... For many, unease with genealogy stems from its exploitation by eugenicists. In Nazi Germany, for instance, one’s family tree, embedded in official identification papers, became literally a matter of life and death. Eugenicists have rationalized some of history’s most heinous acts with the science of ancestry. But eugenics, Kenneally reminds us, was born as and remains a distortion of science. Its founders and champions were elites who took inherent differences as a given — and themselves as humanity’s highest form. The problem was not the theories but “the way they were used to give longstanding social divisions a scientific rationale.”

In America, discomfort with genealogy is sharpened by our inspiring delusion that one can live free of history — as Willie Nelson sings, “It’s nobody’s business where you’re going or where you come from, and you’re judged by the look in your eye.” At its extreme, Kenneally writes, this “reflex against the idea that the past must have meaning . . . became a belief that the past has no meaning.”

Yet the past’s value and meaning are rendered powerfully clear in [Christine] Kenneally’s stories of people whose pasts were erased by history: African-Americans, Jews, orphans. Genetic analyses and genealogical databases like those at Ancestry.com and Family Tree DNA, especially when combined with earnest exploration in archives, now enable people to reconstruct lost lineages. Far more often than not, their reconnection with both family and history changes and deepens their lives. Other sleuthings examine broader puzzles: One chapter describes a fascinating study in which maps of genetic differences within the British Isles neatly match cultural and linguistic distinctions....