THE PROBLEM: In humans and other placental mammals there appears to be an extremely heavy biological cost to being female relative to being male. Basic genetic theory and simple logic dictate that males can father children at a tiny fraction of biological cost born by the mother. Then why would any organism "choose" to be born female? The generalized answer is that, as intelligence increases, the ability of the mother to selectively assure the survival of her offspring skyrockets. Nursing her young, and using her intelligence to teach them her learned secrets of survival, as well as directly defending her babies gives her offspring an immensely greater chance of survival than that of a randomly promiscuous male.
But the recent discovery of the fact that fetal cells persist in the mother’s blood for decades in humans creates an entirely different scenario. What if these cells create some direct benefit to the mother? There are (among mammals) some other biological parallels.
Among wolves and countless other pack and herd animals there are often lifelong relationships between mother and child that facilitate both the longer term survival of the mother and that of her other offspring. When adult children can become hunting partners or additional eyes and ears for the mother (and her other offspring), they are then able to recompense some of her biological cost in bearing children by a synergistic relationship.
Complex organisms are made up of thousands of tissues, each a separate cell line performing some specialized and usually life critical function. Critical cell lines frequently come under attack from bacterial or viral organisms, and then can suffer further attacks from the immune system set up to defend the body. Diabetes is an example of one of a host of conditions brought about by the death of single critical cell line. Many cell lines can be replaced by the maintenance of unspecialized stem cells, which (upon some evidence of massive cell death) can specialize to become a new replacement cell line for the critical tissue. In long lived organisms, however, these stem cell lines (which must by necessity be kept small in number) can themselves become extinct, or present a danger of becoming unspecialized and very dangerous cancers or other tumors.
SOLUTIONS: A fetus, growing within a mother, begins entirely made up of stem cell lines. There is incontrovertible evidence that some of these take up long term and circulating residence inside the mother. Being 50% foreign, they could in both theory and practice create immense immune system havoc. It would not take too biologically sophisticated a system to purge these from the mother’s body once the baby is born. (It must be noted that doing so while the baby is in the womb would undoubtedly present a hazard to the baby.) The fact that this is not done (in spite of the biological cost of foreign cells in one’s body) indicates that there must be biological benefits accruing to the mother for not doing so. There are a number of possible candidates.
First, there is the simplest explanation, based on reproductive biology. If the mother is basically monogamous, the tolerance of foreign genetic material with a high correlation to that of future babies may help assure that these future babies ¼ot be attacked by the mother's immune system responses. Basically they could serve as a means of keeping a record of her ¼mate’s biological structure to help prevent her body from rejecting future pregnancies. This solution, though bizarre, is statistically quite likely PROVIDED that the species (in this case, human) is relatively monogamous during childbearing years. It is only likely to be a solution IF a similar lingering of fetal cells does not occur in truly promiscuous species. A relatively simple testing of several other species would either prove or disprove this solution. The fact that very different reproductive strategies have been practiced among humans tends to mitigate against it. But it obviously can’t be ruled out entirely.
Second, the most radical solution is that fetal stem cell lines may be capable of becoming specialized to take over the function of some dying or dead cell line of the mother. This appears the most straightforward explanation - but seems incredibly unlikely according to our knowledge of immune responses. Yet, however unlikely, Occam’s razor insists it is a likely solution. Let’s look at the evidence.
We know that there is an immense biological advantage for a mother not to mount an immune response to an infant in the womb. If she tolerates cells in her blood, she obviously won’t attack them in her womb. But why would they be in her bloodstream in the first place?
One explanation for such lingering foreign cells might have to do with lactation. If a mother were to mount an immune response to a baby's foreign cells during her nursing period, antibodies deadly to the baby would find their way into the milk. One would logically think that foreign cells in the blood would INCREASE not decrease the likelihood of a mother’s immune attack. It is at least faintly possible that there is some feature of such circulating stem cells that play a role in preventing an immune response. Surely if they encouraged such a response, they would be an immense liability to the baby.
One almost has to assume that the mother’s body has a tendency to define as not foreign that which is "family". As in the first solution such response tends to reduce a female’s likelihood of allergic reactions to her mate’s sperm or her future children’s genetic structure. But the extended period of lactation, particularly in humans, provides motivation to tolerate the foreign cells for an extended time. It is not entirely unreasonable for the time to be longer than absolutely needed – except for the health risks involved.
IF a mother tolerating the foreign cells reduced the chance of failed pregnancy, and then over a longer time, prevented antibodies against her baby in her milk, then there has to be a long term mechanism for not rejecting the cells. The same exact mechanism could then be called upon to prevent the rejection of offspring of the cells if they created new cell lines to replace damaged maternal tissues.
Humans commonly in the past and in the third world today often nursed to past the age of two. Combined with the length of pregnancy, this implies the requirement of toleration of foreign cells for close to three years. It is known as a fact that powerful immune responses would normally occur in time periods orders of magnitude shorter than that. Therefore an active suppression system must be in place.
It is also wise to look at the biology the other way around. Why would the baby inject it’s own cells into the mother’s blood unless they contained an active immune suppressing capacity against the mother’s immune defenses. In fact, there are only two reasons for the cell to be there, from a baby’s perspective. The first might be to actively produce some hormone that when released in the mother's blood will facilitate the child’s survival. Say, a hormone that would discourage another pregnancy during lactation, or a hormone that would exaggerate some motherly instinct. The second is that the cells provide an ACTIVE role in inhibiting the mother’s immune response to the child, in or out of the womb. It would be logical to be far less likely to find such cells in non-lactating animals, since the need to suppress immunity in the mother ends at birth.
If the fetal cells are not just accidental (and there are virtually no accidents in biology) then it is very likely the baby actively injects cells to suppress maternal immune response. But, for them to be tolerated beyond weaning there must be a benefit to the mother. The suppressing of aging in the mother at least to the end of her biologically possible child raising years gives the properly synergistic explanation. It benefits both mother and child, which is a necessary requirement for such a chimera to exist.
In virtually all human populations, when childbearing deaths are allowed for, women survive significantly longer than do men. It has also been recently noted that bearing children late in life increases a woman’s chance of living to a hundred years old by an order of magnitude. From what is being learned about aging and death, it currently seems quite likely that (among healthy people) many or even most deaths from old age result from the death of small but critical tissues in the body. The kind of tissue where several hundred or at most a few thousand cells release some critical chemical into the body. It is precisely that kind of tissue most likely to be colonized and replaced, at least marginally, by juvenile stem cells.
Assume in a normal human culture that a woman approaching fifty has a baby. That baby has a vested interest in its mother maintaining her health till the child reaches maturity. In fact, since grandparents seem to play an important role in the survival of grandchildren in their earliest years, a child probably has a vested interest in their mother surviving in fair health at least till the child is in its mid twenties or thirties or even longer. Lingering fetal cells could postpone the leading causes of death. I propose that the seven or eight year survival edge of women (corrected for birth related deaths) over men is due to chimerical (two cell line) tissues in the mother. Specifically that certain critical extinct cell lines are replaced by her child’s stem cells circulating in her body. The existence of even one entirely foreign tissue anywhere in the mother’s body would probably be enough evidence to prove this solution and this theory. Like the original discovery, the detection could probably rely on male tissues found in a female’s body.
Finally, the third possible solution to the chimera mother (the mother containing two different cell lines in her body) is that the fetal cells in the mother’s blood contribute genes back to the mother's somatic cells to repair broken cellular mechanisms. This is not likely to be a popular proposal. It violates standard genetic dogma. But lateral gene flow, even between different species, is a proven fact. It is at least vaguely possible that virus like particles are able transfer useable genes from circulating fetal cells into maternal somatic or even reproductive cells. This could be a highly evolved mechanism for genetic repair in higher animals, but one that (at least in the first generation) only benefits females.
Many mathematical geneticists have floundered trying to explain the benefits of sexual reproduction, particularly for the mother. It seems sexual reproduction puts too much biological cost on the mother, and offers her only 50% returns on her investment (only half the genes of her offspring come from her). Logic would dictate that a genetic plan would be developed that would give her a genetic input more proportionate to her biological expenses. It is obvious that some degree of sexual reproduction is needed for diversity, biological immunity, fast adaptation, and numerous other reasons. But it is by no means obvious why the genes need to be sorted exactly at the 50% ratio. Indeed, there are creatures that alternate sexual and non-sexual reproduction to benefit the maternal gene pool. But they are not that numerous, and certainly not in the higher animal kingdom.
If, however, a mechanism existed for a mother to extract genetic advantage from fetal cells circulating in her body, then the fetal genes must be sufficiently alien to be able to replace her own faulty genes. It turns out that the ideal balance between reproduction and such gene healing is much closer to 50%.
So – what then is the advantage to the female of using sex to reproduce? Genetics indicates that full sexual reproduction must more than double the number of surviving offspring before it is worthwhile for the female. Rewarding females with a longer life, both for reproduction, and for enabling the survival of her offspring seems like a trade nature may have made.