Making the weird news last week was Yamini Karanam, who underwent surgery in Los Angeles after being diagnosed with a brain tumour. The benign lump became a sensation when it turned out to be a grotesque growth containing bone, hair and teeth. Known to medical science as a teratoma (from the Greek for “monstrous tumour”), news wires quoted Karanam’s post-surgical joke that it was her “evil twin sister” who’d been torturing her.

Her sentiment was understandable – the condition must have been torturous for the 26-year-old student who had moved from Cyberabad, India, to Indiana to pursue her doctorate in computer science. The headaches and confusion that worsened as the tumour grew were compounded by her inability to afford the surgery. Happily, after a massive fundraising effort by well-wishers and the removal of the growth, Karanam seems back on track, and, as her GiveForward page now announces in a celebratory tone: “After successful surgery Yamini found her twin.”

As with most cancers, Yamini's torturer was likely of her own flesh, initiated by a tiny, terrible misunderstanding

Although it might have been one, this growth of hers was no deformed, misplaced sibling. As with most cancers, her torturer was likely of her own flesh, initiated by a tiny, terrible misunderstanding. When she was an embryo, perhaps before her mother even knew of the pregnancy, some of the tissue from Karanam’s tiny developing body would have broken away in error and become folded up into the tube that was already destined to become her brain. Each cell at that early stage carried the potential to build every part of every structure of a new body. Misplaced, in her brain, it became as human as it could.

Yet when it comes to tales of the monsters, chimaeras and mutations documented by medicine to have shared the human body, cases like Karanam’s are by no means the most bizarre. In up to 20 out of every 100 diagnoses of abnormal growths (neoplasias) in the ovaries, teratomas are to blame; and very occasionally, among these often shiny balls of skin, bone and hair emerge something almost recognisable as a person in its own right. Ovarian teratomas are the result of cells that should become our eggs going awry, as if attempting a run at becoming a baby without first waiting to be fertilised.

In 2003, Japanese doctors operating on a 25-year-old virgin identified the most fully formed ovarian teratoma yet found – a small, doll-like body, mostly complete. Like any normal foetus, the body was covered with fine, downy hair, but the homunculus was unmistakably deformed – it had spina bifida (“split spine” in Latin) and its brain failed to divide into two normal hemispheres. In the centre of its forehead was a single soft, spherical, fluid-filled “eye” cloaked by thick, long eyelashes. This strange “foetus” had one ear, all its limbs, a brain, a spinal nerve, intestines, bones, and blood vessels – even a jaw, already ruptured by several teeth, emerging from beneath the skin. It also had what looked like a phallus, positioned neatly between its legs.

Apart from teratomas, babies sometimes turn up with growths of a “parasitic twin” – a conjoined headless sibling, as in the “sister” famously detached from little Lakshmi Tatma four years ago in India. More rare still are the Russian doll-like formations of foetuses within foetuses, where a baby is born with its malformed, lifeless twin enveloped within its body. But sometimes, in an adult, as Karanam suggested in jest, no one may ever know that there was even a twin there at all.

In a case from Boston, US, a woman had gone to the hospital for blood tests ahead of a kidney transplant. Her three biological sons were potential donors. But the tests said that two of them were not actually hers, though her husband was their father. That, she knew, was impossible, so further checks were done. They discovered that her body was composed of two genetically distinct groups of cells. It seems she had actually been one of a pair of non-identical twin girls whose embryos had fused, very early on. Presumably her blood was carrying DNA from one twin, while other tissues carried DNA from the other. In which case, genetically speaking, her unborn twin was the parent of two of her boys. Like Karanam’s benign tumour – not evil, but you might say, she sort of found her twin.