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A recent study in genetic variance - The Ex-Communicator

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September 27th, 2008


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01:15 pm - A recent study in genetic variance
andyduckerlinks links to this new scientist study. I read about this research somewhere else over the summer (wish I could remember where) and I was thinking about it a lot while I was in Brittany. The basic research as I understand it shows that there is twice as much variance in the DNA we have inherited from female ancestors as from male (this x2 figure isn't given in the linked article, sorry I can't find the reference). But in any case, roughly, we have twice as many female ancestors as male ancestors.

The way the male scientists report this is 'Some men mated with lots of women', which is one way of putting it. Another way of putting it is that for an extended period in prehistory men were twice as likely as women to die without reproducing themselves. And when you consider how dangerous it must have been for women to give birth in a cave, this means that men must have been dying at a rate we don't see in any population these days: more than 50% of men must have died without any offspring at all. If you think that more that two thirds of the men and women alive in the Middle Ages have offspring alive today (despite the black death etc.) then you can see that there must have been a male-skewed die-off quite unlike anything in recorded history.

It's possible there was some terrible disease or parasite which affected men only - those immune have left offspring. Or perhaps there was some male-only occupation (hunting or war perhaps) which was way, way more dangerous than the most dangerous hunting or war we know of. So perhaps the tribes consisted largely of women, with a small number of adult men, many of them severely injured or sick.

Or perhaps there were lots of adult men, but a few of them exercised such iron fist control over the rest that most men never had sex. There is nothing like this in any human society today, except a small number of extreme religious cults. It's hard for me to imagine how this could be enforced in a hunter-gatherer society, I would imagine that young men and women would just run off. But perhaps it happened.

Whatever the reason, daughters would be twice as likely as sons to pass on DNA, so in genetic terms they were a much more secure 'bet'. Food and other resources may therefore have been directed more at girls, and this may have resulted in a feedback loop - girls are more likely to survive, so they are given more resources, so they survive etc. Thus the small natural survival advantage of girl babies over male may have been amplified by child rearing practice.

So, there are many possible ways that we could have ended up with this strange pattern of variance. I think all we can conclude is that ancient society was quite unlike anything that exists nowadays. A society of scarce and damaged men, or a society of male warrior-virgins, or of pampered daughters, nobody knows. But 'men mated with lots of women' doesn't really capture the level of strangeness these findings imply.

(of course it's possible the whole study is founded on incorrect notions of DNA creep and repair, or their model is way off, but I'm taking it here at face value)

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From:hafren
Date:September 27th, 2008 03:22 pm (UTC)
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And when you consider how dangerous it must have been for women to give birth in a cave

Maybe it wasn't. After all, the non-naked apes get by without medical intervention. Perhaps human parturition got more dangerous as life generally got more complicated.
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From:communicator
Date:September 27th, 2008 03:39 pm (UTC)
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I do wonder about this sometimes. In countries where there is little medical treatment available a lot of women do die. I've heard that about one in seven births end in the mother's death, if there is no medical intervention.

I don't think we have brilliant data on this though, because women who can't get medical treatment often have other problems to deal with, like undernourishment.

I personally would have died, and that colours my views pretty strongly.

Edited at 2008-09-27 03:40 pm (UTC)
From:mathew5000
Date:October 4th, 2008 09:36 pm (UTC)
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It’s true that hardly any female gorillas (for example) die in childbirth; the reason is that newborn gorillas weigh only about 2 kg. Human newborns are larger than gorilla newborns, and the human newborns are much larger than gorilla newborns considered as a fraction of their mothers’ size.

The large relative size of human newborns is the reason that humans die in childbirth at such a high rate compared to other primates. This is not something that began recently.
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From:hafren
Date:October 6th, 2008 05:53 am (UTC)
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That's fascinating - do you know why it should be - other, of course, than the fact that the human female body was clearly designed by Bloody Stupid Johnson? Intelligent design, my arse....
From:(Anonymous)
Date:October 6th, 2008 06:04 am (UTC)
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There must be some evolutionary tradeoff but I don’t know what it is.
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From:communicator
Date:October 6th, 2008 08:16 am (UTC)
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not sure if you are mathew500?

My feeling is that it's because Evolution is ruthless - the big brain gave enough of a species advantage to outweigh the deaths of a significant proportion of young women. If there had been a competitor species with big brain and better birth arrangements, it might have replaced us, but there wasn't.

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From:fjm
Date:September 27th, 2008 04:22 pm (UTC)
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First, it's a myth that everyone mates. For much of human history one had to have money or property to get married/mate. Lots of people matched late (average age of first child for a woman is pretty consistent from about 1100 as early twenties) and many, many didn't match at all, instead staying in the houses where they accepted work. Any levels of polyamy reinforce this pattern even if technically it is serial monogamy caused by death.

Second, war takes out a lot of men. There seems to have been a man shortage in Europe from about 1790 thru to the 1920s (made much, much worse by WWI).

Third, occupational injury takes out a lot of men: in colonial Virginia women married three or four times.

Fourth, men *are* weaker than women and more vulnerable to disease. Of the 110 men born to every 100 women, 10 of the men won't make it to 15.

Fifth, when genetic surveys have been done of closed, and very rigid communities, almost all the adultery is high status men to lower status women.

Hope that helps.
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From:executrix
Date:September 27th, 2008 04:50 pm (UTC)
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One reason that prostitution is nicknamed "the oldest profession" is that in many cultures, contacts with prostitutes were very common among unmarried as well as married males, so financial inability to marry is not necessarily tantamount to never having sex. Cultures vary greatly in their attitudes about the sinfulness of nonmarital sex, and individuals vary greatly in how much attention they pay to these strictures.

I think the prevalence of death while giving birth is part of--well, depending on who you think is in charge--God's or Nature's way of maintaining population equilibrium; fewer surviving mothers means fewer maternal and potential-subsequent-infant mouths to feed.

However, what God, Nature, etc. really has a vendetta against is fertilized eggs--I believe about three-quarters of them fail to implant.
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From:fjm
Date:September 27th, 2008 05:28 pm (UTC)
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Re the first: where prostitution is legal, it tends to be a temporary occupation.

I like the way you put the last point!
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From:communicator
Date:September 27th, 2008 04:54 pm (UTC)
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But the research reveals (or claims to reveal) a pattern of reproduction in the past which is radically different from that found anywhere in the world today. Of course not everybody reproduces, but in the world nowadays the same number of men as women reproduce. This despite for instance the two world wars, and different occupational patterns, and a small amount of polygyny, official or otherwise.

In the past this was not the case. At least according to the research. It is the difference which is significant.

The scientists are describing the difference in one way, I am pointing out that exactly the same thing could be described in a different way.

Thus - yes, men are weaker - hence my hypnothesis that in the past this was exacerbated by different social choices, whereas nowadays the disadvantage is compensated for (for instance by child rearing practices).

ETA - I should say that my guess is that their mathematical modelling is bad, and the differences they have detected are due to the cumulative effect of minute multi-generational bias, so that neither male reproductive failure or mating patterns were as different from today as they assert. But that is only a guess, because I haven't seen their workings.

But for the sake of discussion I'm assuming that they are right.

Edited at 2008-09-27 05:13 pm (UTC)
[User Picture]
From:emeraldsedai
Date:September 27th, 2008 05:20 pm (UTC)
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Whatever the reason, daughters would be twice as likely as sons to pass on DNA, so in genetic terms they were a much more secure 'bet'. Food and other resources may therefore have been directed more at girls, and this may have resulted in a feedback loop - girls are more likely to survive, so they are given more resources, so they survive etc. Thus the small natural survival advantage of girl babies over male may have been amplified by child rearing practice.

This almost sounds like an evolutionary "reason" for the control human society generally seems to require over women's bodies.
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From:communicator
Date:September 27th, 2008 05:31 pm (UTC)
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I should say that I'm kind of taking the study mock-seriously, because I think the maths is probably wrong. But if it were true I think it would overturn what happens nowadays, because girls are undervalued, and the emphasis is on male reproduction. But logically, if the study is correct, the implication is that girls were much more important in the past than they are now.
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From:emeraldsedai
Date:September 27th, 2008 05:44 pm (UTC)
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I agree on all counts, and particularly on the "wait and see if they did the numbers right" part. I would only argue that "girls being important" and "girls being protected from their own free will" aren't necessarily mutually exclusive ideas.
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From:azdak
Date:September 27th, 2008 06:39 pm (UTC)
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I simply don't know enough pre-history to comment on this in an informed fashion, but there are two points I'd like to make which may or may not be relevant. Firstly, the difference in body size between men and women ("sexual dimorphism") implies competition between males for access to females - the fact that the differences aren't as great as between, say, male and female elephant seals, suggests that more human males managed to mate than do bull elephant seals; but since the difference is great than that between males and females of species who are strictly monogamous, it's seems likely that being able to beat up the opposition was an important factor in successful mating. And that in turn implies that the big blokes had a lot more sex than the skinny runts. Secondly, in biblical Israel the high status males had lots of wives and the low status ones no wives, which would fit exactly with both the pattern of sexual dimorphism and the lower degree of variance in male DNA. So perhaps it wasn't such a weird and unlikely social structure.
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From:communicator
Date:September 28th, 2008 01:14 pm (UTC)
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Yes, my argument isn't that it didn't happen (my suspicion as I say is that they might have messed up the maths, but for the sake of argument, let us say they are right). My argument is that you can look at the same picture two ways - you can say 'men had multiple wives' or 'most men didn't have a wife at all'. Perhaps these spare men were driven out of the community to live alone? Perhaps they were actually killed?

Perhaps the attempt to control women in modern societies is so that men can live together without always trying to kill each other?
From:mathew5000
Date:October 4th, 2008 10:02 pm (UTC)
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These research results do not necessarily lead to the conclusion that “more than 50% of men must have died without any offspring at all”. There are several other explanations, for example philandering: suppose that a certain percentage of men (say 5%) are extreme philanderers, while other men are either moderate philanderers or monogamous. Then in a particular time period, you might have had that small group of extreme philanderers fathering, on average, 30-40 offspring in a lifetime, while other men might father, on average, 2-3 offspring, and women might have, say, 3-5 offspring. This would account for the higher rate of variation in the X chromosome than in other parts of human DNA, without polygyny and without a male-skewed die-off.
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From:communicator
Date:October 5th, 2008 07:02 am (UTC)
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Yes, this was what I meant by the comments above that I think they may have screwed up the maths. Their assertion is that their findings prove a time of widespread polygyny, whereas like you I was thinking it could be the cumulative effect of a bias which is small but always in one direction. I wondered whether it couldn't be the result of a consistent but minor trend for some men to father children in more than one generation, for instance. This is consistently gender-skewed, because of the menopause, so it might build up over time (or not - I can't grasp the mathematical model well enough).

But I suppose the main point of what I wrote was to say that the scientists hadn't really thought through the implications of what they said. They were interpreting the evidence through their own gender and social bias. The decision to say 'men had two wives' rather than 'half the men didn't reproduce' is a biased decision - and teasing out the implications of the latter way of expressing it makes it clearer what a strange society that would be. The former way of expressing it slips easily into acceptance because it harmonises with modern hierarchies of power; the latter doesn't.

FWIW I mentioned the cross-generational thing. I also wonder whether the concept of marriage itself might be relatively new. Perhaps women were at liberty to choose men on a short term whim, and a particularly attractive man would end up fathering a lot of children.

Anyway, I could go on generating scenarios which might be consistent with the evidence because it's fun, but the point is that when studying humans people are not as free-thinking or objective as they like to say they are.

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