National Geographic : 2009 May
the largest genome to be synthesized was only a thousandth the size of the mammoth s. Once scientists have functional mammoth chromosomes in hand, they could wrap them in a membrane to create an arti cial cell nucleus. en they could follow the approach pioneered in creating Dolly, the sheep cloned in 1996 by scientists at the Roslin Institute in Scotland: Remove the nucleus of an elephant s egg and replace it with the rebuilt mammoth nucleus, electrically stimulate the egg to trigger initial cell division into an embryo, and eventually transfer the embryo into an elephant s womb for gesta- tion. Each of these steps has signi cant question marks of its own. No one knows, for example, just how to build a mammoth nucleus. Harvest- ing an elephant egg is di cult, and bringing a mammoth fetus to term in an elephant uterus is fraught with uncertainties. Some scientists are tackling a less daunting challenge: cloning endangered or recently extinct animals. The San Diego Zoo and the Audubon Center for Research of Endangered Species in New Orleans both maintain "frozen zoos," where the DNA of a growing number of endangered species is stored in tanks of liquid nitrogen at minus 320° Fahrenheit. In 2003 sci- entists at Advanced Cell Technology used cells stored at the San Diego facility to successfully clone across the species barrier. ey created two bantengs, an endangered Southeast Asian ox, by inserting banteng DNA into domestic cow eggs and placing the resulting embryos in cow foster-mothers. ere is talk of using simi- lar methods to clone endangered giant pandas, African bongo antelopes, and Sumatran tigers. Ultimately scientists hope to re-create extinct species like the Pyrenean ibex and the thylacine, or Tasmanian tiger. Today the thorniest questions about clon- ing extinct species may be less technical than ethical. "Mammoths, like elephants, were intel- ligent, highly social animals," says Adrian Lister, paleontologist and mammoth expert at the Natural History Museum in London. "Cloning would give you a single animal, which would live all alone in a park, a zoo, or a lab---not in its native habitat, which no longer exists. You re basically creating a curio." Tom Gilbert, an expert in ancient DNA at Copenhagen University who with Schuster and Webb pio- neered the harvesting of mammoth DNA from hair, admits that as a student of mammoths, he d be the rst to go see one trundle across a paddock. But he questions both the utility and the wisdom of cloning extinct species. "If you can do a mammoth, you can do anything else that s dead, including your grandmother. But in a world in global warming and with limited resources for research, do you really want to bring back your dead grandmother?" j COURTESY RICHARD BEHRINGER In 2008 scientists reported they had brought a piece of Tasmanian tiger DNA back to life by inserting it into a mouse embryo. A special dye revealed that the extinct DNA turned on in the forming skeleton.