The idea of cloning had occupied biologists since the late 19th century, but producing a true copy of an adult mammal was considered impossible. Embryonic cells were versatile — frogs had been cloned from frog eggs in the 1950s. But an adult somatic cell (skin, muscle, mammary tissue) had already specialised; it could not revert and produce an entire organism the way an embryo cell could. This assumption was a half-century dogma of biology.

At the Roslin Institute near Edinburgh, embryologist Ian Wilmut and cell biologist Keith Campbell's team set out to break it. The method was called somatic cell nuclear transfer (SCNT): the nucleus of a mammary-gland cell from a six-year-old Finn Dorset ewe was fused by an electric pulse into the enucleated egg cell of another sheep; the resulting embryo was implanted into the uterus of a third sheep, the surrogate mother. 277 attempts were made, 29 embryos began to develop, only one made it to full birth. On 5 July 1996 a lamb was born — its DNA identical not to the genetic mother but to the six-year-old donor of the nucleus. Wilmut named the lamb after the country singer Dolly Parton (a nod to the mammary cell of origin).

The team stayed silent for eight months to verify the result. The paper published in Nature on 23 February 1997 caused a global shock. The headlines all asked the same question: are humans next? Pope John Paul II and President Bill Clinton both issued statements banning human cloning the same week; the U.S. cut federal research funding, and the Council of Europe adopted a protocol prohibiting human cloning in 1998. Dolly lived for six and a half years — about half the life of a Finn Dorset — and was put down on 14 February 2003 because of advanced lung disease. Her body was preserved and is displayed at the National Museum of Scotland in Edinburgh.

Dolly's real legacy was psychological: it had been proved that adult cells could be "reprogrammed". That opened the door to stem-cell biology. In 2006 the Japanese scientist Shinya Yamanaka showed that adult skin cells could be reverted to embryo-like stem cells (iPSC — induced pluripotent stem cells) simply by activating four genes — a discovery that earned him the 2012 Nobel Prize in Medicine and opened a far ethically cleaner road (no embryo required). Therapeutic cloning, regenerative medicine, and research into growing replacement organs all advanced down this line. Reproductive human cloning has remained banned worldwide; but in 2018 the Chinese scientist He Jiankui's announcement of the birth of twin girls whose genes had been edited with CRISPR was a reminder that the question Dolly first put on the table — "can we / should we" — is still without an answer.