It is widely believed that all organisms can be traced back to a common ancestor. However, over time, the minute evidence of ancestry has exponentially declined - for example, in a slightly oversimplified version, a person receives half of his genetic material from each parent; hence he has 25% from each grandparent, 12 and a half percent from each great-grandparent and so the pattern continues. Imagine the percentages some thousand years before - almost indistinguishable!
Common ancestors then can change - hence the constant debate over the relative relation between humans and chimpanzees, for example. . The term MRCA (most recent common ancestor) is used to trace and identify a common ancestor from which a set of organisms (such as humans) is directly descended. A "pedigree" is thus established.
DNA is used to trace lineage but of course, there is no DNA of those living thousands or hundreds of thousands of years ago. However, DNA passed on through the female line (only) , from a mother to all her children, or, similarly, the male line (only) allows for tracing back to less recent common ancestors.
Distinct lineages are created through DNA the most widely celebrated of which is mitochondrial DNA. New variants created through mutations actually survive and create these lineages making it possible to trace back, in this case, through the female line, female lineages through hundreds of generations. Hence, a common ancestor is found.