed to have taken place 50,000-80,000 years ago.
One explanation given for this discrepancy in the dates of Adam and Eve was that females have a better chance of reproducing than males due to the practice of polygyny. When a male individual has several wives, he has effectively prevented other males in the community from reproducing and passing on their y-chromosomes to subsequent generations. On the other hand, polygyny doesn't prevent most females in a community from passing on their mitochondrial DNA to subsequent generations. This differential reproductive success of males and females can lead to fewer male lineages relative to female lineages persisting into the future. These fewer male lineages are more sensitive to drift and would most likely coalesce on a more recent common ancestor. This would potentially explain the more recent dates associated with Y-chromosome Adam.[9][10]
The 2011 study by Cruciani et al. found that Y-chromosome Adam lived about 142,000 years ago, significantly earlier than previous estimates, such as the 59,000 years ago estimate proposed by Thomson et al. 2000. The older TMRCA was due to the discovery of additional mutations and the rearrangement of the backbone of the y-chromosome phylogeny following the resequencing of Haplogroup A lineages. According to the study, determining the precise date when Y-chromosome Adam lived depends on the accuracy of the mutation rate used. But the repositioning of the MRCA from the root of haplogroups A and BT to the root of Haplogroups A1b and A1a still entails that Y-chromosome Adam is older than previously thought. According to Cruciani et al., the much older date is easier to reconcile with models of human origins.[5]
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Evolutionary tree of Human Y-chromosome DNA (Y-DNA) haplogroups |
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| most recent common Y-ancestor | ||||||||||||||||||||||||||||||||||||||||||||||||||
| A | ||||||||||||||||||||||||||||||||||||||||||||||||||
| A1b | A1a-T | |||||||||||||||||||||||||||||||||||||||||||||||||
| A1a | A2-T | |||||||||||||||||||||||||||||||||||||||||||||||||
| A2 | A3 | BT | ||||||||||||||||||||||||||||||||||||||||||||||||
| B | CT | |||||||||||||||||||||||||||||||||||||||||||||||||
| DE | CF | |||||||||||||||||||||||||||||||||||||||||||||||||
| D | E | C | F | |||||||||||||||||||||||||||||||||||||||||||||||
| G | H | IJK | ||||||||||||||||||||||||||||||||||||||||||||||||
| IJ | K | |||||||||||||||||||||||||||||||||||||||||||||||||
| I | J | LT | K(xLT) | |||||||||||||||||||||||||||||||||||||||||||||||
| L | T | M | NO | P | S | |||||||||||||||||||||||||||||||||||||||||||||
| O | N | Q | R | |||||||||||||||||||||||||||||||||||||||||||||||