Lizzie Valverde and Katy Olson were strangers when they enrolled at Columbia University a few years ago. Ms. Valverde is from New Jersey, while Ms. Olson had grown up mostly in Florida and Iowa.

Their lives crossed in January 2013, on the first day of a writing class, when they took part in one of those familiar around-the-table introductions that by the end had led them to a stunning realization.

These strangers were sisters.

The two women had come to Columbia to learn the finer points of storytelling and wound up in the middle of a doozy: an intertwined tale of their own that they say they could never have conjured.

Their shared story line — a chance reunion three decades after being born to the same troubled mother in Florida and then raised by adoptive families in different parts of the country — has been knitted together by years of curiosity on both women’s parts about their origins.