Scientists from the University of California have successfully created embryos that art part human, part pig, reported The Guardian. The experiment is a part of a larger attempt to grow human organs inside pigs to deal with a scarcity of donors. Researchers combined human stem cells and pig DNA and left the embryos to mature for 28 days. They then analysed the tissues that were formed.

However, many biologists have argued that the presence of human cells in pigs might affect the animal's brain and behaviour. “I’m nervous about opening up a new source of animal suffering. Let’s first get many more people to donate organs. If there is still a shortage after that, we can consider using pigs, but on the basis that we eat less meat so that there is no overall increase in the number of pigs being used for human purposes,” Peter Stevenson, from Compassion in World Farming, said, according to the English newspaper.

Other scientists have countered these fears, saying the animals will look and behave like any other pig. “Our hope is that this pig embryo will develop normally, but the pancreas will be made almost exclusively out of human cells and could be compatible with a patient for transplantation,” Professor Pablo Ross, the reproductive biologist leading the research. Ross said there was “very low potential for a human brain to grow” in a similar situation.