Friday, December 17, 2010

Life Lines 86


Our cells, we have several trillion of them, each contain a nucleus with chromosomes and a surrounding blob of cytoplasm that contains organelles. Organelles do the work of the cell and each type is specialized. One of these is your mitochondria. They are unusual in many ways. They look like bacteria (prokaryotes) and some biologists believe they evolved from bacteria inside other bacteria some three billion years ago. They are usually taught to high school students as being “the powerhouse of the cell.” The phrase means mitochondria take the oxygen we breathe, burn up the food we eat, and give our cells the energy they need to tear apart or synthesize molecules.

Their origin from bacteria makes sense because mitochondria have their own DNA (genes) but the proteins that make up mitochondria come from both our nuclear genes and our mitochondria genes. We get our mitochondria only from our mothers. Our father’s sperm does not contribute any mitochondria to us. Geneticists call such a pattern “maternal inheritance.” The closest thing to it in our culture is Jewishness, which is passed from mother to child; which is why Jews consider the children of an interfaith marriage of a Jewish woman with a non-Jewish male Jewish but the children of a Jewish male with a non-Jewish female are not Jewish (unless they convert to Judaism).

Several years ago when the techniques for sequencing genes into their DNA nucleotides became possible, scientists sampled mitochondria from people in different continents. They compared the sequences and looked for mutant variations. About one such new variant shows up every 250,000 years. If an entire population has that variant it was brought in by a settler, a woman who is ancestral to all those alive today in that ethnic group. If the variant is present in only some of the people of an ethnic group, it arose much later. By comparing the variants around the world, the scientists believed they could identify the mother of us all, dubbed “mitochondrial Eve.” She came from Africa and she lived about 250,000 years ago. Other scientists are not as convinced that the story is this clear. All agree however, that mitochondrial Eve was not the first woman, just a woman who is our Ur-grandmother. All of her kin in Africa (perhaps in the hundreds of thousands or millions) did not end up contributing to our mitochondria.

What this implies is an African origin of our species (Homo sapiens) and the departure from there of our species some quarter of a million years ago to the Middle East, Europe, and Asia and onwards across the Pacific and over Alaska into the New World. Those of you reading this, unless you are of native American descent, are of more recent immigrant settlers to the New World.

No doubt molecular anthropologists will have better and larger samples to check out this hypothesis. Other supporting evidence makes it likely. When I visited Kenya a few years ago, after teaching my students on board the SS Universe about mitochondrial Eve, I mentally paid homage to that remarkable mother of five billion descendants.

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