Caesarean Births & Human Evolution

[rynner2]

Caesarean births 'affecting human evolution'
By Helen Briggs BBC News

The regular use of Caesarean sections is having an impact on human evolution, say scientists.
More mothers now need surgery to deliver a baby due to their narrow pelvis size, according to a study.
Researchers estimate cases where the baby cannot fit down the birth canal have increased from 30 in 1,000 in the 1960s to 36 in 1,000 births today.

Historically, these genes would not have been passed from mother to child as both would have died in labour.
Researchers in Austria say the trend is likely to continue, but not to the extent that non-surgical births will become obsolete.

Dr Philipp Mitteroecker, of the department of theoretical biology at the University of Vienna, said there was a long standing question in the understanding of human evolution.
"Why is the rate of birth problems, in particular what we call fetopelvic disproportion - basically that the baby doesn't fit through the maternal birth canal - why is this rate so high?" he said.
"Without modern medical intervention such problems often were lethal and this is, from an evolutionary perspective, selection.
"Women with a very narrow pelvis would not have survived birth 100 years ago. They do now and pass on their genes encoding for a narrow pelvis to their daughters."

etc...

http://www.bbc.co.uk/news/science-environment-38210837

Evolution has increased human brain size, which leads to birth difficulties. But a baby's brain doesn't grow to 'normal' size until after birth, and this rapid growth and development loses our earliest memories. Any woman who needs a Caesarean should probably avoid having more children for the good of the gene pool.

 

[PeteS]

Interesting information that and of course when you think about it absolutely logical. Puts a mockers on the view held by some that the increase in caesarean births is solely down to mothers not wanting to undergo the pain of natural childbirth.

 

[PeteS]

Interesting information that and of course when you think about it absolutely logical. Puts a mockers on the view held by some that the increase in caesarean births is solely down to mothers not wanting to undergo the pain of natural childbirth.

Heard part of an interview on R2 with Dr Mitteroecker today quite logical stuff really. Interesting though that despite a couple of questions on the subject he would not be drawn out on the subject of the increase in caesareans for cosmetic or convenience reasons since that was "not part of his research."

 

[Ermintruder]

Historically, these genes would not have been passed from mother to child as both would have died in labour.

Insufficient generational steps have occured since the inception of caesarian sugical deliveries for this to have factorially-contributed towards meaningful dominant inheritance effects. To conclude that average perinatal skull size has modally increased across the herd via this mechanism is epigenetic claptrap.

Average bipirietal diameters have been driven up through trends towards ultra-term gestations (cf 'overbake'), supra-nominal fetal nutrition, pathological maternal hyperglycemia and; the inarguable main background driver for big-headed sapiomorphic humans (inferentially-cited as such, within the main originating thrust of this thread) being cranial size as an emergent selected-for characteristic.

I think there's illogical causality going on here. Come back and claim this in 2116, and it might be a defendable hypothesis.

 

[Mythopoeika]

Insufficient generational steps have occured since the inception of caesarian sugical deliveries for this to have factorially-contributed towards meaningful dominant inheritance effects. To conclude that average perinatal skull size has modally increased across the herd via this mechanism is epigenetic claptrap.

Average bipirietal diameters have been driven up through trends towards ultra-term gestations (cf 'overbake'), supra-nominal fetal nutrition, pathological maternal hyperglycemia and; the inarguable main background driver for big-headed sapiomorphic humans (inferentially-cited as such, within the main originating thrust of this thread) being cranial size as an emergent selected-for characteristic.

I think there's illogical causality going on here. Come back and claim this in 2116, and it might be a defendable hypothesis.

Yes, I think you have a point there. Caesareans haven't been around for long enough/used often enough to have caused some form of evolutionary change. The causality is all wrong. I think it's far more likely that changes in diet and exercise may have caused babies to put on weight generally (as you note above), and perhaps a woman's exercise regime/different hormonal levels may cause slimmer hips to arise.
I certainly haven't noticed a dramatic increase in cranial size of people. Perhaps when we start to look like The Mekon, then it's time to get worried.

 

[EnolaGaia]

Insufficient generational steps have occured since the inception of caesarian sugical deliveries for this to have factorially-contributed towards meaningful dominant inheritance effects. ...

There's nothing in the study alluding to demographic, much less evolutionary, much less genetic 'dominance'.

The history of incidental caesarean deliveries extends back centuries, and the history of 'relatively successful' practices goes back to the last half of the 19th century. You're right that there haven't been a lot of generations since the practice's widespread propagation (let's say 1950 onward). However ...

Manifesting a shift in genetic / evolutionary terms takes as many generations as are necessary to make a trait prevalent in a line of genetic descent (e.g., 'vertically' down the tree of all family trees). In the case of technologically-influenced factors or traits the rate of overall propagation can be accelerated by parallel adoption (e.g., 'horizontally' across the tree of all family trees), which isn't delimited by generational increments.

Caesarean procedures purportedly account for 33% of births in the USA and 46% of births in China.

 

[EnolaGaia]

... To conclude that average perinatal skull size has modally increased across the herd via this mechanism is epigenetic claptrap. ...

I would generally agree with the thrust of your comments if the cited study had focused on the points your post insinuates it had emphasized - e.g., fetal head dimensions and diachronic trends pertaining to them. However, the study doesn't focus on such specific traits at all.

As is all too common when observing via the lens of generic (as contrasted with technical / scientific) journalism, the news articles I've seen referring to this study seem to unanimously filter or twist the story - particularly with regard to the researchers' scope of concern and espoused implications.

For the record ... The published paper is accessible in PDF format at:

http://www.pnas.org/content/early/2016/11/29/1612410113.full.pdf

Let me mention what I see as some key points to consider ...

(1) The study focuses on the issue of fetopelvic disproportion - the general case in which the fetus cannot 'naturally' exit owing to size issues pertaining to the fetus and / or the mother's pelvic area (and / or her organs therein). The issue of cephalopelvic disproportion denotes that subset of fetopelvic disproportion strictly attributed to the fetus' head / cranial size. Yes - most cases of fetopelvic disproportion involve the fetus' head size. No - it is not the case that (a) all fetopelvic disproportion problems are attributable to the fetus' head size, nor (b) all cases in which head size poses problems are problematic with respect to head size alone.

(2) It 'takes two to tango' ... The 'disproportion' effect qualifying a case for categorization under either or both the above-cited labels results from the comparative relationship between fetus and pelvic sizes. An atypically large fetus can result in fetopelvic disproportion affecting a mother whose pelvis is (statistically) 'normal'. An attribution of disproportion doesn't imply blame uniquely ascribable to one or the other party involved.

(3) The authors clearly state that environmental factors (above and beyond genetically heritable traits) can and / or do play a role.

(4) The authors are presenting an exercise in statistical analysis addressing a wide swath of phenomena and effects as a method for suggesting trends in selective pressures (not necessarily selective outcomes). If anything, it's more an exercise in epidemiology than evolutionary biology per se, and its implications have as much or more to do with socio-technical factors (e.g., increasing reliance on expertise or technology in everyday childbirth) as (neo?-) Darwinian natural selection per se.