Similar sequences are often found along protein polypeptide chains in different species. This similarity is directly linked to the sequence of the DNA (the genetic material of the organism). Due to the improbability of this being random chance, and its consistency too long to be attributed to convergence by natural selection, these similarities can be plausibly linked to the existence of a common ancestor with common genes. This allows polypeptide sequences to be compared between species, and the difference between two genetic sequences can be used to determine – within error – the time at which a common ancestor existed.