The DNA finding that made national news last week, the one that put a number on the oldest argument in American history, runs through a house in Greenport. Dorothy Westerinen, who lives in the village, descends from Eston Hemings, the youngest son of Sally Hemings, the enslaved woman who lived at Monticello. Her family has said for generations that Eston's father was Thomas Jefferson. As of last Tuesday, the Smithsonian says the chance they are right is 99.9 percent.

That figure comes from a 15-year investigation led by Richard Kurin, a Smithsonian Distinguished Scholar and Ambassador-at-Large, and it is chronicled in his book History by a Hair: Thomas Jefferson, Sally Hemings and Solving a 200-Year-Old Mystery, published September 22 by Penguin Random House. On Saturday, October 3, Kurin brings the story to the North Fork Arts Center in Greenport, and Westerinen and her brother Art will be on stage with him.

A lock of hair from the day he died

The search began with strands of hair found tucked in a Jefferson family Bible, and it ended with a lock preserved in Monticello's own collection, labeled as cut on July 4, 1826, the day Jefferson died. For decades that hair was scientifically useless: it had no follicles, and DNA testing of hair once required the root. The technology finally caught up. Kurin's team, which grew to include geneticists, archaeologists, genealogists and forensic specialists, managed to sequence a genome from the rootless strands.

With Jefferson's own genetic profile in hand, the team compared it against DNA from 13 living descendants of the Jefferson and Hemings families, working down the lines of Sally Hemings' sons Madison and Eston. The analysis put the probability that Thomas Jefferson fathered a child with Sally Hemings at 99.9 percent.

What the 1998 test could not say

If this sounds like settled history, it is worth remembering how unsettled it stayed. A 1998 study in the journal Nature matched the Y chromosome of Eston Hemings' male-line descendants to the Jefferson male line, which was enough to end the era of flat denial. But that test could only point to some Jefferson man. For nearly 30 years, skeptics used the gap to argue for the president's brother or his nephews instead.

The new work closes that gap in the most direct way possible: not by triangulating around Jefferson, but by reading Jefferson himself, from hair cut at Monticello while his household still moved around him. It is the difference between a strong inference and a comparison, and it is why the finding traveled around the world in a week.

The family that carried the story

For the Westerinens, the announcement is less a revelation than an arrival. Dorothy's mother, Julia Jefferson Westerinen, was among the descendants who gave DNA to the project before her death. Julia's branch of Eston's line crossed the color line generations ago, and she learned only in adulthood that the family she grew up in descended from Sally Hemings. She spent years afterward telling that story publicly, at commemorations and family reunions on both sides of a divide the country is still learning to talk about.

Her daughter now gets to sit on a stage in her own village while a Smithsonian scholar explains that the science reads the way the family always said it did. The oral history came first. The genome agreed.

Saturday afternoon on Front Street

The North Fork Arts Center hosts the discussion Saturday, October 3 at 4 p.m. at 211 Front St. in Greenport. Kurin will walk through the investigation, joined by Dorothy and Art Westerinen, two of the descendants featured in the book, in a conversation moderated by Liz Welch, who collaborated on the book and co-founded Coming to the Table, an organization that connects descendants of enslaved people and enslavers. General admission is $20, and anyone 25 and under gets in free. Burton's Books of Greenport will have copies on hand, and Kurin will sign them after the talk.

For most of the country, this is a story about a founding father and a laboratory. On the North Fork it is about a neighbor, a mother's persistence and a village theater where a 200-year-old question gets talked through on a Saturday afternoon. History has a way of feeling like something that happened somewhere else. This weekend, one of its biggest corrections is happening on Front Street.