text: What an awesome question. They do that by looking at families and ancient DNA.\n\nLet's say you have a type of gene that is passed down ONLY by your maternal family line, they can take blood/lymph samples from you, your mother, grandmother , great grandmother, aunts, great aunts whatever.\n\nIn reality this exists in your Mitochondrial DNA/RNA.\n\nIf you have a long family line like lets' say your ancestor was Charlemagne's sister or some famous person from whom they had tissue - hair / blood whatever. Usually they have to find relatively uncontaminated samples so tissue from roots of teeth or inside of large undamaged bone or if the tissues are in some other ways preserved. They enhance this DNA by making many copies of the tissue and then use that to do the comparison.\n\nThey can compare the genes from that old ancestor and see what kind of changes occur between the generations and your distant relative whom you know you are related to.\n\nThen you see what has changed between those older samples and the newer samples. That lets people know what the overall error or difference is. \n\nBy checking with everyone else or using multiple samples from multiple family tree's they compare and get a common set of changes between everyone. \n\nNow you can figure out the rate of change or put another way, how many years or how many generations is it before you can expect to see 1 error/change or 10 or 50 or 100. \n\nFor Mitochondrial RNA/DNA that is very well figured out and there is a great book about this subject called "The 7 Daughters of Eve".\n\nComparing the older DNA of a few individuals to people today has had some AMAZING results.\n\nFrom those results they have discovered some amazing facts,  For instance,\n\nHumanity was almost wiped out as a species 2 times in the last 200,000 years (most recently about 70,000 years ago) , scientists determined there were no more than 5,000 people left after some event caused the population to dwindle. \n\nIn a way the story of Adam and Eve were right,(if I remember correctly) although Eve lived 80,000 years ago in Africa somewhere and Adam lived 40,000 years ago in the European Steppe.\n\nAll women decend from no more than 7 individuals who lived at various times in the last 40,000 years.\n\nAfricans are the oldest branch of the human genetic tree and show almost 10 times more genetic diversity than their indoeuropean and asian cousins.