Mutations are changes in DNA which happen naturally and are the basis for evolution. A mutation can occur when DNA polymerase makes a mistake when copying the DNA strand during cell division, resulting in a slight change in the DNA of the newly created cell. When a cell divides (during a process called meiosis), the DNA in the original cell must make a copy of itself for the new cell. It does this through a process known as DNA replication. During DNA replication, an enzyme known as DNA polymerase makes a copy of the DNA using the original DNA strand as a template. Although this process is usually very accurate and results in an exact replica of the original DNA, on occasion, the DNA polymerase may miscopy a section of the DNA, resulting in a slight change in the newly formed DNA strand. This change is called a mutation and although mutations do not occur frequently, they are an important part of evolution and occur naturally.
Mutations play a very important role in the study of ancient human history and human migrations. Mutations can act as "time and date stamps" because scientists can follow the path of occurance of mutations in indigenous populations from around the world to re-trace the ancient migration path of humans over a period of hundreds of thousands of years. In the case of Y-DNA and mtDNA, scientists can examine the pattern of mutations called SNPs (single nucleotide polymorphisms) to generate a phylogenetic tree which shows how humans branched out from common ancient ancestors originating in Africa over 100,000 years ago.