The most visible example might be a case of conjoined twins sharing a brain, says Kramer, but even regular twins could have shared organs without realising it. During early development, cells can be passed between twins or triplets. Once considered a rare occurrence, we now know it is surprisingly common. Around 8% of non-identical twins and 21% of triplets, for example, have not one, but two blood groups: one produced by their own cells, and one produced by “alien” cells absorbed from their twin. They are, in other words, a chimera – a fusion of two bodies – and it may occur in many organs, including the brain.

A chimera brain could have serious consequences. For instance, we know that the arrangement of different brain regions can be crucial for its function – but the presence of foreign tissue, being directed by different genes carrying a different blueprint, may throw that intricate design into disarray. This may explain, for instance, why twins are less likely to be right-handed – a simple trait that normally relies on the relative organisation of the right and the left hemispheres. Perhaps chimerism has upset the balance.

Even if you do not think you ever had a twin, there are many other ways you might be invaded by another human’s cells. It’s possible, for instance, that you started off as two foetuses in the womb, but the twins merged during early development. Since it occurs at such an early age of development, the cells can become incorporated into the tissue and seem to develop normally, yet they are carrying another person’s genetic blueprint. “You look like one person, but you have the cells of another person in you – effectively, you have always been two people,” says Kramer. In one extreme case, a woman was surprised to be told that she was not the biological mother of her two children (See “Brother from another mother”). Alternatively, cells from an older sibling might stay around the mother’s body, only to find their way into your body after you are conceived.