Mitochondria, the oxygen-processing factories of eukaryotic cells, have their own DNA, a relic from the distant past when they were free-living organisms.
- Evidence of this is the slightly different genetic codes found in nonplant mitochondria. For example:
||Amino Acid (standard)
|All nonplant species
(For those who may be interested, here is the entire sequence of a human mitochondrial DNA.)
Another significant difference in mitochondrial DNA is that it is circular, like the DNA of prokaryotes.
Mitochondrial DNA is inherited ONLY in the maternal line; all of the mitochondrial DNA in any living human came from that individual's mother.
- Thus, it is not altered by sexual reproduction; that is, it does not undergo recombination.
- Changes come only from mutations during cell division.
- Mutations that occur in the control region tend not to be repaired, since that region does not code for any specific product
- Only mutations that occur in female germline cells (those that become eggs) are passed on to offspring
Bryan Sykes, at the Institute of Molecular Medicine in Oxford, has summarized the work of his group and several others:
- A method was developed by Sykes and others for extracting DNA from ancient bones, and amplifying it using PCR (polymerase chain reaction)
- By comparing numbers of mutations in the control region with time scales known from paleontology a clock is established: one mutation roughly every 10,000 years.
- Applying network analysis to the data from hundreds of individuals, Sykes found seven genetic clusters among people from all over Europe, the Middle East, and Asia.
- The seven clusters had ages ranging from 10,000 to 45,000 years
- This is the time for all the mutations observed to arise from a single founder sequence
- That is, one woman is the ancestor of each of these clusters
- We all are descended from one of those seven women
Sykes gave these women names, and in a book called "Seven Daughters of Eve", explained the methodology and presented imaginary lives for the women.
Notice in the graphic that the lines converge in the upper center. Now apply this logic:
Sykes calls this woman Mitochondrial Eve, and estimates that she lived about 130,000 years ago, in East Africa.
- Imagine that we know exactly how a large group of people are related
- Work backward in time:
- The lines in brothers and sisters converge in their mother
- After two generations they converge in their mother's mother
- Another generation converges in a maternal great-grandmother
- When we go back far enough, only two women would have descendants alive at our starting point
- Finally, the lines would converge in a single woman
Another interesting case: Otzi, the Tyrolean iceman.
His mummified body was found frozen in a glacier in the Italian Alps.
- Carbon dating gave an age of about 5300 ybp
- He had been killed: an arrow or spear in his back, a wound in his hand, finished off with a club to the face.
- Rollo and coworkers [Curr. Biol., 2008, 18, 1687] sequenced his complete mitochondrial genome and placed him in the web of modern European populations:
Similar analyses can be applied to the Y chromosome:
- It is passed only from male to male
- Except for two small areas, it does not undergo recombination
- Shen and coworkers [Proc. Nat. Acad. Sci., 2000, 97, 7630] looked at variation in three genes in the Y chromosomes of 50-70 individuals from all continents
- Analysis resulted in a tree of descent:
- Y-Chromosomal Adam lived in East Africa about 50,000 years ago
- More recent estimates [Science, 2013, 341, 562, 565] move Adam back to 148-190,000 ybp
Cavalli-Sforza and collaborators [Science, 2008, 319, 1100] have taken the tracing of our ancestry in a more general direction.
The data are consistent with a single origin for humanity in sub-Saharan Africa, with homo sapiens spreading around the world during the past approximately 100,000 years.
The message from our DNA is clear: we are all brothers and sisters.
[A good review of both the genetic and paleontological data on the human migration: Lalueza-Fox and Gilbert, Curr. Biol., 2011, 21, R1002.]
This page last modified 1:19 PM on Tuesday January 28th, 2014.
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