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Tuesday, Nov 24, 2009
DNA reveals a lot about human evolution, and some family secrets, too.
Apr 29, 2005 | Just for a moment after unrolling the piece of paper, I was struck speechless. Among the multicolored lines, the computer-generated graphs and the maps was an innocuous string of letters, beginning GCTTCTCGCG.
G, A, T and C, the four letters of the genetic code -- representing the four bases that make up every strand of DNA -- have become famous in the past decade. As scientists unlock the DNA codes of everything from bacteria to humans, the scale of the sequences -- humans have 3 billion pairs of these bases in their genome -- can make them seem abstract. But, for me, seeing this sequence brought genetics into focus.
The letters I was looking at came from my own DNA, sequenced by Mark Jobling, a geneticist at the University of Leicester in the U.K. for the 20th anniversary of the invention of DNA fingerprinting by the university's Alec Jeffreys. More specifically, the DNA was from my mitochondria: the powerhouses in the cells that convert glucose into usable energy so that I can function. The mitochondria were bequeathed to me by my mother, who got hers from her mother, and so on.
There was more: a profile of my Y chromosome, the bit of DNA that set me off down the path to becoming a man. This chromosome is identical to the one my father has. And, again, identical to his father's, his grandfather's, etc.
These clear lines of descent are a powerful tool for tracking one's ancestry. For scientists, they offer the ability to piece together how the human race has evolved and spread. Mitochondrial DNA and the Y chromosome are particularly useful for this task because, as they pass through the generations, they hardly change at all.
All our DNA is curled up inside 23 pairs of chromosomes -- 22 are the autosomes, which produce all the characteristics such as hair color and predisposition to disease, and the 23rd pair are the sex chromosomes -- two X chromosomes for a woman, one X and one Y for a man. One of each pair comes from your mother, the other from your father.
But the chromosomes your parents donate are not facsimiles. Instead, the chromosomes in the sperm and egg that met to create you are a reshuffled version of your parent's pair -- the process is called recombination. "Because of recombination, all of the autosomes you carry come from a great deal of ancestors," says Jobling. "If you want to use bits of DNA on these chromosomes to trace history, it's quite difficult to do because they are reshuffled every generation."
All chromosomes go through recombination except the Y, which is passed from father to son without any change. "It's a simple line back through fathers in time," says Jobling. The same goes for mitochondria. The DNA of these organelles takes the form of a circle and is passed directly from mothers to their children without any recombination.
Follow these lineages back far enough and you get to what scientists call the Adam and Eve of genetics. "These are individuals who really lived in the past. There was really some woman at some point in the past who was the common ancestor of all modern mitochondrial DNA, and there really was one man who was the common ancestor of all Y chromosomes," says Jobling. "They didn't necessarily live at the same time -- the evidence suggests they didn't -- but they were both probably within Africa."
Y-chromosomal Adam probably lived 65,000 years ago, and mitochondrial Eve some 150,000 years ago. The people living around the same time as these two individuals would have passed on their genes like Adam and Eve, but their Y chromosome and mitochondrial DNA lines would have eventually died out. "You only need a man who doesn't have any sons or a woman who doesn't have any daughters for one of those lines to die," says Jobling. (I have no sisters, so I am the end of the line for my mother's mitochondrial DNA.)
Mitochondrial DNA might be passed directly from mother to child but, occasionally, it will mutate. Over time, this explains why everyone in the world doesn't have the same DNA and, therefore, can give clues to maternal lineage. It's done by measuring how individual bases differ between different people's mitochondrial DNA -- differences called single nucleotide polymorphisms (SNPs). "There are thousands, indeed millions, of these in our genome," says Jobling. "But the chance of any changes in one generation is about one in 1 billion -- so when you see that two individuals share a particular SNP, that implies shared ancestry."
For the Y chromosome, scientists can study parts of the DNA where the sequence stutters, called short tandem repeats (STRs). This means that a particular base sequence -- say, GACA -- is repeated over and over again at a point on the chromosome. In different people, the number of repeats differs. The chances of these changing in any one generation is around one in 1,000. By recording the occurrences of patterns of STRs on the Y chromosomes at different places around the world and logging where different types of mitochondrial DNA exist, scientists have been able to picture how our species has changed and moved across the Earth.
"On a global continental scale, telling whether someone comes from Africa or the Americas is relatively straightforward," says Jobling. "As you come closer to home, in Europe, it becomes less straightforward."
