Human Y chromosome markers have proven to be a rich source of information for un
ID: 69058 • Letter: H
Question
Human Y chromosome markers have proven to be a rich source of information for understanding the history of how men have moved around the world in the past 100,000 years or so. Most of the Y chromosome doesn't undergo genetic recombination, so the Y chromosome is similar to mitochondrial DNA in that inferences about relationships are not complicated by sexual reshuffling of markers. The following table contains a small subset of data from a study (Underhill et al. 2000 Nature Genetics 26: 358-361) where about 160 markers were assayed in Y chromosomes from a world-wide sample of 1062 men. The markers are identified by numbers that indicate the order in which they were discovered. All but the following two markers were single-nucleotide point mutations. Marker #1 involves the presence (+) or absence (-) of a 300bp mobile element (called the Alu element) inserted at a particular site on the Y chromosome, and Marker #17 involves an insertion/deletion polymorphism where a G nucleotide was either present or absent.
Based on analysis of more distant outgroups (other primate species), the first haplotype in the table (haplotype A from Mali) can be identified as the outgroup to all the remaining haplotypes, so its markers should be considered to represent the ancestral condition for these traits.
marker number:
haplotype(source) 31 60 109 42 40 168 145 1 64 17 173 174
A (Mali) C C C A G C G - A G present A T
B (Ethiopia) G T C T G C G - A G present A T
C (Sudan) G T T T G C G - A G present A T
D (Japan) G C C T G T A + G G present A C
E (Taiwan) G C C T G T A + A G present A C
F (Mali) G C C T A T A + A G present A T
G (Middle East) G C C T A T A + A G present A T
H (Siberia) G C C T G T G - G G deleted C T
I (Pakistan) G C C T G T G - A G deleted C T
J (Ethiopia) G C C T G T G - A G present C T
1. What is the most parsimonious tree for these data? For each trait, mark the branch on the tree where changes occurred in that trait.
2. The most parsimonious tree involves exactly one trait that showed convergent evolution. Which trait shows convergence, and on what branches of the tree did the convergent changes take place?
3. According to this tree, at what point did humans leave Africa? What is the earliest ancestral branch in the tree that is indicative of human colonization of non-African locations?
Explanation / Answer
In paleoanthropology, the recent African origin of modern humans, or the "out of Africa" theory (OOA), is the most widely accepted model of the geographic origin and early migration of anatomically modern humans. The theory is called the "out-of-Africa" theory in the popular press, and the "recent single-origin hypothesis" (RSOH), "replacement hypothesis", or "recent African origin model" (RAO) by experts in the field. The concept was speculative before it was corroborated in the 1980s by a study of present-day mitochondrial DNA, combined with evidence based on physical anthropology of archaic specimens.
Genetic studies and fossil evidence indicate that archaic humans evolved to anatomically modern humans solely in Africa between 200,000 and 60,000 years ago, that members of one branch of Homo sapiens left Africa at some point between 125,000 and 60,000 years ago, and that over time these humans replaced other populations of the genus Homo such as Neanderthals and Homo erectus. The date of the earliest successful "out of Africa" migration (earliest migrants with living descendants) has generally been placed at 60,000 years ago based on mitochondrial genetics, but this model has recently been contested by simulations of mitochrondrial DNA data, 125,000-year-old Arabian archaeological finds of tools in the region and the discovery of Homo sapiens teeth in China, dating to at least 80,000 years ago.
The recent single origin of modern humans in East Africa is the predominant position held within the scientific community. There are differing theories on whether there was a single exodus or several. An increasing number of researchers think that "long-neglected North Africa" may have been the original home of the first modern humans to migrate out of Africa.
The major competing hypothesis is the multiregional origin of modern humans, which envisions a wave of Homo sapiens migrating earlier from Africa and interbreeding with local Homo erectus populations in multiple regions of the globe. Most multiregionalists still view Africa as a major wellspring of human genetic diversity, but allow a much greater role for hybridization.
Genetic testing in the last decade has revealed that several now extinct archaic human species may have interbred with modern humans. These species have been claimed to have left their genetic imprint in different regions across the world: Neanderthals in all humans except Sub-Saharan Africans, Denisova hominin in Australasia (for example, Melanesians, Aboriginal Australians and some Negritos) and there could also have been interbreeding between Sub-Saharan Africans and an as-yet-unknown hominin (possibly remnants of the ancient species Homo heidelbergensis). However, the rate of interbreeding was found to be relatively low (1–10%) and other studies have suggested that the presence of Neanderthal or other archaic human genetic markers in modern humans can be attributed to shared ancestral traits originating from a common ancestor 500,000 to 800,000 years ago
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