How Old Is Humanity?

By David Plaisted, Ph.D. ([Footnote:] Dr. David Plaisted is Professor of Computer Science at the University of North Carolina, Chapel Hill. He has written numerous papers dealing with mathematics and computer science.)
New evidences suggest that the human race is very young. The journal Science reported that the age of the human race is roughly 1,000 to 10,000 generations. Other information about mitochondrial DNA mutation rates gives an even younger age than 1,000 generations.
Age estimates are obtained by observing differences between the DNA of different individuals and calculating the time of divergence using estimates of mutation rates. Mitochondrial DNA is often used, since it is separate from the bulk of DNA found in the cell nucleus. Mitochondrial DNA has about 16,000 base pairs and mutates, apparently, much faster than nuclear DNA. Human mitochondrial DNA has been completely mapped, so all the coding regions are known, as well as the proteins or RNA for which they code. Some areas of mitochondrial DNA known as “control regions” do not code for anything. A control region is a non-coding section that seems to have some kind of regulatory function. Because variation among humans is greatest here, scientists think this region mutates faster than any other region.
Mitochondrial DNA mutation rates in the control region were measured directly by comparing mitochondrial DNA from siblings and from parents and their offspring. Mitochondrial DNA was found to mutate about 20 times faster than previously thought, at an approximate rate of one mutation (substitution) every 33 generations. The control region studied has about 610 base pairs. Humans typically differ from one another there by about 18 mutations. By simple mathematics, it follows that the human race is about 300 generations old. If one assumes a typical generation of about 20 years, this gives an age of about 6,000 years.
This calculation is done as follows: Assuming all human beings initially have identical mitochondrial DNA, consider two randomly chosen human beings. After 33 generations, two such random humans will probably differ by two mutations, since there will be two separate lines of inheritance and statistically one mutation along each line. After 66 generations, two randomly chosen humans will differ by about four mutations. After 100 generations, they will differ by about six mutations. After 300 generations, they will differ by about 18 mutations, the typically observed value in humans today.



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