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Why are human babies' heads so big?


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Human mothers have a hard time giving birth to babies compared to other mothers in the animal kingdom because human babies' heads are so big. One often reads that this is so because we humans have such big brains. But what does grown-up brain size have to do with baby brain size? After all, the huge human brain ultimately comes from one single cell, the zygote, so why don't human mothers simply give birth earlier? The brain can grow as much as it wants after birth, so why does it have to be already so big at birth?

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If you have a species with selective advantage in a large brain,  then a lot of neural development has to happen in utero,  in order to get a large brain.   The brain has to be somewhat prepared for perception and learning when it emerges from the womb,  for a variety of reasons you can probably figure out.  One example is the bonding with the parents. 

Another issue is bipedalism.   To achieve that,  narrower hips are needed.   If we had stayed quadrupeds, squeezing out those bigheaded babies would have been much easier.   

Also bear in mind neurons are extremely specialized cells that don't have time or resources to replicate themselves.      

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Iirc, the brain and body develop at different rates due to different requirements. We need a big enough head for the brain to fulfill various functions, but not so big that we can't fit through the birth canal, which is restricted in humans partly due to bipedalism. We start out with about 25% of our brain size at birth, and grow the rest after.

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30 minutes ago, Tristan L said:

The brain can grow as much as it wants after birth 

Can it? Or does it need some minimal functionality to make sure the body is working?

 

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17 hours ago, J.C.MacSwell said:

The human brain needs a head start.

Looking around society these days, it's safe to say some need this more than others.

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Of course the human baby's brain needs to have some basic ability at birth, but for that basic functionality, such a huge baby brain is a total overkill, isn't it? A chimpanzee baby's brain is also up to the job, so why don't human mothers give birth once their babies have chimpanzee-level intelligence, and then the babies' brains grow to grown-up human proportions as the child grows ip? Indeed, a reptilian brain is enough to perform all the bodily functions, so what would be wrong with giving birth while the human baby still has reptilian-level intelligence?

All the higher functions, e.g. bonding, can come later on, can't they?

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17 minutes ago, Tristan L said:

Of course the human baby's brain needs to have some basic ability at birth, but for that basic functionality, such a huge baby brain is a total overkill, isn't it? A chimpanzee baby's brain is also up to the job, so why don't human mothers give birth once their babies have chimpanzee-level intelligence, and then the babies' brains grow to grown-up human proportions as the child grows ip? Indeed, a reptilian brain is enough to perform all the bodily functions, so what would be wrong with giving birth while the human baby still has reptilian-level intelligence?

All the higher functions, e.g. bonding, can come later on, can't they?

The other factor is the mother. There is a huge metabolic cost for the mother developing a brain that will double in size within a year after birth. Ape brains are about 40% adult size at birth, compared to us at around 25%. At a certain point, the metabolism of the fetus exceeds the mother's ability to keep them both alive, and the mother gives birth.

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5 hours ago, iNow said:

Looking around society these days, it's safe to say some need this more than others.

Some of this fits us a little more than we'd like it to:

image.thumb.png.a0b35ea6159a0377490a321fe60e9eed.png

At least we have bigger walnuts...

...so maybe there's hope?

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I am not sure "Why?" is a valid question - babies with large heads in comparison with other mammals is how humans are when born; it is in our developmental genetics. "How?" might be better - which may answer why the balance between pre-birth and post-birth development shifted during evolution, which simply means those with the bigger heads at birth went on to survive and reproduce but those with smaller heads didn't. It is not just about the baby but the carrying mother; it is safe to assume those with heads too big/mothers too small did not survive. It does not even follow that those with smaller birth size failed because of their smaller birth size; the crucially significant baby born with genetics for smaller births size but capable of growing a big brain may have gotten eaten by a hyena and all human evolution after that was altered forever.

Those that were born better able to function more independently at birth and needed less parental care somehow did not go on to thrive - and not necessarily because of any direct relationship; other factors can and will be in play. We can list the advantages and disadvantages and hope we haven't overlooked some but it cannot really give a definitive answer

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8 hours ago, Ken Fabian said:

I am not sure "Why?" is a valid question - babies with large heads in comparison with other mammals is how humans are when born; it is in our developmental genetics. "How?" might be better - which may answer why the balance between pre-birth and post-birth development shifted during evolution, which simply means those with the bigger heads at birth went on to survive and reproduce but those with smaller heads didn't. It is not just about the baby but the carrying mother; it is safe to assume those with heads too big/mothers too small did not survive. It does not even follow that those with smaller birth size failed because of their smaller birth size; the crucially significant baby born with genetics for smaller births size but capable of growing a big brain may have gotten eaten by a hyena and all human evolution after that was altered forever.

Those that were born better able to function more independently at birth and needed less parental care somehow did not go on to thrive - and not necessarily because of any direct relationship; other factors can and will be in play. We can list the advantages and disadvantages and hope we haven't overlooked some but it cannot really give a definitive answer

Great answer IMO.

Very interesting aspects have surfaced here that no doubt played a role in evolutionary history of humans and their brains. But as Ken points out --in my own words-- we must keep in mind that evolution is multi-factor, intricate, arguably chaotic. A particular development that could be advantageous if you, as an animal, have a lifestyle 'on-the-run', could be detrimental if you are sedentary; advantageous if you're a predator, detrimental if you're more of a gatherer, ineffectual if you're a scavenger; advantageous if you live in the tropics... Etc. You get the picture.

I think what the OP suggests as a possibility could well have happened --had one or many of the circumstances that constrained human way of life been different. Maybe modern humans would be like pandas, which have some of the most extreme ratios in size between infants and adults.

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Panda walks into a bar,  orders lunch and a beer.   When it's finished its meal,  it draws a pistol from a holster and fires it into the ceiling,  then walks out. 

A patron nearby yells,  "what the hell was that about? "

Bartender produces a dictionary,  shows patron the entry for "panda. "

Patron reads panda (n) : an Asian mammal that eats shoots and leaves... 

 

Anyway,  with all the adaptive complexity of thriving in some ecological niche, I think a key factor may be the challenge of producing new neurons (they don't replicate,  recall, but are grown from ESC) when struggling with the challenges of being ex utero.   So if a hundred billion is adaptive,  they are best in place from Day One.   ( if you've followed research on stroke and TBI victims, you have heard that new neurons can appear in some areas,  but this is a slow laborious process that uses stem cells)(so wouldn't apply well to newborns who are quickly developing their "software")  Remember that babies first develop their brains by growing (then pruning,  after age two) synaptic  connections rather than by neurogenesis.  

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On 8/18/2021 at 12:59 PM, Tristan L said:

Of course the human baby's brain needs to have some basic ability at birth, but for that basic functionality, such a huge baby brain is a total overkill, isn't it? A chimpanzee baby's brain is also up to the job, so why don't human mothers give birth once their babies have chimpanzee-level intelligence, and then the babies' brains grow to grown-up human proportions as the child grows ip?

You make it sound as if bad planning was involved, when in fact NO planning was involved. Evolution starts with what exists, it doesn't say "since I'm about to develop big brains I want a 'do-over' on the pelvic width developed so we don't run into any birthing issues due to larger heads."

Since there was no 'plan' for human development we have to accept that many body parts and functions are not a good fit with other body parts and functions.

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  • 2 weeks later...

Thank you all for your interesting and information-giving answers!

@zapatos
Of course no planning is involved, that's correct. It's just easier to say the short and onefold/simple "Feature A evolved so as to do function F" than the cumbersome "By random chance, some individuals had A and others didn't, and since A does the bootful/advantageous function F, individuals with A-giving genes spawned more successfully/spowfully than individuals without them, and so A-giving genes became commoner in the population over time".

I'm asking why we didn't evolve to give birth earlier. Of course it could be accidental, but since the fit of the baby's head size to the mother's birth canal size is a tight one, there's a good chance that there's a reason for this. Is it that smarter brains don't simply go through stages that correspond to less smart ones? That is, might it pehaps be the case that the human brain never is at a chimpanzee brain's level - when the human brain is as functional as the chimpanzee's, it's already smarter, and when it's as smart as the chimpanzee's, it's not yet functional enough?

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