Two people have a genetic relationship if one is the ancestor of the other, or if they share a common ancestor. Each of someone's ancestors will have contributed to their DNA. In evolution, species that have evolved from the same ancestor are said to be of common descent. However, this concept of ancestry does not apply to some bacteria and other organisms capable of horizontal gene transfer. DNA testing is used in determining ancestry. This can be for testing, to determine the parents of a child for legal purposes. DNA is also used to better understand the ancestry and history of all humans over the last 50,000 years. This was one of the benefits of the Human genome project.