Siamese twins go way back before P.T. Barnum, of course. The earliest reports of Siamese twins come from Peruvian pottery dated from 300 C.E. They have a loooong history in Europe; a pair of Armenian twins were deemed an act of God, Mary and Eliza Chulkhurst of England were the first named conjoined twins, and a French couple put their conjoined girls on display in Paris for all eight months of their short lives. Barnum is not the only one who liked showing this off; the record of conjoined twins is remarkably well-documented, especially in Europe. They pretty much make news whenever they happen (1 in every 50,000-100,000 births; somewhat higher in Southeast Asia and Africa).
One of them likes Starbucks, the other likes Caribou. How DO they share a body?
Despite the condition's publicity, nobody is really sure what happens to Siamese twins in the womb. There are two competing theories as to what causes the fused bodies: In the "fission" version, the fertilized egg simply does not completely divide; in "fusion," the egg completely separates, but because stem cells seek similar, the twins' stem cells bind to each other. Occam's Razor says that the simplest solution is probably the correct one, and incomplete fission sounds hella simpler.
Speaking of fission, it is not always a good idea to separate Siamese twins. Remember how I said that conjoined twins can be joined anywhere? No kidding; although the popular place is usually somewhere on the body, Siamese twins can be merged in any number of places. Some of them are attached at the skull, making surgery exceedingly difficult; others have one twin as a parasite, i.e. the one twin CANNOT survive without the other. Many of the times, they share organs. These different types of twins all have very long Greek names that I will not hurt your brains with and, usually, have bitchingly complex surgeries to match. The survival rate of Siamese twins is approximately 25%.
By the way, this can happen in animals, too. There are a few stories floating around about a duck with six legs or snakes with two heads. Polycephaly (that is, multi-headedness) is most common in turtles and snakes, but can theoretically occur in any animal species. There are also a lot of fake PhotoShop jobs of such things. When it is real, it's the same deal as Siamese twins.
|Only a multi-headed dog would look more like a chthonic horror.||Those exist, by the way.|