This story is a supplement to the feature "The Migration History of Humans: DNA Study Traces Human Origins Across the Continents" which was printed in the July 2008 issue of Scientific American.

Methods: Genetic Prospecting
Digging through DNA to find the origins of the first modern humans began 20 years ago through inspection of genetic material in the cell’s mitochondria and later in the Y chromosome. Today investigations can scan sections of the whole genome contained in the cell nucleus to compare differences, or polymorphisms, in large numbers of individual nucleotides, the “letters” of the DNA alphabet.

Many Ways to Slice a Genome
Scientists continually seek genetic markers—characteristic patterns of nucleotides—that
differ from one population group to another and that can be used when comparing whole genomes. Microsatellites, short repetitive nucleotide segments found on all the chromosomes (top), have served as markers  for a number of years. A new type of whole-genome analysis looks for what are called copy number variants—deletions or duplications of up to one million nucleotides (bottom).