The chimera was a zoological monstrosity of Greek myth: a fire-breathing beast with the head of a lion, the body of a goat and a serpent for a tail. It was vanquished by Bellerophon, who dropped lead into the creature’s mouth, inducing molten suffocation.
Scientists are now breathing life into the concept of chimeras by creating organisms containing the cells of humans and animals. Several teams in the US are inserting human tissue into early pig and sheep embryos. The journal MIT Technology Review estimates that 20 such hybrid embryos have been produced, an early proof-of-principle step towards growing human organs inside animal tissue. The hybrid embryos have been observed in early development but, crucially, are never brought to term. The Salk Institute in California, together with the universities of Stanford and Minnesota, are among the institutions pioneering the as-yet-unpublished research....