Mitochondrial Eve

Mitochondria are subcellular organelles involved in energy generation that have their own greatly simplified genome, separate from the DNA in chromosomes found in the nucleus. In humans mitochondria contain an approximately 16k base pair circular DNA microchromosome and are inherited strictly maternally. Assuming more-or-less constant rates of mutation, and unidirectional population migration, it is hypothetically possible to infer matrilinear descendant trees by analyzing similarities and differences among present-day mitochondrial sequences from present-day human populations. This type of analysis is the construction of a PhylogeneticTree?. This analysis applied to present-day human mitochondrial sequences can be used to construct such a tree, which bases in SubSaharanAfrica?.

Doubts about the veracity of the tree exist however, since multiple trees of substantial difference can be constructed depending upon the parameters chosen for the construction of the tree, suggesting that the FossilRecord? may be a more accurate way of reconstructing human migratory patterns. 'MitochondrialEve' represents that female individual (or more likely a collection of individuals with near identical mitochondrial sequence) to whom all lines of present-day human mitochondrial descent can be ascribed.

I've heard it said that "MitochondrialEve" doesn't represent "The" common ancestor, she just represents "a" common ancestor. She's the KevinBacon of our family tree.

Also the title of a short story by GregEgan. In the story, a geneticist is caught up in a battle of gender politics as a result of his work on tracing mitochondrial lineages by an innovative method involving quantum entanglement. Well worth a read, as all of GregEgan's stuff.

It is believed that mitrochondria are the descendants of bacteria which became incorporated in primitive organisms a very long time ago, at about the same time as chloroplasts were acquired by ancestral organisms for photosynthesis. This process, as well as the hypothesis on the history of organisms, is referred to as EndoSymbiosis?.

This is not quite true... bacteria are current-day organisms, so bacteria technically have no 'descendants'. One possibility is that an autonomous or semi-autonomous organism (the "proto-mitochondrion") fused with a proto-bacterium giving rise to a symbiont organism which then became the ancestor to the eukaryotic kingdom. The fact that bacteria and humans share the same GeneticCode even today suggests that they are more closely related to each other than either of them are to mitochondria, which have a slightly different GeneticCode. -- AndyPierce

I beg to differ in three increasingly technical points:

: (1) Many current-day organisms (including your and my parents), have plenty of decendants. But seriously, there is no need for an ancestral group of organisms to die out or even differentiate after it gives rise to a derived group of organisms, as you imply. (I believe Andy was using the term as a class, not an instance. Our parents are instances of the class human, and have descendants. The class human, on the other hand, has as yet no descendants.)
: (2) We should be more careful when using the term "Bacteria", a grouping term no longer used in modern biology. Among the Archaea, the group of single-celled organisms most closely related to the hypothesized proto-mito and proto-chloro, there is in fact still some variation in the GeneticCode.
: (3) The argument that the slightly different GeneticCode in human mitochondria implies remote relationship between mito and nuclear genes is confounded by the fact that mitochondrial genetic codes of closely related groups are different from one another. See for instance http://prowl.rockefeller.edu/aainfo/gencode.html. Since we are relatively sure that all the mitos in these disparate groups are relatives and not from separate EndoSymbiosis? events, it implies something WildAndCrazy? is going on that allows the evolution of the GeneticCode in mitos.

But by now this should head over the BioWiki?, wherever that is. -- BillTozier

Bill, you're mistaken on point 2. Mitochondria are generally considered to have originated from proteobacteria, and chloroplasts definitely come from cyanobacteria, which are all eubacterial. Since bacteria is as often used as a short-hand for Eubacteria, specifically excluding the Archaea, instead of for prokaryote, which includes both, there's no problem. And I don't see what's wrong with saying mitochondria originated from ancient bacteria, symbiotic within an ancient eukaryote (no, not all eukaryotes have mitochondria).

There is a close link between mitochondria and Typhus (Rickettsia prowazekii). Interestingly, the method of infection of typhus is that the bacterium invades the cell, bypassing the cell membrane, and replicating inside the cell (similar to a virus, without the DNA/RNA spoofing), which can give you a mental picture of how such a symbiosis could have occurred in the ancient past.

See http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v396/n6707/full/396109a0_r.html (I'm sure there are many other articles to be found, this is just from a quick Yahoo search "typhus+mitochondria").

Another very interesting thing which I read (I believe in a recent ScientificAmerican): It seems that the age of the MitochondrialEve is much less than the age of the first humans and their ancestors (which had already spread around much of the world). This implies that all current modern humans must come from a small isolated region in the not-so-distant past (I seem to recall between 100 000 and 40 000 years ago). It also implies that these humans took over the world from their hominid and human cousins. There were other groups of humans existing, and modern humans wiped them out, they didn't interbreed (if they had, the mitochondrial eve would be much much older; millions of years). This article hypothesized that there was a group of isolated humans (about 2000 or so) who, through genetic drift over hundreds of years of isolation, had a single young maternal ancestor. This small group then started spreading until eventually, here we are today.

In StarWars Episode I, "midichlorians" are symbiotic organisms which inhabit the cells of all living things and which give rise to TheForce. Mitochondria seem to be the obvious inspiration for midichlorians.