If you could cross a monkey with a human, what might that creature tell us? We are beginning to find out. In a pioneering and controversial experiment, scientists successfully created just such a hybrid: a chimeric combination of monkey and human cells, existing together in a living embryo that otherwise would never have been conceived by nature alone. Ethically fraught science? Yes. Mad science? No. While research into human-animal hybrids has a long and questionable history, in recent years researchers have pursued chimeric organisms to probe questions of biology that stand to offer significant gains in fields such as regenerative...