Researchers have successfully transplanted human neurons into the brains of rats. The recent, groundbreaking study described how the human cells took root inside the rat brains, hooked up to their blood supplies and tapped into neural circuitry. Rather than create a kind of super-rat, the ultimate aim is to better understand neuropsychiatric disorders such as epilepsy and schizophrenia, and examine the effects of drugs in real time. But do the potential benefits outweigh the ethical questions that come with combining human cells with other animals? Ian Sample speaks to Prof Julian Savulescu about how the scientists managed to transplant the neurons, what this means, and how we decide where to draw the line in such an ethically complex field of research. Help support our independent journalism at https://www.theguardian.com/sciencepod">theguardian.com/sciencepod