Mixed messages: is research into human-monkey embryos ethical?

Biologists recently created a chimera with both human and monkey cells. But not all scientists are happy to blur species boundaries

When King Minos of Crete was given a magnificent bull by the sea god Poseidon for a sacrifice, he could not bring himself to kill it. In anger, Poseidon enchanted Minos's wife Pasiphae to be filled with lust for the creature. The result of their trans-species mating was the bull-headed monster the Minotaur.

Hybrids of humans and animals throng within myth and legend: centaurs, mermaids, goat-footed Pan. We're both fascinated and uneasy about the boundary that separates us from other animals - and whether it is leaky.