In 1997, Drs. Ian Wilmut and Keith Campbell shocked the world by announcing the birth of Dolly. Dolly was just an ordinary lamb, but the way in which the two scientists had conceived her was extraordinary.

Drs. Wilmut and Campbell removed the nucleus from a sheep egg, leaving the egg without chromosomes and thus without any nuclear DNA. Then the scientists used electricity to fuse the egg together with a cell taken from the udder of an adult sheep. The effect was to substitute the nuclear DNA of the adult sheep for that which had been taken out of the egg. After the fused product subdivided into an embryo, the scientists implanted that embryo into a surrogate mother sheep. Several months later, Dolly was born. In effect, she was the later-born identical twin of the adult sheep that donated the nuclear DNA for the procedure.

Dolly's birth was scientific heresy. For years, biologists believed it to be impossible to clone mammals. Later, when it was discovered that mammals can be cloned from cells taken from embryos, biologists adjusted their beliefs slightly, asserting it to be impossible to clone mammals from adult cells that had taken on specialized functions such as skin, muscle, organs, and so on. Skeptics refused to believe that Dolly could have been cloned from an adult cell.