These two rhesus monkeys are called Roku and Hex. They look like regular old rhesus monkeys, and indeed they were born completely normal and healthy. But they are, in fact, chimeras, each the result of splicing six different genomes together.
Animals created by mixing together genetically distinct material are called chimeras after the mythological beast that was a combination of a fierce lion, a deadly snake, and, for some reason, a goat. These are the first primate chimeras, as most previous examples were limited to mice. You can see a video of the monkey chimeras in action up top.
Scientists at the Oregon National Primate Research Center created these chimeras by basically gluing together cells from different rhesus monkey embryos. Once they had completed one of these mixed embryos, the scientists implanted it inside the womb of a surrogate mother. The key to the experiment's success was the use of very early stage embryos that contained totipotent stem cells, meaning these stem cells are capable of not just form different types of tissues but a complete animal, plus vital extra-embryonic tissues like the placenta.
The researchers initially tried to produce monkey chimeras the same way it's done in mice, which involves taking previously created embryonic stem cell cultures and injecting them into a single monkey embryo. This attempt failed, and so they had to start working solely with living embryos.
That finding has some implications for broader stem cell research, as it may mean that stem cells taken from living human embryos are much more potent and flexible than their cultured counterparts, some of which have been sitting in laboratories for up to two decades. In the monkeys, the embryos appeared to block the full expression of cultured stem cells. Researcher Shoukhrat Mitalipov adds:
"We need to go back to basics. We need to study not just cultured embryonic stem cells but also stem cells in embryos. It's too soon to close the chapter on these cells. We cannot model everything in the mouse. If we want to move stem cell therapies from the lab to clinics and from the mouse to humans, we need to understand what these primate cells can and can't do. We need to study them in humans, including human embryos."
On that last point, it should be pointed out - and indeed Mitalipov made sure to stress this point - that there are no plans and indeed no practical reason to create human chimeras.