History is littered with legends and folklore of Chimeras, a mysterious creature that is half human and half beast. According to the ancient Greeks, a Chimera was a fire breathing creature with a head of a lion and the tail of a serpent. The Centaur, a strange creature with the body of a horse and the head, arms, and chest of a man was believed to be the follower of Dionysus.
Urban legends of beasts that are half human and half animal abound. Science fiction writers and the gaming industry sport mysterious beasts, usually with the intellect of humans and various features of some known animal.
But these bizarre humanoid creatures only exist in the imagination of humans, don't they? Perhaps not.
Human Chimeras do exist, just not in the way they have been portrayed throughout history. A human Chimera, in simple terms, exists when one human being possesses the genetic make up of two separate and distinct individuals.
The concept of human Chimeras was brought to light when Jane, 52 year old woman, underwent routine testing for a possible kidney transplant. The tests were conducted on her, her husband, and her three grown children.
The test results shocked and astounded her. The tests revealed that two of her three children were the offspring of her husband, but were not her biological children. A first, there was speculation that her children had been switched at birth but the odds of two children at two separate times being accidentally switched at birth we too high. There must be another answer.
A team of experts worked diligently for two years before the riddle was solved. By testing the cells of various tissues, experts finally concluded that Jane was a human Chimera. Her body contained the genetic makeup of two distinct human beings.
Experts were aware of a condition called blood Chimeras where fraternal twins
are genetically unique, but may share two distinct blood types. This occurs prior to birth when blood forming tissue is somehow exchanged between the twins. It is estimated that 8% of fraternal twins are actually blood Chimeras.
But Jane didn't have a twin -- that she knew of. Jane did; however, begin her life as a fraternal twin. The two embryos merged in utero leaving only one fetus. The genes in Jane's body are a mosaic of genes from both of the original embryos. Her cheek cells contain the genes of one embryo and her ovary cells contain the genes of another.
Jane brought the condition to light, but she is not an isolated case. In 1998 a patient was referred to the doctors at the University of Edingburg. The patient presented with an undescended testicle. Upon examination, doctors were unable to find the second testicle. What they did find was an ovary and a fallopian tube. The patient was a human Chimera created from the fusion of a male and female embryo.
Not all Chimeras are this dramatic. Most have more subtle signs that they may be a Chimera. Common indicators are eyes of two different colors or patches of hair that are a different color or multicolored hair. Although rare, approximately 30 cases of Chimeras have been documented world wide. It is likely that many more cases exist.
Most of us will never know if we are a Chimera, unless we, like Jane, happen upon an occasion to seek genetic testing and are handed a verdict that can not be explained. Only then may we know for sure that another human being lurks inside of us.
So the next time you look in the mirror and you catch a glimpse that resembles someone else, don't be alarmed. It just might be your long lost twin that coexists within you.
The Stranger Within. New Scientist vol 180 issue 2421 - 15 November 2003, page 34