Evolution 101

Saturday, June 10, 2006

Who Was The Mitochondrial Eve?

You may have heard, at some point in time, about the “Mitochondrial Eve.” This title refers to the most recent common female ancestor of all humans currently in existence. That is, a woman who lived in the past of whom all modern humans are descendents. Some of you may be wondering- did such a woman really exist? And the answer is yes, but- she wasn’t who most people think of when they think of someone named, “Eve.” The mitochondrial Eve was someone quite different.

Before searching her out, let’s first go over the word, “mitochondrial.” A “mitochondrion” is an important part of every animal and plant cell in existence. You can think of a cell like a large machine with a number of different systems within it that contribute in an interconnected way to the overall function of the cell, much in the same way that a car is a large machine with a number of different systems that all work basically together to make the car go. You can also think of your body- it’s composed of a number of subsystems, which contribute different functions to the overall performance of the body. Each system is composed of a separate structure, called an organ, which houses that particular function. In the same way, the separate subsystems of a cell are also made of individual structures, and biologist call them organelles, because they’re like organs, except they’re, to use a technical phrase, very very tiny.

One of these organelles is the mitochondrion. The mitochondrion is a small, oval-shaped structure that provides the cell with the energy that it needs to carry out all its other functions. You might remember from way back in high school biology class, the basic formula for the generation of energy. Sugar, or glucose, plus oxygen becomes carbon dioxide, water, and energy. Seems basic enough, sure, but there’s a lot of details that are sort of glossed over in the word, “becomes.” I’m not going to do much better either- I don’t really have time for an overview of the Krebs cycle, but I do want to point out that the “becomes” part takes place (mostly) in the mitochondrion. The mitochondrion contains a lot of really cool proteins that play a kind of chemical shell game with the electrons that are found in the carbon bonds of sugar, with the eventual result being the formation of a molecule called “adenosine triphosphate,” or simply, “ATP.” This molecule is the basic energy currency for all the important chemical processes that take place in the cell- if something exciting is happening, it’s using ATP.

So the mitochondrion is an absolutely essential part of the cell- it seems nearly impossible that plants or animals could have evolved without them. And the story of how we got them is an interesting little aside, but it’s also relevant to the subject of Mitochondrial Eve, I promise. Mitochondria don’t really seem as if they belong in cells- they chug along, virtually self-sufficient, really only relying on the rest of the cell to supply it with sugar and oxygen, and remove the waste products. In fact, it almost seems like a kind of parasite. But not really a parasite- parasites don’t give anything back to their host. Mitochondria are more like a symbiote, that is, they’re in a symbiotic relationship with the cell around them in which they provide things that the cell needs (ATP), and the cell provides things that the mitochondrion needs (sugar and oxygen). But if the mitochondria in your cells are part of a symbiotic relationship, when did that relationship start? Very likely, a long, long time ago. Probably before there were even multicellular organisms. Mitochondria actually resemble bacteria in a number of ways, and it’s likely that at some point in our evolutionary history, a species of bacteria that was very good at converting organic carbon to a simpler kind of energy currency was engulfed by a larger cell, that perhaps was pretty good at collecting organic carbon, but not so good at breaking it down. Since this pairing was of selective benefit to both species, they continued to associate, and since the selective pressure for the bacteria to function outside the larger cell was reduced, it lost that ability, and became stuck. Not that it cared, of course- all it cared to do was convert energy and replicate.

One strong piece of evidence for this scenario is mitochondrial DNA. That’s right- mitochondria have their own DNA, completely separate from what we typically consider as the center of DNA in the cell- the nucleus. Mitochondrial DNA contains most of the genes that mitochondria need to make their proteins and replicate (some have migrated to the nuclear genome, actually), and it replicates itself completely separately from the rest of the genome. Mitochondrial DNA even uses a slightly different genetic code from nuclear DNA- you’ll remember from the molecular biology primer that DNA is read in three-nucleotide segments called, “codons,” each of which correspond to an amino acid for the synthesis of proteins. In mitochondria, a DNA codon that would correspond to some particular amino acid in the nuclear genome would instead corresponds to a completely different amino acid.

So what do mitochondria, fascinating as they are, have to do with our mothers, and our mothers’ mothers, all the way back to the most recent common mother? Well, that has to do with two significant aspects of biology. One, as I’ve already mentioned, is that mitochondrial DNA replicates only in mitochondria, and doesn’t interact with the rest of the genome. This means that not only are mutations occur completely separately, but they also won’t be covered over by recombination with genomic DNA. The other thing is that sexual reproductions involves two gametes, or sex cells. One, the female, is very very large. The other, the male cell, is very very small. So small, in fact, that it really doesn’t contribute anything to the next generation other than its DNA. So what does that mean? That’s right- mitochondria are only present and passed down in the female gamete, which means that every mitochondria in your body right now is shared with your mother only, and not your father.

Now, this presents a very interesting opportunity. Your genomic DNA is composed equally of DNA from your mother and your father, so it’s not always that easy to figure out genetic ancestry, especially if you go back many generations. However, if you use mitochondrial DNA, you’re guaranteed a source of DNA that is only passed down from mother to child, and that is not complicated by recombination with genomic DNA. Now comes the fun part. The idea of an “Eve,” or a most recent common female ancestor, isn’t really that hard to grasp, nor is it dependent on genetics. It’s just common sense. Think about it- family trees tend to get wider and wider as they progress down the generations, so it stands to reason that they will also get narrower and narrower as you go back in time. If you go back enough generations, eventually every single human living today will have the same female name in their family tree. That’s just logical deduction. The same is true of all organisms, not just humans- there’s a bear “Eve,” a sparrow “Eve,” and an aardvark “Eve.”

The trick, then, is to take a look at the mitochondrial DNA from a wide sample of humans, and upon calculating the mutation rate, work backwards until you figure out when the most recent female ancestor would have lived. And to spare you the trouble of going over the calculations, I’ll just give you what’s been discovered- the mitochondrial Eve lived about 150,000 years ago. That’s quite a bit older than most people associate with the name “Eve,” but that isn’t the only difference. The mitochondrial Eve wasn’t the only human woman alive at the time- if she had been, then it’s likely humans would have gone extinct soon after. In actuality, the mitochondrial Eve was one of many women alive at the time, and the only thing that makes her distinctive is the fact that there is an unbroken chain of female descendents going from her to each and every one of you listening to this podcast today. Other women living at the same time may have had only sons, which means that their mitochondria wouldn’t have been passed on, even though their genomic DNA would have. Still other women would have had daughters, but their daughters might have had only sons, with the same result to the flow of mitochondrial DNA. For many years, in fact, the honor of mitochondrial Eve would have switched from one woman to the other, as different lineages either died out or produced only males.

Speaking of males, we men have something to offer to the study of gender-specific heredity- our Y chromosomes. Since Y chromosomes are only inherited from father to son, and since the Y chromosome doesn’t travel in pairs, it’s also an excellent source of information for paternal inheritance in human history- a marker of the Y-chromosome Adam. Interestingly, the data so far seem to indicate that the male who would have been the Y-chromosome Adam would have lived many years after the mitochondrial Eve, so there was virtually no chance they would have even lived in the same time, let alone known each other. And with that, the last of the comparisons to the mythical Eves and Adams dies.

So, to review, the concept of the “mitochondrial Eve” refers to the woman in human history whose mitochondria have been inherited by all humans living today. This is due to the fact that mitochondria remain somewhat separate from the rest of the cell, and carry their own DNA separately from the nuclear genome. Comparison of mutations in mitochondrial DNA from modern humans indicates that the mitochondrial Eve lived about 150,000 years ago, although her male genetic counterpart, the Y-chromosome Adam, lived much later.

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