Consciousness and how it got to be that way

Tuesday, July 30, 2013

Evolution and Rh Negativity in Historical Populations

A Kleihauer-Betke stain showing fetal blood cells circulating in maternal blood. Even 0.5 mL of fetal red
blood cells can be detected with this method. It can also be detected by the mother's immune system.

The Rh antigens on human blood cells always bothered me; what a terrible system. If a mother who does not have the Rh protein on her blood cells reproduces with a male that does, she may have a baby who also has the Rh protein. The first such baby is fine; but the mother stands a chance of getting enough of the baby's blood cells in her own circulation during the birthing process that her immune system sees it. Now, if she has another baby that is Rh+, her immune system will attack the baby's blood cells and destroy them. 5% of babies who are the second Rh+ baby of an Rh- woman will get hemolytic disease of the newborn (HDN), and be very anemic and sick; possibly die.

Structure of Rh factor. That's a channel-looking protein if ever there was one. From Mike Merrick at the John Innes Centre.

So what happened, I always wondered, before Rhogam? (The injection that stops an Rh- woman from making antibodies to Rh+ baby's blood cells.) Did we go through a hundred thousand years until 1960 with stillbirths and sick, anemic babies affected for their whole lives being born left and right, and life was just so scary and incomprehensible and miserable that we just accepted this?

I looked into this during my OB-GYN rotation and ended up giving a short talk about it. It turns out that Rh negativity is most a European thing; especially, it turns out, a Basque thing. Europeans are 15% Rh negative, Basques about 36%. The interesting connection is that the function of the Rh protein is an ammonia channel that, among other things, confers resistance to toxoplasma - which is spread through the feces of cats - which did not exist in pre-Holocene Europe. There's our answer! There was no pressure for the Rh factor be maintained once Europeans were in Europe, and the longer they were there, the more they lost it. So if the Rh factor is lost, no big deal, right?

Well, this still bothers me, for the simple problem that where we find Rh negativity, we don't find only Rh negativity. The Rh factor didn't disappear completely, so there's still a price to pay in terms of HDN and sick babies. In an all Rh- population, things would be fine; there would be no Rh-caused HDN, and no disadvantage to being an Rh- homozygote. But in the absence of an advantage to being Rh- (or even heterozygous) - which we're not aware of - it's a disadvantage, balanced against no advantage. I used to tell myself that maybe the mortality numbers were low so it didn't matter, but nature keeps close score. Even if Rh negativity is a little bit deleterious in terms of HDN, it should be selected out - eventually.

Using the Hardy-Weinberg equation, we can construct a simple model that tells us how rates of Rh negativity should change over time.

- Assume mated pairs have the same number of births on average, regardless of Rh status.

- Assume that for the second and further Rh+ babies born to an Rh- mother, 5% have HDN, and all these babies die. (They wouldn't have all died, even in the paleolithic, so the gene will disappear more slowly than this model predicts.) Future Rh+ babies get HDN at a higher rate than 5% but again, as long as I assume 100% death for them the gene will disappear more slowly than the model predicts.

- Assume there's no advantage to Rh negativity; its effect is entirely negative through HDN as above.

- Plug in current gene frequencies in Europe.

Since the gene will disappear more slowly than this model predicts due to the assumptions outlined above, we can project back based on current gene frequencies, and get dates that are probably slightly more recent than reality.

This model tells us that for the gene frequency to drop to 1% will take 565 generations - between 8,475 to 11,300 years, using a short generation time of 15 to 20 years. For the current Basque frequency of 36% Rh- (assuming it has remained static) to come down to the wider European frequency of 15% would have taken 208 generations, or between 3,120 and 4,160 years.

Assuming that Rh- was at fixation in proto-Basque populations in Iberia and Gascony, the introduction of just 1% Rh+ homozygotes would have taken 22,110 to 29,480 years to get the Basques to 36%. It doesn't escape notice here that these date ranges are all post African exodus, and some of them are within the scope of antiquity of the Near East.

Finally and most interestingly: assuming about an 8.6% injection in a 0 Rh- population of Rh+ 2,025 to 2,700 years ago, this would give us 36% today. That period coincides with the establishment of Phoenician and later Roman colonies in Iberia, and quickly established colonies and armies could easily introduce 1/12th the human DNA in the sparsely-populated Iberian peninsula. This assumes that during this time, Semitic and Indo-European people were Rh+.

It seems impossible without further research, particularly on ancient DNA, to distinguish between these three possibilities:

1) Rh negativity confers no advantage, and there was a founder effect that gradually eroded. Rh positivity was lost completely in a small ancestral Basque population, and very gradually Rh+ through HDN has been decreasing the proportion in the population; in turn Rh negativity has spread throughout the western half of the Old World.

2) Rh negativity confers no advantage, and a dramatic amount of Rh positivity was introduced in antiquity by migrants from around the Mediterranean. The high Rh negativity also seen in some parts of Africa could support a Phoenician mechanism of gene flow (Rh+ into Iberia, Rh- out).

3) Rh negativity does confer an advantage that partially or totally counterbalances the HDN problem that we are so far unaware of. Not exclusive of #1 or 2.

Rh factor isn't nearly the only immune incompatibility between maternal and fetal blood that can cause HDN, and we certainly have a lot to learn about the function of these markers.

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