The hope is to implant human stem cells in an animal embryo so that it will grow specific human organs. The approach could, in theory, provide a ready-made replacement for a diseased heart or liver – eliminating the wait for a human donor and reducing the risk of organ rejection.  It's going to open up a new understanding of biology 

These bold and controversial plans are the culmination of more than three decades of research. These experiments have helped us understand some of the biggest mysteries of life, delineate the boundaries between species, and explore how a ragbag bunch of cells in the womb coalesce and grow into a living, breathing being.

With new plans to fund the projects, we are now reaching a critical point in this research. "Things are moving very fast in this field today," says Janet Rossant at the Hospital for Sick Children in Toronto, and one of the early pioneers of chimera research. "It's going to open up a new understanding of biology."

That is, provided we can resolve some knotty ethical issues first – questions that may permanently change our understanding of what it means to be human.

For millennia, chimeras were literally the stuff of legend. The term comes from Greek mythology, with Homer describing a strange hybrid "of immortal make, not human, lion-fronted and snake behind, a goat in the middle". It was said to breathe fire as it roamed Lycia in Asia Minor. Previous attempts to produce a hybrid "interspecific" chimera often ended in disappointment. The embryos simply failed to embed in the uterus, and those that did were deformed and stunted, and typically miscarried before they reached term.