Deoxyribonucleic acid (DNA) is the blueprint for construction all of the cells of living beings. During conception most of the DNA from male and female mix and mingle to result in a truly unique offspring. However, one contribution of DNA from the male and the female change only very occasionally in a know manner which leads to these changes becoming a genetic clock. If you take samples from today's living population and determine how much they vary now, you can extrapolate this information backward in time for when the first human ancestor for appeared on earth. This video will explain details of how these tests are done along with a sample result.
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Video Length = 24 minutes