"WHY" - CHROMOSOMAL ADAM?

The documentary “Journey of Man” is an ambiguous title, which is suggestive of the journey as outlined in the previously viewed documentary “The Real Eve,” as “Mankind,” but also suggests the journey of the male sex literally. The title in itself is a micro-summary of the entire theory of the journey of the human race through the tracking of the male Y- chromosome.

Staying with romanticisms, “Adam” fits in perfectly as the politically correct candidate to lobby the plight of future male offspring…but here is the interesting thing, if there is race to the finish between the genes, who is the most successful, is it man or woman? Can mitochondrial DNA outlast Y-chromosomal DNA in remaining relatively unchanged through generations in evolutionary time, or, is mitochondrial DNA dominant over Y-chromosomal DNA, which is “submissive” because of all the changes which it faces in the same length of time…maybe this explains why stubbornness is a common and unresolved trait amongst so many women today (your comments ladies?). Is it just written in their genes? And it looks as if us males have genes, which just cannot commit…

Lets look at some of the facts again.

Y-chromosomal Adam represents the most recent common ancestor from whom all male sex chromosome are descended [1], and lived approximately 60 000 to 90 000 years ago [1]. The Y-chromosomal DNA (deoxyribonucleic acid) is unique to males because females have two X-chromosomes. The sex chromosomes have two variants, namely male denoted as XY and female XX. The Y-chromosomal DNA, because it is passed from one generation to another, can indicate a purely male historical line of descent [1]. So apart from the sex difference, what else is different in Y-chromosomal DNA and mitochondrial DNA? Y-chromosomal DNA is found in the nucleus, unlike the mitochondrial DNA.

As I have mentioned previously, the chromosomes are macromolucules containing genetic information. This genetic information is supplied in a sequence of codes called alleles, which code for a specific gene [2] like hair colour, eye colour etc. When you put a series of theses alleles together seen on specific locations on the chromosome, this is known as a haplotype, or haploid genotype [3]. Large groups of haploid genotypes form haplogroups [3]. Differences in the Y-chromosomal DNA of people determines the haplogroups [3]. The variation patterns often seen in human DNA is generally accepted as resulting from both neutral processes [4], where mixing of genetic material creates new re-combinations of shared genetic material from mother and father, mutation, and the migration of a group of people to expand their genetic “diversity,” and natural selection [4]. Natural selection is survival of the fittest, and can be seen as “purifying selection,” where useless variants or mutations are removed [4], “balancing selection,” where the most successful variant remains constant [4], or “directional selection,” where trends form in the variants [4]. Directional selection is indicated as rare in human genetics [4], but a reflection of this is the ability of generations of people living in malaria areas having acquired immunity to the parasite for example [4]. Interestingly, looking at this example, Directional selection must therefore be reliant on not only balancing selection, but purifying selection as well since not all progeny toward the successful variant will be successful, and, only the strongest will survive. These directionally selected variants display a signature on the genome, indicated as an increase in the frequency of the selected allele (coding for this gene) and all its subsequent results from other links, which form a haplotype that is found more often than what can be expected normally [4]. These signatures are now also thought to be produced by the results of human activities, since we have to adapt to our new surroundings, particularly during migration, where food items change, diseases are encountered, and changes in climate etc. [4]. If the changes in the variations or mutations can be recognised and mapped, with time-frames given, tracing the migration routes through different geographical areas is possible. What is needed to be more accurate though is the time it takes for normal variations and these accelerated variations to take place, and, how do accelerated variations play a role in swaying time-result-assumptions of migratory processes in humans? A possible answer to this question is the study of population-specific Y-chromosome haplotypes that can act as markers of founder events which happen in the population [6]. This research was sparked by the fact that there is actually reduced variation in the Y-chromosome, less than in the X-chromosome [6].

While digging even further, I was interested to find an echo of what is suggested in “Journey of Man,” that the Khoisan people from Southern Africa genetically may well be the closest living relatives to Y-chromosomal Adam [5]. This is explained by the finding and recording of the types of variations or polymorphisms on the Y-chromosome. A primitive variation “A” is found in 15% of the Khoisan men sampled [5], while “A” is found in 5 – 10% of Sudanese men sampled [5]. All people found living outside Africa have the “T” variation on the Y-chromosome, as do most Africans [5]. It is thought that the “A – T” mutation occurred very early on in human evolution after humans split from Apes [5], with the majority of modern humans now having the “T” variation [5]. This supports the idea of an African origin, but with some differences to that of Mitochondrial Eve.

Also of interest is the Y-chromosomal tool being used to determine the ancestral origins of those African Americans whose ancestors were taken as slaves during the slave trade during the 18th century [7]. From this study, in support of the suggestion of a difference in the mutation variance in the Y-Chromosome of men in Africa, those with an African origin in America have been shown to have a significantly different lineage [7].

Of utmost importance, and something that comes through from most sources is the fact that through Y-chromosome tracking of evolutionary past, the routes of our actions as humans determines not only how we adapted to our new environment physically, but also how we adapted to each other as different populations with different cultures and language, and the spreading of the idea of farming and trade. Our ecological footprint began with us, and is written in our genes. We initially impacted ourselves by changing the environment around us, but look at us now. Presently our ecological footprint has “evolved” from an indication of human presence and ingenuity to ecological erosion!

Thought-provoking!

References

[1] Wikipedia contributors. Y-chromosomal Adam [Internet]. Wikipedia, The Free Encyclopedia; 2006 Jun 29, 09:22 UTC [cited 2006 Jul 28]. Available from: http://en.wikipedia.org/w/index.php?title=Y-chromosomal_Adam&oldid=61169096.

[2] Wikipedia contributors. Allele [Internet]. Wikipedia, The Free Encyclopedia; 2006 Jul 18, 23:25 UTC [cited 2006 Jul 28]. Available from: http://en.wikipedia.org/w/index.php?title=Allele&oldid=64560556.

[3] Wikipedia contributors. Haplogroup [Internet]. Wikipedia, The Free Encyclopedia; 2006 Jul 23, 12:46 UTC [cited 2006 Jul 28]. Available from: http://en.wikipedia.org/w/index.php?title=Haplogroup&oldid=65369165

[4] Zerjal T., Xue Y., Bertorello G., Spencer Wells R., Bao W., Zhu S., Qamar R., Ayub Q., Mohyuddin A., Fu S., Li P., Yuldasheva N., Ruzibakiev R., Xu J., Shu Q., Du R., Yang H., Hurles M. E., Robinson E., Gerelsaikhan T., Dashnyam B., Mehdi Q., and Tyler-Smith C. 2003. The Genetic Legacy of the Mongols. American Journal of Human Genetics 72 (Report. pages not given).

[5] Nelson D. Lecture Notes For Evolution (2) [Internet] 2001 Dec 19. 8:15 [cited 2006 Jul 28] 23:02. Available from: http://drnelson.utmem.edu/evolution2.html

[6] Poloni E. S., Semino O., Passarino G., Santachiara-Benerecetti A. S., Dupanloup I., Langaney A., and Excoffier L. 1997. Human Genetic Affinities for Y-Chromosome P49a,f/Taql Haplotypes Show Strong Correspondence with Linguistics. American Journal of Human Genetics 61: 1015 – 1035.

[7] Parra E. J., Marcini A., Akey J., Martinson J., Batzer M. A., Cooper R., Forrester T., Allison D. B., Deka R., Ferrell R. E., and Shriver M. D. 1998. Estimating African American Admixture Proportions by Use of Population-Specific Alleles. American Journal of Human Genetics 63: 1839 – 1851.

David Vaughan
Senior aquarist, Quarantine
Two Oceans Aquarium
Cape Town, South Africa
dvaughan@aquarium.co.za
davidvaughan.b-logging.com