In February 1997, Ian Wilmut and his colleagues at the Roslin Institute in Scotland announced the stunning news that they had cloned a lamb from the cells of a mature sheep. Although many animal cloning experiments had been performed in the past, they had succeeded only in cloning the cells of embryos. This latest experiment was remarkable because it was the first successful attempt to clone a mature mammal, producing an exact physical replica of the adult animal.

Wilmut and his lamb, whom the researchers named Dolly, became instant celebrities and found themselves at the center of intense debate. However, the uproar that resulted from the scientists' announcement was focused not on the creation of Dolly, but on the possible application of cloning technology to other mammals—specifically humans. Indeed, the arrival of Dolly brought the realization, as the British scientific journal Nature stated, that "cloning humans from...