Each child is born with a complete set of genes, the genome. Half of anyone’s genes come from the father, and half from the mother. (That is, ignoring mitochondrial DNA for the moment.) These genes were shuffled before they were passed on, and half of them discarded, and they’re often not expressed the way they were in the parent, but they’re there. Each parent, of course, had half of his or her genes from each grandparent, and so on, back into time. Therefore each of us carries samples of the genes of our ancestors. Anthropological geneticists use our modern DNA, our gene sequence, carried in every living person to trace the migrations of humanity.
But the samples are scrambled in the genetic processes that create a baby. How do scientists find a group of genes that has not been mixed and recombined? Such a sample would be a great help in researching the people who came before us.
One place to find an unmixed sample is on the Y chromosome. Humans have twenty-three chromosome pairs in each body cell, the structures that carry their genes. Twenty-two of those are pairs similar in shape and size each to the other, and in reproduction they recombine. The twenty-third pair, the sex chromosomes, is different. In a female, the two chromosomes are both X, one from each parent. In the male, there is one X, from the mother, and one Y, from the father. The Y chromosome can only come from the father; genetically speaking, it is what makes him male. Most of the Y, the non-recombining region, is never mixed with the genes of the female parent. So it carries a sample of unmixed male DNA from far back in time. The only changes to the DNA in the non-recombining region of the Y are through random mutation.
The mitochondrial genome, the mtDNA, plays a similar role in tracing a child’s genetic heritage through the female line. The majority of human DNA is carried in the nucleus of each cell. That is the DNA which anthropologists trace back through the male line. But there is another store of DNA in the human body. It is found in the mitochondria, small organs, organelles, that produce energy within every human cell except the mature sperm. Because mitochondria are not present in the sperm cell, the mitochondrial DNA shows only the maternal line. So the changes in mtDNA are also exclusively from mutation.
So scientists can isolate prehistoric genes passed down through time, changed only by mutation. How does that help trace the migrations of humanity? Genetic anthropologists track the mutations.
When people reproduce, their DNA is copied. Because these are long complicated molecules, there are sometimes mistakes. These “copying errors” are mutations. Some mutations are harmful or fatal. Some, a very few, may carry an advantage for the children born with them, so these will tend to increase in a population. Most have essentially no effect. When people’s DNA is analyzed, these mutations are tracked.
They serve as genetic markers for the populations in which they are found. Once they have been identified they can be traced back in time to the common ancestor of the group sharing that marker. When a mutation arises, it quickly spreads to most of the people in the area where it arose. Then when that group leaves a particular region it carries its genetic signatures with it. So by studying populations, and the various markers found within them, scientists can tell where a particular person came from.
Scientists can estimate the date at which a group left an area by assuming that mutations in a specific piece of DNA occur at a constant rate. (Since the mechanism responsible for the copy error hasn’t changed it makes sense that the frequency of error wouldn’t change.) They use this rate as a molecular clock to time migrations. However, many factors can perturb the molecular clock: tiny populations, extra large populations, hard times, useful mutations, and other factors. Therefore, the clock is usually calibrated by comparing it to other evidence, such as the fossil record. Comparing DNA of groups from many places can reveal the order and number of mutations since their common ancestor. So knowing the rate of change (more or less), and counting the amount of change gives an estimate of the time involved.
Genetic anthropologists study our DNA to show us where we came from, what routes our journeys have led us on, and when we came to be where we are. Their tools are inside of each of us, in the DNA that makes us who we are. Their discipline proves that in spite of our differences we are all close kin.
http://www.ornl.gov/sci/techresources/Human_Genome/elsi/humanmigration.shtml
https://www3.nationalgeographic.com/genographic/
http://www.sciencedaily.com/releases/2003/05/030528081109.htm
