Look in a mirror. There is a good chance you are looking at more than one “person”.
I first learned about human chimeras watching the CSI episode “Bloodlines”, where a rapist goes free to later kill his victim because his DNA checked in the lab did not match the DNA from the crime scene. But because of the closeness of the match, the CSI team determined the attacker was the suspect’s brother. It turns out the killer was his own brother. The killer was a merger of two people! Two twins had merged in the womb.
It turns out that human chimeras exist. In fact, there are many different degrees of chimerism, and it is far more common than you might imagine. You may very well be one yourself.
Carl Zimmer’s recent book “She Has Her Mother’s Laugh” has a great section on chimerism. The examples below are largely taken from his book.
Tetragametic chimerism occurs when two fraternal twins (two eggs fertilized by different sperm) merge early during their development in the womb. Only one human is born, but that human is literally made up of two different persons. Different sets of genes are distributed throughout the body. Test one tissue in the body, find the DNA of person A. Test another tissue, and you might find the DNA of person B.
A famous tetragametic chimera is Taylor Muhl, where the different persons that make her up can actually be seen by changes in her skin color. See “I am my own twin: Model and musician Taylor Muhl comes out as a chimera” for some fascinating photos.
The stories of Lydia Fairchild and Karen Keegan show how this can cause legal trauma for families when DNA tests indicate the mother isn’t the mother. See “She’s Her Own Twin”.
It doesn’t take much thought to realize roughly half of tetragametic chimeras are a blend of both male and female persons - true hermaphrodites.
How rare is this? No one really knows. Most chimeras, like Lydia Fairchild and Karen Keegan, probably go through life without any indications they are composed of multiple persons.
Fraternal Twin Chimerism
Even when fraternal twins do not merge into a single human, each twin is often a merger of more than one person, especially in their bone marrow (and thus their blood). If you look carefully at their blood, it is made up of cells from two persons. One German woman had 100% male blood from her male twin.
One study looking for multiple blood types in people calculated at least 8% of twins and at least 21% of triplets have this level of chimerism. When they bleed, they bleed cells from two or more different persons.
Even if you do not have a fraternal twin, there is a good chance you still have some chimerism. A study of the blood of 154 young girls (10-15 years old) found 13.6% of them had at least small amounts of male cells in their blood. One source of these male cells may have been from older brothers. The brother’s cells crossed into the mother, survived in the mother for years, and then the cells were passed down into the younger sister. The test only looked for other human cells that were male because it was an easy test. Presumably if they also looked for foreign female DNA (e.g., from an older sister), that number would have climbed to over 26%.
These small levels of other persons’ cells circulating in a human are called microchimerism.
Almost all mothers are microchimeras. They have the cells of their children embedded inside their own body in many different tissues including their brains. These cells from their children can often be found decades after birth.
Who are you?
So, look in a mirror and ask yourself, “Are there multiple people inside of me?”