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

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 Pasiphaë 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.

So the recent report by a team in the US and China of embryos that contain a mixture of human and monkey cells mines an ancient seam of anxiety. What strange hybrids are we creating, and why?

Living entities containing the cells or tissues of more than one species are technically called chimeras – the name of a legendary monster that Homer described in The Iliad as “lion-fronted and snake behind, a goat in the middle”.

We’ve long known that these mosaic-style animals are possible. A chimera with a mixture of goat and...