Scientists have for the first time shown it's possible to create human-animal hybrids using existing genetic editing techniques.
It's hoped the breakthrough will lead to technologies allowing human organs to be grown inside other animals, rather than relying on human donors.
"The ultimate goal is to grow functional and transplantable tissue or organs, but we are far away from that," says Juan Carlos Izpisua Belmonte, professor at California's Salk Institute for Biological Studies.
"This is an important first step."
Researchers have spent decades trying to grow human organs from stem cells in petri dishes, without success.
"It's like when you try to duplicate a key. The duplicate looks almost identical, but when you get home, it doesn't open the door. There is something we are not doing right," says Dr Belmonte.
"We thought growing human cells in an animal would be much more fruitful."
Scientists started with rats and mice, managing to produce a hybrid - or chimera - by introducing rat stem cells into mouse embryos.
They did this by using a technique known as CRISPR editing to "delete critical genes" in fertilised mouse egg cells.
The rat stem cells filled the gap, creating whatever organs the mouse was missing as a result of the genetic editing - be it an eye, pancreas or heart.
While rats and mice diverged on the evolutionary tree 18 million years ago, they're a lot more closely related than humans and pigs, which proved much more difficult to combine.
Pig babies only spend a few months in the womb, so the researchers needed "perfect timing" in introducing the human stem cells.
"It's as if the human cells were entering a freeway going faster than the normal freeway," says Dr Belmonte. "If you have different speeds, you will have accidents."
Once they'd narrowed down the timing, the researchers were able to successfully produce a human-pig chimera. Generally, they contributed genetic material to the embryos' muscles and organs - thankfully not the brain.
"One concern with the creation of human/animal chimeras is that the chimera will be too human," Dr Belmonte explains.
"Researchers don't want human cells to contribute to the formation of the brain."
Pigs were chosen because they're cheaper and easier to experiment on than cows. The embryos were destroyed after four weeks of gestation inside surrogate sows.
"This is long enough for us to try to understand how the human and pig cells mix together early on without raising ethical concerns about mature chimeric animals."
Only around 0.01 percent of the cells were human, the rest were all pig.
The study, published in journal Cell, comes a day after scientists revealed they'd grown mouse pancreatic islets inside rats, and successfully transplanted them back into diabetic mouse.
The Salk Institute's work was funded privately, as US law currently prevents public funding for this kind of research.