The world is a little closer to one of the ethical dilemmas we have put off thinking about. Embryos have been produced combining DNA from pigs and humans to make a so-called “chimera”. They were allowed to develop for weeks before being destroyed. Almost certainly, such chimeric embryos will soon go further, but technical challenges have made their formation harder than scientists expected.

Chimeras are organisms formed from two fertilized cells or zygotes, particularly those from different species. They sound like they should stay in the pages of J.K. Rowling books or ancient mythology, but there are good reasons why some want to create them. Most notably, people are dying from a shortage of donor organs such as hearts and kidneys. Chimeras, particularly those made by combining pig and human fertilized cells, could offer a solution, producing organs that are similar enough to our own to be transferred effectively.

The idea strikes many as horrendous, but others argue it is no worse than raising animals, often in shocking conditions, purely to eat them. It also might be hard to tell someone whose only hope of survival rests with a chimeric liver that the idea is too repulsive to implement. Science fiction writers and philosophers have spent some time wrestling with the issues, but political institutions and the wider public have generally put it in the basket of difficult things we don't have to worry about just yet.

So the announcement in Cell of the successful creation of a pig-human chimera embryo serves as a wake-up call that we can't put off thinking much longer about whether this is something we should do.

The efforts of lead investigator Professor Juan Carlos Izpisua Belmonte of the Salk Institute and his team have showed that the obstacles are not just ethical. “The ultimate goal is to grow functional and transplantable tissue and organs, but we are far away from that,” Belmonte said in a statement. “This is an important first step.”