They walk among us. By the mid nineties, science had only smoked out a few dozen of them worldwide but there are undoubtedly many more. You may well know some of them. You may even be one yourself. Perhaps you're sometimes haunted by the sense that there is someone lurking very near, invisible and silent but sharing that odd thought, impression, or fear from time to time. A more solid clue might be found in the skin on your back. Sometimes you can see this plainly though they say it's usually only discernable under UV light.

I'm talking about what might be looked upon as the most extreme form of Siamese or conjoined twinning, one in which both bodies have merged completely into a single individual. Scientists call the perplexing result a chimera, after the mythological beast described by Homer and others that sported the head of a lion, the body of a goat and the rear of a dragon or snake. What happens is that you have two fertilized egg cells that converge very early in the gestation process. If the cells were identical twins there would probably be no way to identify such two-in-one individuals and the world may well be full of them. But if they're fraternal, things can get more interesting.

If the two are of opposite sex you can end up with a true hermaphrodite, though this seems to be exceedingly rare. In January of 1998 doctors in Scotland reported the birth of a child, originally conceived through in vitro fertilization, who ultimately tested out to present both female (XX) and male (XY) chromosomes and corresponding equal-opportunity genitalia. Most of the time the consequences are much more subtle. In 1953 an English woman named Patricia McDonnell underwent some routine tests when she became pregnant and discovered she carried both Type O and Type A blood in a ratio of about 13 to 1. After considerable study her doctors concluded that the minority Type A was her own and the Type O was what was left of her twin brother.

Sometimes a chimera will have a left and right eye of different colors (like Jane Seymour and Joe Pesci... hmm, do ya think?), while others as alluded to in the first paragraph may display marbling or streaking patterns on their backs, called Blaschko's lines, which suggest an imperfect blend of two differing complexions. Researchers call the latter individuals mosaics. They're intrigued with the phenomenon because they suspect certain afflictions may arise from it such as cancer, Alzheimer's, and autism. (Indeed, Susan Folstein of Tufts University School of Medicine in Boston reports that about one in ten autistics show Blaschko's lines. The inference is that there is an antagonistic mingling of chimeric brain cells that have trouble communicating with each other.)

Beyond all of this, surgeons and researchers can and do intentionally create interspecific chimeras, where they combine tissues from different animal species. Considering that about five hundred prospective transplant patients die in the U.S. every month waiting for human donors, this can obviously be a very good thing. Pig and cow heart valve transplants are already quite common.

Here are two links at least tangentially related to chimeras. This one deals with a rather eerie, way-out aspect in a book by Bill Chalker; while this one explores purported interspecific hybrids involving domestic cats.