DNA and ancient remains
Every cell in your body contains mitochondria, tiny sausage-shaped power generators. They have their own small amount of DNA, separate from the DNA found in the cell nucleus. Because each cell has around 500-1000 mitochondria but only one nucleus, there are many more mitochondrial DNA molecules than nuclear DNA molecules. So a short phrase of mitochondrial DNA is often the only sort present in sufficient quantities to be detected in ancient remains.