Conservation Biology

Thursday, July 27, 2006

MITOCHONDRIAL EVE...SO WHERE'S THE APPLE?


The documentary “The Real Eve” left me with quite a warm feeling deep within me, like a sense of belonging to something away from the chaos of everyday life, of a time when man was as vulnerable as the organism he exploits ironically today. In addition, and as mentioned in the documentary, there was for me a sense of spiritual attachment to the concept of “Eve” and the beginning of our modern lineage from a woman whose progeny were the most successful. There is just something about the topics which we are given as material to review and study at the University which inevitably has me constantly re-evaluating who I am and how I got here…not just in the sense of today, but rather, what did it take in the entirety of so many generations which came together in chance, to make me who I am?

The romantic suggestion of a great and wonderful journey of human success for me is extremely humbling, especially amidst the depression and all-too-familiar barbarism of today’s society of war, murder, deceit and politics. Like Darwin, a man who challenged what we though we knew, there are a handful of specialists, molecular anthropologists who will re-write the paradigm that is the mould from which we are all cast.

According to Max Ingman, a PHd candidate studying medical genetics at the Uppsala University of Sweden, molecular anthropologists have been comparing the DNA of modern living humans to determine evolutionary trees for the last 15 years or so [1]. The most commonly used DNA for this construction of evolutionary trees has been the use of Mitochondrial DNA, which resides as external to the nucleus of a cell. Max Ingman also indicates that mitochondria have their own genome of about 16500bp. So what does this mean? Well, a genome is all the hereditary information of an organism encoded within the DNA, including the more familiar term “genes” and non-coding sequences, which at this stage, science does not really understand fully. Non-coding DNA is thought to be a kind of evolutionary artefact that is no longer required, or has been superseded [2]. To be more correct, a genome is a full set of chromosomes, which are macromolecules of genetic information [3].

Mitochondria are the energy producers of the cells, and depending on the activity of the cell, there may be quite a number to only a few mitochondria present in the cytoplasm. Each mitochondrion contains 13 coding genes and 22 tRNAs and 2 rRNAs. tRNA’s or “Transfer” RNA is a relatively small chain of RNA, or ribonucleic acid made up of between 74 to 93 nucleotides (the small bits stuck together in sequence to make the whole code functional), which transfers a specific amino acid to a polypeptide chain under construction during the decoding of messenger RNA to produce the chain specifically to the specifications of the code sequence [4]. This sounds a lot like a badly run governmental office, but lets look at it a bit differently to explain. Basically if you had to buy a new car, which was delivered to you in all its parts (the messenger RNA), the manual to put all of the pieces together in sequence to make everything work, would be the requirements of the code, and the transfer RNA would be the mechanic to do the job. rRNA or “ribosomal” RNA is the central component of the ribosome [5]. The ribosomes are the small blobs (please excuse my obvious crude reproduction of the exact science of genetics as I understand it) which are often found “attached” to the mitochondria, termed rough mitochondria, which function to manufacture the proteins of all living cells [5]. It is the ribosomes, which provide the mechanism to decode the messenger RNA into the amino acids which are taken up by the tRNA for the construction of the polypeptide chain (the car)[5].

So what does this all mean then?

The important stuff is that mitochondrial DNA can only be inherited from the mother, which actually facilitates the tracing of genetic ancestry very accurately, and, the higher rate of mutation or substitution of a neucleotide with another, moreso than in the case of nuclear DNA (the stuff you learned about in grade 11 biology), makes it easier to resolve differences between closely related individuals [1]. Mitochondrial DNA is also available in high numbers in a cell, especially very active cells, and is therefore also easy to obtain, making fewer samples necessary for the requirements of the scientists [1]. Additionally, the mitochondrial DNA does not recombine like nuclear DNA, which swops around from mother and father to create a new genetic combination of the two. In this way information remains relatively constant through the lineage passed down from mother to daughter and so on [1].

The D-loop

DNA evidence from a variety of studies suggest that there was a recent African origin of man, which has also seen its share of criticism as a result of a lack of supportive statistical analysis [1]. Max Ingman suggests that the reason for such criticism is due to the focus of past studies on a section of the mitochondrial DNA called the D-loop which includes about 7% of the entire mitochondrial genome [1]. The D-loop also has a high rate of mutation, which is debated to actually hinder the acquisition of the true genetic-historical information [1]. Specific problem areas with using data from the D-loop section include areas where there is evidence of a higher rate of mutation than other areas. This could skew the results, but Max Ingman also suggests a solution to this. Data obtained from the total genome length can still provide more informative sites [1], and the rate of evolution of the rest of the genome (excluding the D-loop) was found to be relatively even [1]. So, does this mean that more accurate information can be obtained from mitochondrial DNA with the omission of the D-loop data? If I understand it correctly, then yes, and this is what modern scientists are now working towards.

Interestingly, the study done by Max Ingman suggests that the most important date, the time when all mitochondrial information focuses on one single origin, the “Mitochondrial Eve,” falls at a time 171 500 years ago [1]. This time fits well with the proposal of the Recent African Origin hypothesis, and rejects the Multi-Regional hypothesis because a much older date for this would have to be accepted, which would represent Homo erectus rather than Homo sapiens sapiens [1].

So who is “Eve” anyway?

Rebecca Cann and her collaborators supported the Out Of Africa hypothesis with their original paper on Mitochondial Eve in 1987 suggesting that the mitochondrial diversity of today’s peoples, can be traced back to a single woman who lived in Africa about 200 000 years ago [6, 7]. Interestingly, Cann et al. (1987) indicates that only 147 people were sampled for mitochondrial DNA for this study, and, with exception to those sampled in Africa, the four remaining representative geographical areas (Asia, Europe/America, Australia (native) and New Guinae) from which material was sampled from the placentas of pregnant woman, were colonised repeatedly and from different origins [7]. This suggests that humans must have moved out of Africa to colonise the rest of the world [7, 9], but my question is: Can 147 samples indicate an accurate genetic variety reflective of the world’s population?

From the results, Cann et al. (1987) indicates that seven African “daughters” (lineages) can be derived from the original “Eve”[7].

Interestingly, and possibly a weak point exploited by the critics of this original paper, is the indication that only 2 of the 20 “Africans” from which the placenta was sampled, were actually born in Africa. The rest were born in American hospitals, and already, up-to-date information suggests that Southern Africa has some of the oldest lineages of people in the world. Argumentatively, if it is possible through the generation for mutation, even in the mitochondrial DNA as indicated by Max Ingman, African-Americans could indicate a higher mutation record as opposed to true African lineages from Africa…so is Cann’s paper valid?

Mark Stoneking proceeds with the investigation into mitochondrial DNA and human evolution in a review of the work by Cann et al. (1987), published in the Journal of Bioenergetics and Biomembranes in 1994, where he includes new information which supports the initial theory but also includes a new method of analysing mitochondrial DNA and suggests that the human population expanded fairly rapidly about 40 000 years ago [8].

Although debate still reigns regarding the evidence suggested by opposing scientific communities debating the Multiregional and Out Of Africa hypotheses, one constant remains, of relative certainty, that human-kind was destined to become the master of his and her environment through the change in thinking and implementation of will and knowledge through language and interpretation. Human diversity is a tribute to the success of the human being in mastering change – something that should inspire all of us in these troublesome times.
David Vaughan
Senior aquarist, Quarantine
Two Oceans Aquarium
Cape Town, South Africa
davidvaughan.b-loggin.com


References:

[1] Max Ingman http://www.actionbioscience.org/evolution/ingman.html Accessed: 18:31 27 July 2006.

[2] Wikipedia contributors. Junk DNA [Internet]. Wikipedia, The Free Encyclopedia; 2006 Jun 28, 21:53 UTC [cited 2006 Jul 27]. Available from http://en.wikipedia.org/w/index.php?title=Junk_DNA&oldid=61091410

[3] Wikipedia contributors. Chromosome [Internet]. Wikipedia, The Free Encyclopedia; 2006 Jun 23, 16:37 UTC [cited 2006 Jul 27]. Available from http://en.wikipedia.org/w/index.php?title=Chromosome&oldid=65394396

[4] Wikipedia contributors. Translation [Internet]. Wikipedia, The Free Encyclopedia; 2006 Jun 25, 02:07 UTC [cited 2006 Jul 27]. Available from http://en.wikipedia.org/w/index.php?title=Translation&oldid=65672553

[5] Wikipedia contributors. Ribosomal RNA [Internet]. Wikipedia, The Free Encyclopedia; 2006 Jul 21, 15:20 UTC [cited 2006 Jul 27]. Available from http://en.wikipedia.org/w/index.php?title=Ribosomal_RNA&oldid=65041687

[6] Howard Hughes Medical Institute, Question archives: http://www.hhmi.org/ Accessed: 20:57 27 July 2006.

[7] Cann R., Stoneking M., & Wilson A. C. 1987. Mitochondrial DNA and human evolution. Nature 325: 31 – 36.

[8] Stoneking M. 1994. Mitochondrial DNA and human evolution. Journal of Bioenergetics and Biomembranes 26(3): 251 – 259.

[9] Watson E., Forster P., Richards M., & Bandelt H. J. 1997. Mitochondrial footprints of Human Expansions in Africa. American Journal of Human Genetics 61(3): 691 - 704.

7 Comments:

  • Regarding the mutation rate in Africans vs. African-Americans, have any studies been done to test for differences?

    A technical point: when you quote Cann et al. put the date in brackets (1987).

    By Blogger Gwen, at July 28, 2006 8:56 AM  

  • Thanks Gwen, I was suggesting that isolated pockets of African tribes for example, because of thier isolation from mixing with other groups genetically, probably have a lower overall mutation history from being maintained in small populations as opposed to more cosmopolitan societies who have had more chance of genetic exchange with their own mutation histories combined. What is your take on this subject?

    Thanks for the tip regarding brackets. I enjoyed your portrait on the Conservation biology chapters...

    Cheers, see you at the Uni.

    David

    By Blogger davidvaughan, at July 28, 2006 11:32 AM  

  • Hi Gwen

    Changes were made as suggested (brackets).

    Cheers

    David

    By Blogger davidvaughan, at July 28, 2006 11:38 AM  

  • Mitochondrial DNA is not exposed to recombination (exchange) but the mitochondrial lineage to which an individual of mixed race would belong would be determined by female ancestry and may differ from the racial category to which the individual is assigned.

    By Blogger Gwen, at July 28, 2006 1:32 PM  

  • Hi

    On the referencing for presenting on the Internet - I like the number in Square brackets, even cooler is to hyperlink the number if it is available on-line.

    Aside from the detail - I think you presented this very well, I have some material to put up on genetic markers, this area is new to me as well - but is very important.

    By Blogger Rich Knight, at July 28, 2006 3:40 PM  

  • Hey David

    I hate to just echo Gwen and Richard, but I have to agree, nicely done, especially as regards the relevant molecular aspects of it all.

    One quibble, though...just what, exactly, is an mtDNA Eve? And how is this woman any different from the rest of her contemporaries?

    Oh, and while you have peer-reviews references, you don't have any that are very recent. In a field as controversial and fast-changing as human origins, it's best to be as up-to-date as possible.

    Cheers
    Nick

    By Blogger NcK, at July 28, 2006 4:01 PM  

  • Thanks everyone!

    Nick, I will see if I can dig up some of the newer juicy stuff and call in a few favours from overseas. Lets see what happens.

    I do agree with your question of what makes mitochondrial Eve so special, or different to her contemporaries...I think she was just the lucky one really, who prospered. It is a lovely thought though isn't it? To think that she had no idea that the entire human race's future relied on her and the decisions she must have had to make.

    Gwen, your comment is supportive of what we discussed briefly this afternoon at the University. I will see if I can get some of the authors of these papers to answer the question, and post it up. If you think of anything else, please let me know.

    Cheers

    David

    By Blogger davidvaughan, at July 28, 2006 4:55 PM  

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