Scientists announced today that they have created the first successful human-animal hybrids. The project proves that human cells can be introduced into a non-human organism, survive, and even grow inside a host animal, in this case, pigs. This biomedical advance has long been a dream and a quandary for scientists hoping to address a critical shortage of donor organs. An international team of researchers led by the Salk Institute ... created what's known scientifically as a chimera: an organism that contains cells from two different species. Such experiments are currently ineligible for public funding in the United States. Public opinion, too, has hampered the creation of organisms that are part human, part animal. The Salk-led group [used] the genome editing tool called CRISPR to hack into mouse blastocysts-the precursors of embryos. There, they deleted genes that mice need to grow certain organs. When they introduced rat stem cells capable of producing those organs, those cells flourished. The mice that resulted managed to live into adulthood. Pigs have a notable similarity to humans. Not that these similarities made the task any easier. In order to introduce human cells into the pigs without killing them, [the team] had to get the timing just right. When those just-right human cells were injected into the pig embryos, the embryos survived. Then they were put into adult pigs, which carried the embryos for between three and four weeks before they were removed. In all, the team created 186 later-stage chimeric embryos that survived.