On Christmas Day last year I was reading an article about cattle breeding, recalled the novel Brave New World by Aldous Huxley, and suddenly an explanation for human homosexuality came to mind. As a fairly new parent, this issue, along with others concerning how people grow and what they ultimately become, has been on my mind. There are many theories, both religious and scientific, that have been proposed as explanations for the occurrence of homosexuality, but none seemed all that compelling to me. Some of the scientific theories are rather far-fetched and go through all sorts of convoluted reasoning to explain how something that renders people less fertile could be selected for by evolution. The most popular religious explanation of today is that it is a choice, which, being a man and knowing how little control I have over my own sexual desires, I cannot accept. The best idea I’d seen was Cochran’s pathological origin hypothesis. In fact, I pretty much accepted that explanation for some time. However, when reading a 19th century book on American Indians and their societies just a few days prior to coming up with this idea, I found that one of the pieces of data he’d relied on was strongly contradicted by the behavior of certain tribes. Cochran suggested that among hunter gatherers homosexuality was rare, even unheard of. However, the Koniagas of Kodiak Island and the Aleuts both practiced homosexuality to the extent that it was a culturally-sanctioned institution. Both groups could fairly be described as hunter gatherers, as could the Californian tribes who were also notorious for homosexuality. So he was wrong at least on that point, and that gave me second thoughts, which led me to keep my mind open to other possibilities.
When I first came up with the idea I had a “Eureka!” moment, but when I looked further into the subject of embryology I came to realize just how strange and complex an environment the womb is. It seems we can say more certain things about the Martian environment than we can about where we all started our lives, so I began to realize why it is so difficult to understand certain aspects of human development, especially those things that occur before birth. Convinced that someone else must already have thought of this I searched diligently through the literature on homosexuality, but although I found plenty of good information and numerous studies, the idea that homosexuality may be a result of chimerism did not come up. Eventually, it was a search into the phenomenon of twinning, which is the main cause of chimerism (excluding fetal cell chimerism in mothers), that led me to the work of Dr. Charles Boklage, a geneticist who is at the forefront of research into chimerism and twinning. Boklage is of the opinion that chimerism is at the root of many poorly-understood conditions and traits, including homosexuality. His research, although considered controversial, was essential to a better understanding of the complex subject of embryology. Furthermore, Richard Lippa, who discovered associations between hair whorls, handedness and homosexuality, introduced the idea of selection for a gene that might predispose people to homosexuality, counter-clockwise hair whorls and left-handedness. His explanation was that perhaps what’s being selected is phenotypical diversity, but he left out the association between twinning and the characteristics he studied. Twinning suggests increased fertility, which is always a factor in selection. So perhaps homosexuality is incidental to increased maternal fertility in the rather unique environment of the human womb, which is not designed for plural pregnancies.
What fascinates me is the idea that homosexuality may be the combination of genetic and environmental factors working together, and that it could be explained in terms of human evolution. Whether it is actually a result of sibling stem cells or not, the association of homosexuality with plural pregnancies is a fascinating phenomenon, and provides a potential link between a genetically advantageous trait and homosexual expression.
The Chimera Hypothesis
For over a century, scientists and thinkers have been trying to figure out the origin of homosexuality. Sir Richard Burton, one of the first to propose a biological basis for homosexuality, theorized that it was prevalent in a geographical region he called the “sotadic zone.” Psychologists attempted to explain it as a result of the action of various mental influences, and religious leaders believed it to be a moral issue.
Recent research has focused on genetic and environmental variables. Could it be the result of hormones in the womb? Could it be a genetic condition? Gregory Cochran has proposed that, because homosexuality is mathematically counterselective on a genetic basis, it must be due to another factor. He posits that it is the result of an infection at some point very early in human development, when the brain is developing sexual characteristics.
However, I see another explanation. Homosexuality is not the result of solely genetic or environmental factors, but rather a combination of the two. This combination results from the complex conditions in the womb during early development, when cells are exchanged in a medium in which the individual is not yet fully defined.
As the science of genetics has advanced in recent years, the existence of human chimeras has surprised a number of researchers. There are varying degrees of chimeric expression, and the effects span a fairly wide spectrum. In most cases, they are the result of twinning or blood exchange between mother and fetus. The most extreme form of chimerism, called tetragametic chimerism, is the merging of two genetically distinct embryos to form one organism. Although commonly thought of as mythological beings, men have long known of the existence of real-life chimeras. The freemartin, a chimeric cow with varying degrees of male characteristics, was used for centuries by farmers in the identification of fertile cows. Aldous Huxley’s Brave New World envisioned a society in which 70% of human females were deliberately made to be human “freemartins” through hormonal manipulation (DNA had not been discovered at the time he wrote the book) for the purposes of population control.
Genetic testing has revealed some spectacular examples of human chimeras. In one case in England, it was discovered that a woman’s children were actually not genetically her own, but rather her sibling’s. Apparently, she had absorbed stem cells from a vanishing twin early in embryonic development, leaving some of the cells in her ovaries with genes from her twin. This is referred to as “germ-line” chimerism, and, although very rare in humans, has been found in up to a third of marmosets — which, like humans have a uterus designed for single pregnancies — but through evolution have acquired the tendency to have multiple births. In another case, a man was referred to a doctor for an undescended testicle and found to have not a testicle, but rather an ovary from a sister with whom he had merged! These are extreme examples of human chimeras; in most cases the effects are far more subtle.
Chimerism has been found to be exceedingly common in humans. Sensitive genetic tests have shown that at least 50-70% of healthy adults are chimeric to some extent. Certainly, most mothers are. When women are pregnant, they receive a number of fetal stem cells in their bloodstream, and these cells are found in various tissues throughout the body — in some cases actively repairing damage and creating new tissue. Perhaps women are healthier than men in old age due to the beneficial effects of lingering fetal stem cells. It is even thought that these cells could be involved in the regeneration and repair of brain tissue. But not all of these fetal stem cells are necessarily beneficial to mothers. Some diseases — scleroderma in particular — may be a result. Scleroderma affects women 10 times as often as men, and is similar to a disease that occurs in some bone-marrow transplant recipients, where people receive stem cells from donors’ bone marrow.
However, the key issue here is the structure and function of the brain. Some physiological differences have been found between heterosexual and homosexual men suggesting a slight, overall feminization of homosexual men, but the most compelling and obvious difference is behavior, which is neurologically determined. Humans are extraordinarily complex beings, and much of what we are is expressed by our minds. Our likes and dislikes, our tastes — both artistic and physical — are determined by our neurological makeup. Some of us are timid, some bold. Some are meticulous and some spontaneous. And, as it happens, some of us prefer women and others men. In most cases this can be determined by sex, i.e., most men prefer women and most women prefer men, but this obviously isn’t a hard-and-fast rule: some men clearly prefer men to women and some women prefer women to men.
From a conventional evolutionary standpoint this approaches heresy. How could a man prefer to mate with another man, who cannot bear children? One would think this tendency would be bred out of the population very rapidly, as those with a same-sex attraction would have fewer children. But what if there is a factor that actually contributes a certain degree of selection for this trait? Although the concept is counterintuitive, characteristics of the human womb and embryonic development may hold the key to an explanation.
Recently, it has been shown that blood-type chimerism amongst fraternal twins is at least 8%. This is only one expression of twin chimerism, which suggests that others may make the overall rate of chimerism between twins much higher. Other expressions of the tendency have not been fully investigated, but additional effects are certain to show up in time. Additionally, with the advent of early ultrasound, it has been found that “vanishing twins”, i.e., twins that do not make it to full term, are far more common than previously thought. An ultrasound study showed that one out of eight pregnancies was a multiple pregnancy, and this may underestimate the number, as many embryos probably do not grow to the point where they can be detected by ultrasound. Another study found that only 15% of observed multiple pregnancies actually resulted in multiple births. Thus, it is certain that many people are the result of twin pregnancies that turned out to be single pregnancies due to the failure of one or more sibling embryos. Intriguingly, one of the complications listed for children whose opposite-sex twins vanished is gender-identity confusion. It is probable that humans, who typically give birth to only one child at a time, have evolved an intrauterine pruning mechanism to ensure the survival of only one embryo in most cases. It appears that intrauterine fetal competition is part of this mechanism.
Vanishing twins are said to be absorbed by the mother, but it is likely that their siblings absorb some of their twins’ cells as well. Stem cells are passed from twin to twin not only through blood-vessel anastomoses, but also through the amniotic medium, which recycles nutrients back to the placenta. There is also the possibility that cells could go through a mother’s bloodstream and end up in the other twin. If one twin fails to develop, it disintegrates and is resorbed. Although rather disquieting to think of, absorption of a failed twin could be advantageous to a competing embryo, so absorption would not necessarily be only back into the mother. The absorption of a twin’s stem cells is what causes chimerism among fraternal twins. If the vanished twin is female, some female genetic material will be incorporated into any male twin that absorbs her cells.
Homosexuality has high twin concordance, especially in monozygotic (identical) twins, where it approaches 50%. In fraternal twins it significantly lower, but still high, at 22%. Aside from genetic similarity, one of the factors that sets identical and fraternal twins apart is that most identical twin pregnancies are monochorionic, meaning that they share a placenta, whereas most fraternal twins are dichorionic, meaning that they each have their own placenta. Twins that share a placenta are naturally more likely to receive similar amounts of stem cells if any happen to be circulating through the placenta. Twins with separate placentas are still likely to be exposed to the cells, since they are both in the same uterus. However, each one has a distinct supply of nutrients and cells, so the ratio each twin receives would tend to vary more than in the case of identical twins. Interestingly, in fraternal triplets, the blood-type chimerism rate, at 21%, is significantly higher than that in twins. One thing to keep in mind when considering blood-type chimerism is that it is by no means the only type of chimerism exhibited by twins; it simply happens to be the only kind tested for with equipment sensitive enough to measure microchimerism.
One of the functions of the human immune system is to eliminate foreign cells. This is why compatibility is such a big issue with bone-marrow and certain organ transplants. The closer the genetic match, the better. However, even with close matches such as siblings there can be problems. It is likely that many stem cells, even from siblings, would eventually be rejected as foreign in a developing fetus, or would be weeded out when the child is born and begins to develop a mature immune system. However, certain tissues are exempt from this, including those in the brain. This means that stem cells that made their way into the incipient brain of an embryo could stay there safely, even if they are eliminated from other tissues. In fact, they could, and probably do, persist in the brain throughout the life of the organism. Female stem cells in a developing male infant’s brain could have profound implications for its ultimate form. Just as much of the rest of the body is shaped by our gender, so is the brain, and tests with rats have shown that hormones alone are not sufficient to masculinize the brain. The anatomical differences discovered in regards to homosexual brains, most of which show feminization, could be explained by pockets of genetically female cells, or even a few interspersed throughout areas critical to masculinization. Chimerism in the brain is probably more common than in other regions, and does not usually result in homosexuality, but can when cells colonize certain regions.
Genetic researchers have been using the receptiveness of the brain to stem cells to produce interspecies chimeras for some time. Mice and sheep brains have been injected with human stem cells to produce chimeras for study, and they readily grow genetically human brain tissue. Another example of the brain’s propensity to absorb stem cells is that female mice have been found with their offspring’s stem cells in their brains, happily working away creating tissue and repairing damage, a process that is thought to occur in humans as well.
So how could humans select for chimerism that expresses itself as homosexuality? Fertility is obviously a genetically positive trait, and fertility is the result of eggs being both available and fertile. This means that more fertile eggs during ovulation would be genetically advantageous, but due to human child-rearing logistics the survival of only one fertilized embryo would also be advantageous. Therefore, women prone to having more than one egg fertilized, but whose pregnancies only resulted in only one live birth, would have the optimum level of fertility. A side effect of this could be an increased incidence of chimerism in human children, and in the case of multiple embryos with different genders, this could lead to physiological changes that result in homosexuality in a proportion of the population. The key is that although homosexuality lowers male fertility in a couple percent of the population, even a relatively small increase in female fertility, which is really the limiting factor in human population growth, could outweigh the disadvantage of less fertility in a small number of male infants.
An Italian study that finds higher fertility in the maternal female relatives of homosexual men seems to bear this out. If these women are more likely to have plural pregnancies, then sibling chimerism would be more common among their offspring. In any case, people who are the result of multiple pregnancies are more likely to be homosexual, whether chimerism is the cause or not.
Chimerism may also be the cause of female homosexuality. That more homosexuals are male further supports twin chimerism as a cause of homosexuality, because in cases of twinning the male embryo is more likely to survive to term than the female. Female homosexuals also show opposite-sex characteristics in certain regions in their brains and some generalized masculinization.
- Men with twin sisters are more than twice as likely to be homosexual as those without
- Children known to have had a vanished twin are more likely to have gender-confusion issues
- Both twins and homosexuals are more likely to be left-handed
- At least 8% of twins have blood-type chimerism
- Older women are more likely to have plural pregnancies, as well as have homosexual children
- The brain does not reject stem cells, whereas most other organs’ tissues do
- Maternal relatives of homosexuals have been found to be more fertile than their paternal relatives
- Most plural pregnancies result in a single birth
- Female twins are more likely to vanish
So far, the possibility of maternal stem cells has not been discussed, but it is worth mentioning. Because we all have mothers, a maternal source of chimerism might be the most logical thing to look for, and indeed maternal stem cells have been implicated in some pediatric diseases. However, embryonic stem cells would be more likely to contribute to ultimate phenotypical expression than adult stem cells, and barriers between maternal cells and embryos appear far stronger than those between twin siblings.