Genetic genealogy through DNA testing is the most advanced tool researchers have today, used for either deep ancestry - where our ancestors lived 100,000 years ago - or to connect more contemporary branches of our families over the past several centuries.

It depends on what you want to know.

"DNA carries the story of our ancestors written in its simple code. We inherit our DNA from our parents, and they got it from their parents, and so on - right back to the beginning of life on Earth," says Dr. Spencer Wells, director of The Genographic Project for the National Geographic Society/IBM's five-year, $40 million study of deep ancestry, from one family 60,000 years ago in Africa to the migration of people throughout the world. In his book, Deep Ancestry, he describes the use of DNA as creating a path that links the present to deep ancestry.

A geneticist, anthropologist, writer and filmmaker, interested in both science and history, Wells investigates how science can explore questions of the past as he collaborates with linguists, paleoclimatologists, archaeologists and geneticists. While Wells deals with deep ancestry, most family history researchers don't need to go back that far. They want to know if all families with a certain name are related, how to hop gaps created by missing documents and how to break through genealogical brick walls, confirming or disproving oral history.

While DNA is the latest tool in the researcher's bag of tricks, there are no tricks involved. A paper trail can disappear or documents might be faulty, but our blood contains our history and bridges gaps that can't be filled in other ways.

"DNA is nothing more than a tool in the toolkit for the genealogist who has run into a paper trail roadblock," Family Tree DNA company founder and CEO Bennett Greenspan likes to say. "With DNA testing we are able to unravel that history book that is contained within the cells of all of us."

Family Tree handles the Genographic Project's public testing and has so far processed more than 200,000 kits. Participants can choose to add their results to Family Tree DNA's database, thus helping connect people.

How did this tool become reality?

Greenspan, born and raised in Omaha, Nebraska, has been interested in genealogy from a very young age; he drew his first family tree at age 11.

Most genealogists hit a dead end at some point in their research and it was Greenspan's turn in the 1990s. He had located a possible relative in Buenos Aires who bore the same name and came from the same town, but there was no paper trail to prove the connection. He had read something about DNA, but at the time, no company performed genealogical DNA testing for the general public. Greenspan contacted many people in the field. A University of Arizona genetics professor told him, "I get calls from crazy people like you all the time."

Greenspan had read about the 1997 Cohanim Project, which tested men of Ashkenazi, Sephardi, and Mizrahi origin who identified themselves as Cohanim. The results proved that a significant percentage of Jewish men who claimed this heritage did share a genetic signature, despite coming from widely diverse communities. According to Jewish tradition, Cohanim - the Temple priests - are direct male descendants of Moses's brother Aaron.

Greenspan persuaded Dr. Michael Hammer of the University of Arizona's genomic analysis laboratory to test samples from a group of people, including a known California cousin and the Buenos Aires man. The DNA matched and Greenspan convinced the scientist to provide testing for public genetic genealogy while he handled the business end.

The rest is history. Family Tree DNA became the first company in the field and remains at the cutting edge of the industry.

Family Tree DNA's database is the largest in the world, according to Greenspan. It also includes the largest Jewish comparative databases, with records for Ashkenazim, Sephardim, Levites and Cohanim - a boon for researchers who wish to compare results with other Jewish participants. The company's database also comprises comparative data on Native Americans and many other groups.

These projects can confirm or dispel genealogical theories, confirm traditional research or offer clues to people's origins. Samples are preserved for 25 years, making it easy to upgrade as more extensive tests appear.

As of March 24, 2008, the company site hosted 4,713 surnames or geographic projects, along with 74,499 unique surnames, 121,239 Y-DNA (male DNA passed from father to son), and 63,646 mtDNA (female DNA passed from mother to son or daughter) records.

Over the years, Family Tree DNA has been involved in fascinating stories: Hispanics in New Mexico who carry the Cohanim markers; a Polish man whose infant mother was handed to a Catholic family by Jewish parents on the verge of deportation; an identity crisis among some Lithuanian and Galizianer Jews who probably have Sephardi roots.

The company has also been involved with other prominent non-Jewish cases, such as debunking a Maryland accountant's purported link to Genghis Khan.

Jewish researchers who want to test their family's male Y-DNA should compare their markers against a database and, as Greenspan says, "size does matter." The database with the largest number of Jewish samples is the most suitable, as testing against a database with few Jewish samples will not show the genetic matches that really exist.

Jewish genealogy has taken an interesting turn as some Ashkenazi Jews have begun to discover their Sephardi roots.

"My grandfather always said we were Marranos. It was a story carried through the generations for 500 years that our family left Spain during the Inquisition," New Yorker Judy Simon tells Metro, adding that this derogatory term meaning "swine" in Spanish has been replaced by "conversos," "crypto-Jews" or "anousim."

Her grandfather's family lived in Rezekne, Latvia as far back as the mid-1750s, according to records. Some cousins believed so completely in the family story that, around 1909, they moved "back" to Spain, while her grandfather went to the United States. "For years, we had contact with these Spanish cousins, but this wasn't proof of our Sephardi ancestry."