The technique has been shown to work for growing rat organs, such as pancreases, in mice, and vice versa. The embryo will then use the guest stem cells to make it instead. They created a “niche” for the foreign cells by genetically modifying the animal embryos so that they can’t grow the target organ – a liver, say – on their own. Several years ago, Japanese biologists Hiromitsu Nakauchi and Toshihiro Kobayashi pioneered a trick for getting cells injected into an embryo from another species to develop into just the organ you want. In particular, the shortage of human organs such as kidneys for transplantation – which leads to many potentially preventable deaths from organ failure – could be solved by growing “human” organs in animals. Such experiments might evoke grotesque echoes of HG Wells’ maverick mad scientist Doctor Moreau, who “humanised” animals by surgery. Some of these cells can survive for days. Human embryonic stem cells have previously been inserted into sheep foetuses and mouse, rabbit and pig embryos. The unease provoked by such biological patchworks is considerably stronger when the chimeras contain human cells. In some circumstances, the foreign cells can keep growing and thriving in their new host. Chimeras are generally made by transferring stem cells, capable of growing into a range of tissue types, from one species into the embryo of another. A chimera with a mixture of goat and sheep tissues, called a geep and having a goat’s head but a sheep’s woolly body, was reported in 1984. We’ve long known that these mosaic-style animals are possible.

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”. What strange hybrids are we creating, and why? 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.

Human chimera baby