Hey, smart people. Joe here. In 1960, a baby girl was born in a Seattle hospital. She was healthy, though one of her eyes was brown, and the other was hazel. And her organs were a bit unusual as well. On one side of her body, she had an ovary. On the other, what looked like a testicle. Strangest of all, when doctors tested her blood, they found a mix of two blood types. The genetic tests revealed that the cells in the girl's body came from two distinct individuals, a male and a female. This girl was what we call a chimera. So how did this happen? Well, her mother had been pregnant with fraternal twins, a boy and a girl, who had fused into one embryo, resulting in a single healthy child. But doctors used to think that chimeras were extremely rare. But as genetic testing has become more widespread and advanced, it turns out there's more chimeras out there than we thought. In fact, you might even be one.

[MUSIC PLAYING]

(Describer) Title: It's Okay to Be Smart.

If the idea that humans could be weird mash-ups of two or more different individuals sounds like something out of a monster myth, that's because it is. Luckily, I know a real-life monsterologist. So I called my friend, Dr. Emily Zarka, from the YouTube channel Monstrum so she could tell us more.

(Describer) Zarka:

The word "chimera" comes from the name of a great mythological beast in ancient Greek mythology, the chimaera. Said to have the body and head of a lion, the tail of a snake, and the head of a goat placed randomly in the middle of its back, the monster first appears in "The Iliad." The chimaera is killed, but because of her impossible hybridity, we've carried her name with us through the centuries. Thank you, Dr. Z. I want to point out that Emily really does study monsters for a living, which is so cool, and you should definitely watch Monstrum because it's awesome. Now, in biology, a chimera is not as scary as in the myth. It simply refers to an individual who is an amalgamation of two or more genetically distinct individuals of the same species. No goat heads grown out of their back or anything. Now, For humans, this means a person containing cells from two or more different people. The little girl in our example is called a tetragametic chimera because four gametes or reproductive cells came together to create her. Two of her father's sperm fertilized two of her mother's eggs, resulting in two embryos that merged into one. Now, the fact that the two original embryos were of opposite sexes made this case of chimerism easy to see on the genetic level. But sometimes there's nothing obvious indicating a person might be a chimera. This story is super messed up. In 2003, a woman named Lydia Fairchild nearly lost her four children because DNA tests said she wasn't their mother. Now, a person inherits half of his or her DNA from each parent. So a mother and her child should be at least a 50% match. But when authorities tested DNA from her skin, hair, and saliva, it wasn't a 50% match with her kids. She shared much less than a parent should. So authorities accused Lydia of kidnapping kids that weren't hers. But then, researchers looked at cells from Lydia's cervix. And that DNA matched her children. And because of these results, the court allowed Lydia to keep her kids. Lydia was a chimera too. But in her case, two female embryos had fused in the womb. She was her own fraternal twin sister. Now, genetically, some parts of her body made her the kids' aunt, while others made them her mother. Now, these sound like one-in-a-million cases. But recent research shows that chimerism is far more common than we once thought. In 2012, a group of researchers examined the brains of 59 women who had died, mostly in their 70s. They were looking for a gene that was found only on the Y chromosome, a chromosome that a biological woman's cells shouldn't even have. But they ended up finding the gene in almost 2/3 of the women's brains. These women were chimeras. They had cells with male DNA. And that male DNA most likely came from sons the women had given birth to decades earlier. Scientists learned that during pregnancy, some cells from the fetus enter the mother's bloodstream and migrate to various organs. And they may stay there for decades mixing with the woman's own cells. This can even occur during pregnancies that end in miscarriage or abortion. In other words, there's a good chance that your mom is a chimera, which finally explains the goat head. And if you're a mom yourself, you probably are, too, a microchimera, to be exact, since you likely only have a small number of your children's cells still living inside of you. Carrying around your kid's cells for years on end might make you more prone to autoimmune diseases, where your immune system gets confused about which cells belong to you and start attacking your own tissues. On the other hand, if some of your organs malfunction, your child's healthy cells may actually step in and save the day. When one woman's liver was damaged by hepatitis C, her child's cells grew a whole new chunk of liver for her. When's the last time you got your mom something that nice? Now, chimeras share their bodies with other people. But there's another way that you might share your body with different versions of yourself. In a mosaic, a single cell in one body mutates and then keeps dividing, spawning a whole separate lineage of cells in that body genetically distinct from the rest. The mosaic system can be really obvious. You ever see a tree with a branch or two that look like they don't belong? Well, in medieval Europe, people used to call these strange growths hexenbesen: witch's brooms.

(Describer) Zarka:

Witch's broom is an odd tree growth where short branches grow in closely packed bunches from a central source. They can be a symptom of plant disease or a result of genetic mutation, a mosaic, in fact. While some of them do look like brooms, many of them actually resemble nests. But keep in mind anything odd, especially anything even vaguely resembling a human object, was at one time seen as unnatural, even supernatural. Pretty much anything weird in nature was seen as monstrous. Since witches have been associated with riding brooms since at least the 14th century, people saw these strange tree growths and gave them a name that tied the two things together. Some people even believed the mutated branches could be used by witches for shelter. The reality is much more mundane. A few cells mutated in one part of the tree and just gave rise to a branch that looks different from the rest of the tree. In the case of the witch's broom, the mutations are harmless. But sometimes the mutated cells can cause more serious problems. That's what happened in the most famous case of mosaism, a 19th century Englishman known as the Elephant Man. Born Joseph Merrick as a baby, strange outgrowths appeared on his head, arm, and other parts of his body crippling him to the point that soon the only work available to him was serving as a circus attraction. Eventually at age 27, Merrick's mysterious condition killed him. Well, doctors now believe that Merrick had Proteus syndrome, a condition named after a Greek sea god who could change his shape. But the most dangerous case of mosaism is one that's all too common, cancer. In cancer, a few cells mutate and become different from the body's normal cells, eventually dividing uncontrollably and growing into tumors. In most cancers, it's the body's own cells that mutate and become cancerous. But certain rare types of cancer can actually come from another individual's mutant cells. That's right. Contagious chimeras. That's what happens with canine transmissible venereal tumor. A dog with this cancer develops tumors around their genitals. When this dog mates with a healthy dog, because dogs just aren't that picky, some tumor cells jump over to the healthy animal and soon they're growing tumors on that new dog. These tumors grow in the bodies that carry them. But they didn't come from those dog's bodies. The tumors are genetically distinct from the host. They're actually a piece of another dog that's been passed from dog to dog to dog back through time. Scientists trace them to an ancient canine who lived about 11,000 years ago around the end of the last Ice Age. Yes, a piece of one dog has been alive for 11,000 years, making this tumor the oldest dog ever. Another chimeric cancer is threatening Australia's Tasmanian devils, which are now an endangered species. They pass these facial tumors through biting each other. Now luckily, chimeric cancers are rare. But mosaic cancers, sadly, those are anything but rare. But really even healthy bodies are mosaics, even yours. Each of our bodies is a product of a huge number of cell divisions. Minor changes to our DNA creep in, leading to whole lineages of cells that are a little different from the rest, often in ways that we can't see. Almost all of us harbor multiple mutations, small changes in our bodies that we weren't born with. If you sequence the DNA from cells in your heart and your nose and your left toe, I bet none of them would be an exact match. We like to think of our DNA as a unique marker of our identity that is set in stone for life. For much of human history, people with genetic abnormalities were treated like monsters. But modern science has taught us that, in fact, we're not all that different from the Elephant Man. We're all mosaics, and the longer we live, the more versions of ourself we become.

(Describer) A goat pokes its head up behind him. Accessibility provided by the US Department of Education.

Stay curious.

[MUSIC PLAYING]