The term common ancestor can basically refer to two things. One is the scientific search for a human who is the common ancestor of all people living today. The second is the search for a person whose descendants, when tracing their genealogy, have nothing in common until they reach that person. In both cases, finding a common ancestor involves tracing the history of families back through time.
You can search for common ancestors by tracing the genealogy of two or more people until you find a point where they intersect. For example, you might start with three people not of the same family, but born with the same last name. By tracing their paternal (fathers') lineage, you can eventually discover someone with the same last name to whom all three are related. This person is their common ancestor. Common ancestors don't necessarily share the same names; it is just easier to trace those who do. Perhaps your family can trace its roots back to a particular person. To make it simple, let's say that person is a famous man or woman, such as Benjamin Franklin. Anyone who can trace their family back to Ben Franklin would then be known to have a common ancestor with others who can do the same, even if the name Franklin had not been in their family for centuries.
How to Find Common Ancestors
Searching for common ancestors requires patience and diligence. Begin with what you know about the people you are related to--in particular, your parents, grandparents, and great grandparents. Search family records, town records, or church records for marriages and births. Immigration records can also help lead you to common ancestors. You may need to check for changes in spelling or translations of names from one language into another.
Scientists continue to search for what is sometimes called the "holy grail of genetics"--the common ancestor of every person alive. This search has led down different paths, to early humanoid fossil remains from Africa, to animals that share genetic code, and to the idea of the most recent common ancestor (MRCA). Because of recent findings by statisticians, including Yale professor Joseph T. Chang, that the genealogies of living humans overlap in remarkable ways, and combined with the exponential increases in population in recent times, some scientists currently believe that our most recent common ancestor may have lived only a few thousand years ago.
In 2001, palaeontologists announced the discovery in Ethiopia of what is thought by many to be the oldest humanoid skeleton, a being who lived more than 5.2 million years ago. According to Yohannes Haile-Selassie, who made the discovery, this could be the ancestor of us all, and the first "human" after the still "missing link" said to exist between humans and chimpanzees.
Even Further Back
Some scientists are exploring DNA research to try to find one organism that is a common ancestor to all forms of animal life on our planet. These scientists believe that a common ancestor could be found in the gene pool dating back about 3.9 billion years. The hypothesize that it would probably be a very simple, perhaps single-cell, life form.