Adam of the Y chromosome
Adam of the Y chromosome is a term from archaeogenetics for that primeval man who is related to all men living at a certain later point in time via an uninterrupted lineage of exclusively male descendants. An "Adam of the Y chromosome " is therefore the common progenitor of all men living at a certain point in time, which can be reconstructed if ancestry is defined exclusively through fathers and male ancestors ( paternal ), completely disregarding mothers and female ancestors and from several under these conditions determined common ancestors selects the genealogically youngest.
The term and its meaning result from the fact that the Y chromosome, unlike other chromosomes , is inherited exclusively from father to son. In a "Y-chromosomal Adam" is an increase of the phylogenetically youngest man in the genus Homo , to which all the time in question existing human Y chromosomes decline. He is the male counterpart to mitochondrial Eve , the historically last woman who is related to all people living at a certain moment through an uninterrupted line of exclusively female descendants and from whom all human mitochondria that exist at that moment come .
Even if one assumed in a mind game that all of humanity descended from a specific first man, that there was a real ancestor of all people in the manner of Adam of the Bible , this is one , but not the Adam of Y- Chromosome. If the original Adam has exactly one son, the son becomes the new Adam with the death of the father. If the original Adam has several sons, he initially remains Adam after his death, but subsequently loses this role to one of his descendants as soon as all but one of the "houses" he founded die out, as soon as only one of his Sons of male offspring exist. The new and each subsequent Adam also pass on the role in question under analogous circumstances, whereby theoretically any number of generations can be skipped over each time.
An Adam on the Y chromosome does not have to be the first man or the only man of his generation. Although an Adam on the Y chromosome is by definition the ancestor of all men living at the time in question, in theory it does not necessarily have to be an ancestor of all women living at the time in question . In principle, he doesn't even have to be human.
When inventing the term, recourse was made to the image of Adam and Eve anchored in general knowledge as the first parents of all people. However, it was overlooked that the facts to be presented do not apply to Adam at all, but to another man in the creation story, namely Noah . Due to the narrowing in the flood , which only Noah with his wife, his three sons and their wives survived, all men living after him - and thus their Y chromosomes - come from Noah, while all X chromosomes from Eve and Adam Noah's family go back. So it would be more appropriate to speak of a “Noah of the Y chromosome”.
The current state of relevant research supports the so-called out-of-Africa theory , according to which all of today's humanity descends from a manageable and homogeneous population of early African humans. It thus invalidates the competing hypothesis that today's humanity emerged from a mixture of different groups of people who developed independently of one another from different geographically separated pre-human groups. The result does not provide an answer to the currently much discussed question of whether the current non-African ethnic groups can be traced back to one large or several small African waves of emigration.
Current research indicates that to a certain extent matings between different species of the genus Homo have taken place. In the years 2013 to 2015, the research team led by the Swedish scientist Svante Pääbo published new findings on mixing :
- Improved analysis methods showed a gene flow of up to 4% Neanderthal genes to the gene pool of today's Europeans and Asians.
- Research data on the Homo sapiens fossils from Peştera cu oasis in Romania and Ust-Ischim in Siberia as well as on the discovery of Denisova corroborate these statements.
- However, gene flow has so far only been detected in one direction (mating of Homo sapiens men with Neanderthal women) and not from Neanderthal men to Homo sapiens women.
The Y chromosome does not recombine with the X chromosome for over 95% of its length. The DNA of all men in this area must therefore go back to a single ancestor, i.e. be monophyletic .
Compared to ape the Y chromosome significantly larger sequence differences is than the autosomes . For example, the Y chromosomes of humans and chimpanzees differ by 1.7%, but autosomes differ by only 1.25% on average. Within the species, however, the diversity of the Y chromosome is significantly lower than that of autosomes. The Y chromosome is only inherited via the male germline , in which the mutation rate is slightly higher than in the female. This explains the higher divergence of the sequences between primates .
One of the factors contributing to genetic diversity within the human population is the smaller effective population size . It is only about ¼ as large as that of autosomes and thus roughly corresponds to that of mitochondrial DNA. The age of the mitochondrial Eve and the Adam of the Y chromosome must therefore be similar, but not identical, as the actual effective population sizes can be quite different.
Although the current population size of man is far higher than the other great apes , genetic diversity in humans is much smaller. In a study of a 3,000-base-long DNA segment on the Y chromosome of 101 common chimpanzees and seven bonobos , 23 variable positions were found, while 42 people in the same region showed no differences at all. The orangutan also shows a much higher genetic diversity than humans.
|mtDNA||3 to 4 times as high||higher||higher||higher|
|X chromosome||3 times as high||no data||2 times as high||3.5 times as high|
The gorilla is the only hominoid to show a lower genetic diversity of the Y chromosome than humans, although its genetic diversity is much higher in mitochondrial DNA and the X chromosome. The cause is the special social structure of the gorillas. They live in groups in which a dominant male ( silverback ) mates with all females. This reduces the effective population size of the Y chromosome, which results in its low genetic diversity.
The diversity of the Y chromosome in humans is greatest in Africa. The very old macro haplogroups A and B are only found in Africa , while other macro haplogroups can be found both in Africa and outside Africa. The haplogroups outside of Africa are heavily ramified, which is typical of expanding populations. The only apparently greater diversity of the Y chromosomes in Europe and Asia is due to the fact that the anthropologists divided the macro-aplogroups of Eurasia into younger sub-haplogroups (e.g. the macro-aplogroup F into the haplogroups G to R) in order to understand further, more recent stages of settlement can.
|mtDNA (differences in pairs)||2.08||1.75||1.08|
|Y chromosome (43 markers)||0.841||0.904||0.852|
|X chromosome (nucleotide diversity)||0.035||0.025||0.034|
|Autosomes (nucleotide diversity)||0.115||0.061||0.064|
Note: Africa would also show the highest diversity for the Y chromosome if the genetic diversity is measured as the number of paired differences.
In contrast to mitochondrial DNA, the DNA sequences of the Y chromosome differ very little from person to person. If you wanted to generate a family tree just by comparing sequences, as in the Mitochondrial Eve, very long sequences would be required. This would have resulted in extremely high costs, especially in the early 1990s when the first studies of this type began. Instead, the researchers work with genetic markers . The first markers were discovered in 1985. Since then, more and more markers have been added. The DHPLC technology ( Denaturing high performance liquid chromatography ) makes an important contribution to this.
- The family trees clearly indicate an African origin of the Y chromosome, although not as clearly as in the case of mitochondrial DNA.
- The two oldest branches of the tree (A and B) are practically specific for Africa, but only about 13% of the Y chromosomes in Africa belong to these haplogroups.
Estimating the time to the last common human paternal ancestor is particularly difficult. Older studies showed an astonishingly young age of the Adam of the Y chromosome, albeit with large error intervals. Thomson et al. (2000) estimated the time to the last common ancestor to be around 60,000 years (60,000 to 90,000). If these results are correct, it means that the Adam on the Y chromosome lived only a short time before the first great exodus from Africa. This clearly distinguishes him from the mitochondrial Eve, who lived about 175,000 years ago. The very young age of the Y chromosome would also explain why its genetic diversity is not significantly higher in Africa than in Eurasia and why it is so low overall in humans. It also explains why Africans are not as clearly separated from non-Africans in the family tree of the Y chromosome as is the case with mitochondrial DNA.
However, a study published in 2013 calculated that the Y chromosome of an African American ("Albert Perry") had separated itself from all other Y chromosome lines 338,000 years ago and had similarities with the Y chromosomes of a group of eleven men African Cameroon .
All studies on the ancestry of a locus such as the Y chromosome implicitly assume that the locus is not under selection, i.e. that the various haplotypes are neutral variations. Selection reduces genetic diversity as the beneficial Y chromosomes spread throughout the population and displace less beneficial ones. Krausz et al. (2001) described a haplotype in Danish men that is associated with reduced sperm count . So far, however, there is no real evidence of selection on the Y chromosome.
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