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#1
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With all the furor of the recent article in Nature (in which the MHC
mutation/brain size hypothesis is not being well received in the academic community), this occurred on one of the science usenet groups I read: Big Brains and Bipedalism One of the reasons often quoted for the limit on human brain size, is the restriction placed on head size due to childbirth. In bipedal humans, childbirth poses a serious risk for both mother and child (mortality rates increasing for both), because the newborn must now pass out of a narrow birth canal between the narrower pelvis hips (which have evolved for an upright posture). This is a problem not experienced to the same degree by quadrupeds. It is now well established that bipedalism evolved first followed by large brains. To me this seems something of a paradox because we have a trait, bipedalism (with its associated narrower birth canal and increased mortality) which would seem to place selective pressure on smaller heads (and associated brains), not larger. The adaptive value of larger brains must now be explained as being very significant in order to overcome this reverse trend which would have been expected. I would now like to suggest an alternate hypothesis, which is, that larger brains are a direct result of bipedalism and difficult child birth rather than the opposite. Let me explain. Because of bipedalism and its associated difficult childbirth, evolution has come up with a partial solution for the big head problem, (neoteny being one), and also a softer, (less rigid) more plastic or malleable skull during childbirth. Humans at birth have not only a hole in the top of the skull but also the skull cap is extremely soft with radial unhealed fractures running in all directions. (If you have every seen a newborn immediately after birth, their heads look like squashed prunes). Only bipedal humans have this trait, quadrupeds have much more hard and rigid skulls with little deformation (its not needed), their skulls size (while they still grow) are more set at birth. Could it be that the physiology of brain growth is such that they simply grow to their container size? and that the pressure of a growing brain in a softer more plastic skull has resulted in a larger brain before the skull matures (heals) into a more rigid one. Skulls do not grow to accommodate a predetermined brain size, the opposite is true, brains grow to fit a genetically determined skull size. When you think about it, brains exactly fit skulls. This may seem intuitively obvious, but wait, other body organs cannot follow this growing strategy, because they are not enclosed (and subsequently restrained) in a rigid structure like a boney skull. This is why (as any surgeon will attest) organs can and do vary in size constrained only by an inherited genetic growth limit. Is it possible that brains follow a different growth strategy. The physiology of brain growth may be different from other organs, relying more on the pressure of the container size to restrict growth rather than having a preset genetic size, (the folds in the brain may have something to do with this pressure growth relation). One observation seems to confirm this, which is, have you ever heard of a person with a large skull but a small brain, the answer is of course no, brains always grow to container size. If brain size of an individual is predetermined by inherited genetics (skull size most certainly is) we should expect statistically to see varying sizes of brains to skulls (small brain, large skull, would be one) and of course we don't. If this idea is correct, large Brains would now seem to be a physiological development phenomenon rather than a naturally selected adaptive trait. The traits usually quoted as the adaptive reasons for big brains, speech, mental maps, group hunting, social situation manipulators, heat radiators, tool use, dexterity (opposable thumbs) etc. can now be viewed as exaptations (which are not the reasons for large brains but a consequence of them), which have found utility within the confines of a larger brain. Of course once any or all of the above traits become incipient, a positive feedback loop would set in reinforcing the value of the larger brain. So there it is, 1- Bipedalism evolves for some reason, 2- Childbirth with rigid skulls increases mortality rates, 3- Soft, plastic more malleable skulls are naturally selected for, 4- Neotenist brain growth restricted by the pressure of the container now pushes on a more malleable skull, 5- Large brains. .... Could it be this simple .... Gould would have liked this, he favors more random, contingent processes. Are there any stats to indicate larger [or smaller] brains in c sections? Any correlations with IQ? (I believe there is). A good Grad thesis would be to remove mouse embryos and place radial cuts in skull caps, replace to term and measure brain growth to size. Anyway, this is all off the top of my hard rigid head." Beelzibub The human in us owes fealty to humanity. But the wolf in us acknowledges no master. |
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On Sat, 27 Mar 2004 08:23:50 -0600, elzinator
wrote: With all the furor of the recent article in Nature (in which the MHC mutation/brain size hypothesis is not being well received in the academic community), this occurred on one of the science usenet groups I read: Big Brains and Bipedalism One of the reasons often quoted for the limit on human brain size, is the restriction placed on head size due to childbirth. In bipedal humans, childbirth poses a serious risk for both mother and child (mortality rates increasing for both), because the newborn must now pass out of a narrow birth canal between the narrower pelvis hips (which have evolved for an upright posture). This is a problem not experienced to the same degree by quadrupeds. It is now well established that bipedalism evolved first followed by large brains. To me this seems something of a paradox because we have a trait, bipedalism (with its associated narrower birth canal and increased mortality) which would seem to place selective pressure on smaller heads (and associated brains), not larger. The adaptive value of larger brains must now be explained as being very significant in order to overcome this reverse trend which would have been expected. I would now like to suggest an alternate hypothesis, which is, that larger brains are a direct result of bipedalism and difficult child birth rather than the opposite. Let me explain. Because of bipedalism and its associated difficult childbirth, evolution has come up with a partial solution for the big head problem, (neoteny being one), and also a softer, (less rigid) more plastic or malleable skull during childbirth. Humans at birth have not only a hole in the top of the skull but also the skull cap is extremely soft with radial unhealed fractures running in all directions. (If you have every seen a newborn immediately after birth, their heads look like squashed prunes). Only bipedal humans have this trait, quadrupeds have much more hard and rigid skulls with little deformation (its not needed), their skulls size (while they still grow) are more set at birth. Could it be that the physiology of brain growth is such that they simply grow to their container size? and that the pressure of a growing brain in a softer more plastic skull has resulted in a larger brain before the skull matures (heals) into a more rigid one. Skulls do not grow to accommodate a predetermined brain size, the opposite is true, brains grow to fit a genetically determined skull size. When you think about it, brains exactly fit skulls. This may seem intuitively obvious, but wait, other body organs cannot follow this growing strategy, because they are not enclosed (and subsequently restrained) in a rigid structure like a boney skull. This is why (as any surgeon will attest) organs can and do vary in size constrained only by an inherited genetic growth limit. Is it possible that brains follow a different growth strategy. The physiology of brain growth may be different from other organs, relying more on the pressure of the container size to restrict growth rather than having a preset genetic size, (the folds in the brain may have something to do with this pressure growth relation). One observation seems to confirm this, which is, have you ever heard of a person with a large skull but a small brain, the answer is of course no, brains always grow to container size. If brain size of an individual is predetermined by inherited genetics (skull size most certainly is) we should expect statistically to see varying sizes of brains to skulls (small brain, large skull, would be one) and of course we don't. If this idea is correct, large Brains would now seem to be a physiological development phenomenon rather than a naturally selected adaptive trait. The traits usually quoted as the adaptive reasons for big brains, speech, mental maps, group hunting, social situation manipulators, heat radiators, tool use, dexterity (opposable thumbs) etc. can now be viewed as exaptations (which are not the reasons for large brains but a consequence of them), which have found utility within the confines of a larger brain. Of course once any or all of the above traits become incipient, a positive feedback loop would set in reinforcing the value of the larger brain. So there it is, 1- Bipedalism evolves for some reason, 2- Childbirth with rigid skulls increases mortality rates, 3- Soft, plastic more malleable skulls are naturally selected for, 4- Neotenist brain growth restricted by the pressure of the container now pushes on a more malleable skull, 5- Large brains. .... Could it be this simple .... Gould would have liked this, he favors more random, contingent processes. Are there any stats to indicate larger [or smaller] brains in c sections? Cool, I was already thinking about caesarians before I read this. But it seems like it might take a couple of generations to show up significantly. Guess it would depend somewhat on whether the selection process just leads to hard-headed kids, or if perhaps the birthing process itself is important, that is, priming the skull to be more malleable somehow. Any correlations with IQ? (I believe there is). A good Grad thesis would be to remove mouse embryos and place radial cuts in skull caps, replace to term and measure brain growth to size. Anyway, this is all off the top of my hard rigid head." Beelzibub The human in us owes fealty to humanity. But the wolf in us acknowledges no master. Proton Soup |
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On Sat, 27 Mar 2004 08:23:50 -0600, elzinator
wrote: With all the furor of the recent article in Nature (in which the MHC mutation/brain size hypothesis is not being well received in the academic community), this occurred on one of the science usenet groups I read: Big Brains and Bipedalism One of the reasons often quoted for the limit on human brain size, is the restriction placed on head size due to childbirth. In bipedal humans, childbirth poses a serious risk for both mother and child (mortality rates increasing for both), because the newborn must now pass out of a narrow birth canal between the narrower pelvis hips (which have evolved for an upright posture). This is a problem not experienced to the same degree by quadrupeds. It is now well established that bipedalism evolved first followed by large brains. To me this seems something of a paradox because we have a trait, bipedalism (with its associated narrower birth canal and increased mortality) which would seem to place selective pressure on smaller heads (and associated brains), not larger. The adaptive value of larger brains must now be explained as being very significant in order to overcome this reverse trend which would have been expected. I would now like to suggest an alternate hypothesis, which is, that larger brains are a direct result of bipedalism and difficult child birth rather than the opposite. Let me explain. Because of bipedalism and its associated difficult childbirth, evolution has come up with a partial solution for the big head problem, (neoteny being one), and also a softer, (less rigid) more plastic or malleable skull during childbirth. Humans at birth have not only a hole in the top of the skull but also the skull cap is extremely soft with radial unhealed fractures running in all directions. (If you have every seen a newborn immediately after birth, their heads look like squashed prunes). Only bipedal humans have this trait, quadrupeds have much more hard and rigid skulls with little deformation (its not needed), their skulls size (while they still grow) are more set at birth. Could it be that the physiology of brain growth is such that they simply grow to their container size? and that the pressure of a growing brain in a softer more plastic skull has resulted in a larger brain before the skull matures (heals) into a more rigid one. Skulls do not grow to accommodate a predetermined brain size, the opposite is true, brains grow to fit a genetically determined skull size. When you think about it, brains exactly fit skulls. This may seem intuitively obvious, but wait, other body organs cannot follow this growing strategy, because they are not enclosed (and subsequently restrained) in a rigid structure like a boney skull. This is why (as any surgeon will attest) organs can and do vary in size constrained only by an inherited genetic growth limit. Is it possible that brains follow a different growth strategy. The physiology of brain growth may be different from other organs, relying more on the pressure of the container size to restrict growth rather than having a preset genetic size, (the folds in the brain may have something to do with this pressure growth relation). One observation seems to confirm this, which is, have you ever heard of a person with a large skull but a small brain, the answer is of course no, brains always grow to container size. If brain size of an individual is predetermined by inherited genetics (skull size most certainly is) we should expect statistically to see varying sizes of brains to skulls (small brain, large skull, would be one) and of course we don't. If this idea is correct, large Brains would now seem to be a physiological development phenomenon rather than a naturally selected adaptive trait. The traits usually quoted as the adaptive reasons for big brains, speech, mental maps, group hunting, social situation manipulators, heat radiators, tool use, dexterity (opposable thumbs) etc. can now be viewed as exaptations (which are not the reasons for large brains but a consequence of them), which have found utility within the confines of a larger brain. Of course once any or all of the above traits become incipient, a positive feedback loop would set in reinforcing the value of the larger brain. So there it is, 1- Bipedalism evolves for some reason, 2- Childbirth with rigid skulls increases mortality rates, 3- Soft, plastic more malleable skulls are naturally selected for, 4- Neotenist brain growth restricted by the pressure of the container now pushes on a more malleable skull, 5- Large brains. .... Could it be this simple .... Gould would have liked this, he favors more random, contingent processes. Are there any stats to indicate larger [or smaller] brains in c sections? Cool, I was already thinking about caesarians before I read this. But it seems like it might take a couple of generations to show up significantly. Guess it would depend somewhat on whether the selection process just leads to hard-headed kids, or if perhaps the birthing process itself is important, that is, priming the skull to be more malleable somehow. Any correlations with IQ? (I believe there is). A good Grad thesis would be to remove mouse embryos and place radial cuts in skull caps, replace to term and measure brain growth to size. Anyway, this is all off the top of my hard rigid head." Beelzibub The human in us owes fealty to humanity. But the wolf in us acknowledges no master. Proton Soup |
#4
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![]() "elzinator" wrote in message ... With all the furor of the recent article in Nature (in which the MHC mutation/brain size hypothesis is not being well received in the academic community), this occurred on one of the science usenet groups I read: Big Brains and Bipedalism One of the reasons often quoted for the limit on human brain size, is the restriction placed on head size due to childbirth. In bipedal humans, childbirth poses a serious risk for both mother and child (mortality rates increasing for both), because the newborn must now pass out of a narrow birth canal between the narrower pelvis hips (which have evolved for an upright posture). This is a problem not experienced to the same degree by quadrupeds. It is now well established that bipedalism evolved first followed by large brains. To me this seems something of a paradox because we have a trait, bipedalism (with its associated narrower birth canal and increased mortality) which would seem to place selective pressure on smaller heads (and associated brains), not larger. The adaptive value of larger brains must now be explained as being very significant in order to overcome this reverse trend which would have been expected. I would now like to suggest an alternate hypothesis, which is, that larger brains are a direct result of bipedalism and difficult child birth rather than the opposite. Let me explain. Because of bipedalism and its associated difficult childbirth, evolution has come up with a partial solution for the big head problem, (neoteny being one), and also a softer, (less rigid) more plastic or malleable skull during childbirth. Humans at birth have not only a hole in the top of the skull but also the skull cap is extremely soft with radial unhealed fractures running in all directions. (If you have every seen a newborn immediately after birth, their heads look like squashed prunes). Only bipedal humans have this trait, quadrupeds have much more hard and rigid skulls with little deformation (its not needed), their skulls size (while they still grow) are more set at birth. Could it be that the physiology of brain growth is such that they simply grow to their container size? and that the pressure of a growing brain in a softer more plastic skull has resulted in a larger brain before the skull matures (heals) into a more rigid one. Skulls do not grow to accommodate a predetermined brain size, the opposite is true, brains grow to fit a genetically determined skull size. When you think about it, brains exactly fit skulls. This may seem intuitively obvious, but wait, other body organs cannot follow this growing strategy, because they are not enclosed (and subsequently restrained) in a rigid structure like a boney skull. This is why (as any surgeon will attest) organs can and do vary in size constrained only by an inherited genetic growth limit. Is it possible that brains follow a different growth strategy. The physiology of brain growth may be different from other organs, relying more on the pressure of the container size to restrict growth rather than having a preset genetic size, (the folds in the brain may have something to do with this pressure growth relation). One observation seems to confirm this, which is, have you ever heard of a person with a large skull but a small brain, the answer is of course no, brains always grow to container size. If brain size of an individual is predetermined by inherited genetics (skull size most certainly is) we should expect statistically to see varying sizes of brains to skulls (small brain, large skull, would be one) and of course we don't. If this idea is correct, large Brains would now seem to be a physiological development phenomenon rather than a naturally selected adaptive trait. The traits usually quoted as the adaptive reasons for big brains, speech, mental maps, group hunting, social situation manipulators, heat radiators, tool use, dexterity (opposable thumbs) etc. can now be viewed as exaptations (which are not the reasons for large brains but a consequence of them), which have found utility within the confines of a larger brain. Of course once any or all of the above traits become incipient, a positive feedback loop would set in reinforcing the value of the larger brain. So there it is, 1- Bipedalism evolves for some reason, 2- Childbirth with rigid skulls increases mortality rates, 3- Soft, plastic more malleable skulls are naturally selected for, 4- Neotenist brain growth restricted by the pressure of the container now pushes on a more malleable skull, 5- Large brains. .... Could it be this simple .... Gould would have liked this, he favors more random, contingent processes. Are there any stats to indicate larger [or smaller] brains in c sections? Any correlations with IQ? (I believe there is). A good Grad thesis would be to remove mouse embryos and place radial cuts in skull caps, replace to term and measure brain growth to size. Anyway, this is all off the top of my hard rigid head." Good stuff Elzi. Out of the context of the above argument, what hapens when the skull does not expand as fully as it could. But still is at least normal if not bigger size? Let me explain. I knew a couple people who skull plates did not become rigid in the normal manner. They were told by docs that the plates fused too early while growing. But they have either normal or above normal sized skulls. Both people are very smart. Each has a ridge down the top of the skull. It would seem, out of the context of the above argument, that if these skulls have been allowed to grow larger, their brains would have been bigger. And as such, these individuals may have been even more intelligent. Does this sound reasonable? |
#5
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![]() "elzinator" wrote in message ... With all the furor of the recent article in Nature (in which the MHC mutation/brain size hypothesis is not being well received in the academic community), this occurred on one of the science usenet groups I read: Big Brains and Bipedalism One of the reasons often quoted for the limit on human brain size, is the restriction placed on head size due to childbirth. In bipedal humans, childbirth poses a serious risk for both mother and child (mortality rates increasing for both), because the newborn must now pass out of a narrow birth canal between the narrower pelvis hips (which have evolved for an upright posture). This is a problem not experienced to the same degree by quadrupeds. It is now well established that bipedalism evolved first followed by large brains. To me this seems something of a paradox because we have a trait, bipedalism (with its associated narrower birth canal and increased mortality) which would seem to place selective pressure on smaller heads (and associated brains), not larger. The adaptive value of larger brains must now be explained as being very significant in order to overcome this reverse trend which would have been expected. I would now like to suggest an alternate hypothesis, which is, that larger brains are a direct result of bipedalism and difficult child birth rather than the opposite. Let me explain. Because of bipedalism and its associated difficult childbirth, evolution has come up with a partial solution for the big head problem, (neoteny being one), and also a softer, (less rigid) more plastic or malleable skull during childbirth. Humans at birth have not only a hole in the top of the skull but also the skull cap is extremely soft with radial unhealed fractures running in all directions. (If you have every seen a newborn immediately after birth, their heads look like squashed prunes). Only bipedal humans have this trait, quadrupeds have much more hard and rigid skulls with little deformation (its not needed), their skulls size (while they still grow) are more set at birth. Could it be that the physiology of brain growth is such that they simply grow to their container size? and that the pressure of a growing brain in a softer more plastic skull has resulted in a larger brain before the skull matures (heals) into a more rigid one. Skulls do not grow to accommodate a predetermined brain size, the opposite is true, brains grow to fit a genetically determined skull size. When you think about it, brains exactly fit skulls. This may seem intuitively obvious, but wait, other body organs cannot follow this growing strategy, because they are not enclosed (and subsequently restrained) in a rigid structure like a boney skull. This is why (as any surgeon will attest) organs can and do vary in size constrained only by an inherited genetic growth limit. Is it possible that brains follow a different growth strategy. The physiology of brain growth may be different from other organs, relying more on the pressure of the container size to restrict growth rather than having a preset genetic size, (the folds in the brain may have something to do with this pressure growth relation). One observation seems to confirm this, which is, have you ever heard of a person with a large skull but a small brain, the answer is of course no, brains always grow to container size. If brain size of an individual is predetermined by inherited genetics (skull size most certainly is) we should expect statistically to see varying sizes of brains to skulls (small brain, large skull, would be one) and of course we don't. If this idea is correct, large Brains would now seem to be a physiological development phenomenon rather than a naturally selected adaptive trait. The traits usually quoted as the adaptive reasons for big brains, speech, mental maps, group hunting, social situation manipulators, heat radiators, tool use, dexterity (opposable thumbs) etc. can now be viewed as exaptations (which are not the reasons for large brains but a consequence of them), which have found utility within the confines of a larger brain. Of course once any or all of the above traits become incipient, a positive feedback loop would set in reinforcing the value of the larger brain. So there it is, 1- Bipedalism evolves for some reason, 2- Childbirth with rigid skulls increases mortality rates, 3- Soft, plastic more malleable skulls are naturally selected for, 4- Neotenist brain growth restricted by the pressure of the container now pushes on a more malleable skull, 5- Large brains. .... Could it be this simple .... Gould would have liked this, he favors more random, contingent processes. Are there any stats to indicate larger [or smaller] brains in c sections? Any correlations with IQ? (I believe there is). A good Grad thesis would be to remove mouse embryos and place radial cuts in skull caps, replace to term and measure brain growth to size. Anyway, this is all off the top of my hard rigid head." Good stuff Elzi. Out of the context of the above argument, what hapens when the skull does not expand as fully as it could. But still is at least normal if not bigger size? Let me explain. I knew a couple people who skull plates did not become rigid in the normal manner. They were told by docs that the plates fused too early while growing. But they have either normal or above normal sized skulls. Both people are very smart. Each has a ridge down the top of the skull. It would seem, out of the context of the above argument, that if these skulls have been allowed to grow larger, their brains would have been bigger. And as such, these individuals may have been even more intelligent. Does this sound reasonable? |
#6
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![]() "elzinator" wrote in message ... With all the furor of the recent article in Nature (in which the MHC mutation/brain size hypothesis is not being well received in the academic community), this occurred on one of the science usenet groups I read: Big Brains and Bipedalism One of the reasons often quoted for the limit on human brain size, is the restriction placed on head size due to childbirth. In bipedal humans, childbirth poses a serious risk for both mother and child (mortality rates increasing for both), because the newborn must now pass out of a narrow birth canal between the narrower pelvis hips (which have evolved for an upright posture). This is a problem not experienced to the same degree by quadrupeds. It is now well established that bipedalism evolved first followed by large brains. To me this seems something of a paradox because we have a trait, bipedalism (with its associated narrower birth canal and increased mortality) which would seem to place selective pressure on smaller heads (and associated brains), not larger. The adaptive value of larger brains must now be explained as being very significant in order to overcome this reverse trend which would have been expected. I would now like to suggest an alternate hypothesis, which is, that larger brains are a direct result of bipedalism and difficult child birth rather than the opposite. Let me explain. Because of bipedalism and its associated difficult childbirth, evolution has come up with a partial solution for the big head problem, (neoteny being one), and also a softer, (less rigid) more plastic or malleable skull during childbirth. Humans at birth have not only a hole in the top of the skull but also the skull cap is extremely soft with radial unhealed fractures running in all directions. (If you have every seen a newborn immediately after birth, their heads look like squashed prunes). Only bipedal humans have this trait, quadrupeds have much more hard and rigid skulls with little deformation (its not needed), their skulls size (while they still grow) are more set at birth. Could it be that the physiology of brain growth is such that they simply grow to their container size? and that the pressure of a growing brain in a softer more plastic skull has resulted in a larger brain before the skull matures (heals) into a more rigid one. Skulls do not grow to accommodate a predetermined brain size, the opposite is true, brains grow to fit a genetically determined skull size. When you think about it, brains exactly fit skulls. This may seem intuitively obvious, but wait, other body organs cannot follow this growing strategy, because they are not enclosed (and subsequently restrained) in a rigid structure like a boney skull. This is why (as any surgeon will attest) organs can and do vary in size constrained only by an inherited genetic growth limit. Is it possible that brains follow a different growth strategy. The physiology of brain growth may be different from other organs, relying more on the pressure of the container size to restrict growth rather than having a preset genetic size, (the folds in the brain may have something to do with this pressure growth relation). One observation seems to confirm this, which is, have you ever heard of a person with a large skull but a small brain, the answer is of course no, brains always grow to container size. If brain size of an individual is predetermined by inherited genetics (skull size most certainly is) we should expect statistically to see varying sizes of brains to skulls (small brain, large skull, would be one) and of course we don't. If this idea is correct, large Brains would now seem to be a physiological development phenomenon rather than a naturally selected adaptive trait. The traits usually quoted as the adaptive reasons for big brains, speech, mental maps, group hunting, social situation manipulators, heat radiators, tool use, dexterity (opposable thumbs) etc. can now be viewed as exaptations (which are not the reasons for large brains but a consequence of them), which have found utility within the confines of a larger brain. Of course once any or all of the above traits become incipient, a positive feedback loop would set in reinforcing the value of the larger brain. So there it is, 1- Bipedalism evolves for some reason, 2- Childbirth with rigid skulls increases mortality rates, 3- Soft, plastic more malleable skulls are naturally selected for, 4- Neotenist brain growth restricted by the pressure of the container now pushes on a more malleable skull, 5- Large brains. .... Could it be this simple .... Gould would have liked this, he favors more random, contingent processes. Are there any stats to indicate larger [or smaller] brains in c sections? Any correlations with IQ? (I believe there is). There is more of a correlation )of IQ) with the relative number of glial cells (not Swanns mind you - astrocytes and oligodendrocytes) to neurons than size/volume per se. We know the various ATP-mediated communications mechanisms among the glia and betwixt the glia and neurons, and that that evidence points to/supports several functional cognitive enhancments in which glia are involved over and above their normal scenarios (e.g., Einsteinn's brain). AS for what came first - the history of evolution is one of environmental contingencies contribnuting to parallel development of new functional capabilities *and* the CNS mechanisms to support such. A little bit of this then a little bit of that, then a little bit more of this, then...... There are exceptions (the whole PE scenario of which Gould is the high priest of proseyltizment (new word)), but the soft-skull scenario did not contribute *all at once* to the cerebral cortex we now enjoy. That there are several functional areas fo such, all interacting with different sets of environmental contingencies should be a clue in that direction. |
#7
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![]() "elzinator" wrote in message ... With all the furor of the recent article in Nature (in which the MHC mutation/brain size hypothesis is not being well received in the academic community), this occurred on one of the science usenet groups I read: Big Brains and Bipedalism One of the reasons often quoted for the limit on human brain size, is the restriction placed on head size due to childbirth. In bipedal humans, childbirth poses a serious risk for both mother and child (mortality rates increasing for both), because the newborn must now pass out of a narrow birth canal between the narrower pelvis hips (which have evolved for an upright posture). This is a problem not experienced to the same degree by quadrupeds. It is now well established that bipedalism evolved first followed by large brains. To me this seems something of a paradox because we have a trait, bipedalism (with its associated narrower birth canal and increased mortality) which would seem to place selective pressure on smaller heads (and associated brains), not larger. The adaptive value of larger brains must now be explained as being very significant in order to overcome this reverse trend which would have been expected. I would now like to suggest an alternate hypothesis, which is, that larger brains are a direct result of bipedalism and difficult child birth rather than the opposite. Let me explain. Because of bipedalism and its associated difficult childbirth, evolution has come up with a partial solution for the big head problem, (neoteny being one), and also a softer, (less rigid) more plastic or malleable skull during childbirth. Humans at birth have not only a hole in the top of the skull but also the skull cap is extremely soft with radial unhealed fractures running in all directions. (If you have every seen a newborn immediately after birth, their heads look like squashed prunes). Only bipedal humans have this trait, quadrupeds have much more hard and rigid skulls with little deformation (its not needed), their skulls size (while they still grow) are more set at birth. Could it be that the physiology of brain growth is such that they simply grow to their container size? and that the pressure of a growing brain in a softer more plastic skull has resulted in a larger brain before the skull matures (heals) into a more rigid one. Skulls do not grow to accommodate a predetermined brain size, the opposite is true, brains grow to fit a genetically determined skull size. When you think about it, brains exactly fit skulls. This may seem intuitively obvious, but wait, other body organs cannot follow this growing strategy, because they are not enclosed (and subsequently restrained) in a rigid structure like a boney skull. This is why (as any surgeon will attest) organs can and do vary in size constrained only by an inherited genetic growth limit. Is it possible that brains follow a different growth strategy. The physiology of brain growth may be different from other organs, relying more on the pressure of the container size to restrict growth rather than having a preset genetic size, (the folds in the brain may have something to do with this pressure growth relation). One observation seems to confirm this, which is, have you ever heard of a person with a large skull but a small brain, the answer is of course no, brains always grow to container size. If brain size of an individual is predetermined by inherited genetics (skull size most certainly is) we should expect statistically to see varying sizes of brains to skulls (small brain, large skull, would be one) and of course we don't. If this idea is correct, large Brains would now seem to be a physiological development phenomenon rather than a naturally selected adaptive trait. The traits usually quoted as the adaptive reasons for big brains, speech, mental maps, group hunting, social situation manipulators, heat radiators, tool use, dexterity (opposable thumbs) etc. can now be viewed as exaptations (which are not the reasons for large brains but a consequence of them), which have found utility within the confines of a larger brain. Of course once any or all of the above traits become incipient, a positive feedback loop would set in reinforcing the value of the larger brain. So there it is, 1- Bipedalism evolves for some reason, 2- Childbirth with rigid skulls increases mortality rates, 3- Soft, plastic more malleable skulls are naturally selected for, 4- Neotenist brain growth restricted by the pressure of the container now pushes on a more malleable skull, 5- Large brains. .... Could it be this simple .... Gould would have liked this, he favors more random, contingent processes. Are there any stats to indicate larger [or smaller] brains in c sections? Any correlations with IQ? (I believe there is). There is more of a correlation )of IQ) with the relative number of glial cells (not Swanns mind you - astrocytes and oligodendrocytes) to neurons than size/volume per se. We know the various ATP-mediated communications mechanisms among the glia and betwixt the glia and neurons, and that that evidence points to/supports several functional cognitive enhancments in which glia are involved over and above their normal scenarios (e.g., Einsteinn's brain). AS for what came first - the history of evolution is one of environmental contingencies contribnuting to parallel development of new functional capabilities *and* the CNS mechanisms to support such. A little bit of this then a little bit of that, then a little bit more of this, then...... There are exceptions (the whole PE scenario of which Gould is the high priest of proseyltizment (new word)), but the soft-skull scenario did not contribute *all at once* to the cerebral cortex we now enjoy. That there are several functional areas fo such, all interacting with different sets of environmental contingencies should be a clue in that direction. |
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![]() "Lee Michaels" wrote in message news:[email protected]_s01... "elzinator" wrote in message ... With all the furor of the recent article in Nature (in which the MHC mutation/brain size hypothesis is not being well received in the academic community), this occurred on one of the science usenet groups I read: Big Brains and Bipedalism One of the reasons often quoted for the limit on human brain size, is the restriction placed on head size due to childbirth. In bipedal humans, childbirth poses a serious risk for both mother and child (mortality rates increasing for both), because the newborn must now pass out of a narrow birth canal between the narrower pelvis hips (which have evolved for an upright posture). This is a problem not experienced to the same degree by quadrupeds. It is now well established that bipedalism evolved first followed by large brains. To me this seems something of a paradox because we have a trait, bipedalism (with its associated narrower birth canal and increased mortality) which would seem to place selective pressure on smaller heads (and associated brains), not larger. The adaptive value of larger brains must now be explained as being very significant in order to overcome this reverse trend which would have been expected. I would now like to suggest an alternate hypothesis, which is, that larger brains are a direct result of bipedalism and difficult child birth rather than the opposite. Let me explain. Because of bipedalism and its associated difficult childbirth, evolution has come up with a partial solution for the big head problem, (neoteny being one), and also a softer, (less rigid) more plastic or malleable skull during childbirth. Humans at birth have not only a hole in the top of the skull but also the skull cap is extremely soft with radial unhealed fractures running in all directions. (If you have every seen a newborn immediately after birth, their heads look like squashed prunes). Only bipedal humans have this trait, quadrupeds have much more hard and rigid skulls with little deformation (its not needed), their skulls size (while they still grow) are more set at birth. Could it be that the physiology of brain growth is such that they simply grow to their container size? and that the pressure of a growing brain in a softer more plastic skull has resulted in a larger brain before the skull matures (heals) into a more rigid one. Skulls do not grow to accommodate a predetermined brain size, the opposite is true, brains grow to fit a genetically determined skull size. When you think about it, brains exactly fit skulls. This may seem intuitively obvious, but wait, other body organs cannot follow this growing strategy, because they are not enclosed (and subsequently restrained) in a rigid structure like a boney skull. This is why (as any surgeon will attest) organs can and do vary in size constrained only by an inherited genetic growth limit. Is it possible that brains follow a different growth strategy. The physiology of brain growth may be different from other organs, relying more on the pressure of the container size to restrict growth rather than having a preset genetic size, (the folds in the brain may have something to do with this pressure growth relation). One observation seems to confirm this, which is, have you ever heard of a person with a large skull but a small brain, the answer is of course no, brains always grow to container size. If brain size of an individual is predetermined by inherited genetics (skull size most certainly is) we should expect statistically to see varying sizes of brains to skulls (small brain, large skull, would be one) and of course we don't. If this idea is correct, large Brains would now seem to be a physiological development phenomenon rather than a naturally selected adaptive trait. The traits usually quoted as the adaptive reasons for big brains, speech, mental maps, group hunting, social situation manipulators, heat radiators, tool use, dexterity (opposable thumbs) etc. can now be viewed as exaptations (which are not the reasons for large brains but a consequence of them), which have found utility within the confines of a larger brain. Of course once any or all of the above traits become incipient, a positive feedback loop would set in reinforcing the value of the larger brain. So there it is, 1- Bipedalism evolves for some reason, 2- Childbirth with rigid skulls increases mortality rates, 3- Soft, plastic more malleable skulls are naturally selected for, 4- Neotenist brain growth restricted by the pressure of the container now pushes on a more malleable skull, 5- Large brains. .... Could it be this simple .... Gould would have liked this, he favors more random, contingent processes. Are there any stats to indicate larger [or smaller] brains in c sections? Any correlations with IQ? (I believe there is). A good Grad thesis would be to remove mouse embryos and place radial cuts in skull caps, replace to term and measure brain growth to size. Anyway, this is all off the top of my hard rigid head." Good stuff Elzi. Out of the context of the above argument, what hapens when the skull does not expand as fully as it could. But still is at least normal if not bigger size? Let me explain. I knew a couple people who skull plates did not become rigid in the normal manner. They were told by docs that the plates fused too early while growing. But they have either normal or above normal sized skulls. Both people are very smart. Each has a ridge down the top of the skull. It would seem, out of the context of the above argument, that if these skulls have been allowed to grow larger, their brains would have been bigger. And as such, these individuals may have been even more intelligent. Does this sound reasonable? Yes and no. That things seem is not that they are. Look up phylogeny and ontogeny and then recapitulation. Combine all in a blender and tell us what you get. |
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![]() "Lee Michaels" wrote in message news:[email protected]_s01... "elzinator" wrote in message ... With all the furor of the recent article in Nature (in which the MHC mutation/brain size hypothesis is not being well received in the academic community), this occurred on one of the science usenet groups I read: Big Brains and Bipedalism One of the reasons often quoted for the limit on human brain size, is the restriction placed on head size due to childbirth. In bipedal humans, childbirth poses a serious risk for both mother and child (mortality rates increasing for both), because the newborn must now pass out of a narrow birth canal between the narrower pelvis hips (which have evolved for an upright posture). This is a problem not experienced to the same degree by quadrupeds. It is now well established that bipedalism evolved first followed by large brains. To me this seems something of a paradox because we have a trait, bipedalism (with its associated narrower birth canal and increased mortality) which would seem to place selective pressure on smaller heads (and associated brains), not larger. The adaptive value of larger brains must now be explained as being very significant in order to overcome this reverse trend which would have been expected. I would now like to suggest an alternate hypothesis, which is, that larger brains are a direct result of bipedalism and difficult child birth rather than the opposite. Let me explain. Because of bipedalism and its associated difficult childbirth, evolution has come up with a partial solution for the big head problem, (neoteny being one), and also a softer, (less rigid) more plastic or malleable skull during childbirth. Humans at birth have not only a hole in the top of the skull but also the skull cap is extremely soft with radial unhealed fractures running in all directions. (If you have every seen a newborn immediately after birth, their heads look like squashed prunes). Only bipedal humans have this trait, quadrupeds have much more hard and rigid skulls with little deformation (its not needed), their skulls size (while they still grow) are more set at birth. Could it be that the physiology of brain growth is such that they simply grow to their container size? and that the pressure of a growing brain in a softer more plastic skull has resulted in a larger brain before the skull matures (heals) into a more rigid one. Skulls do not grow to accommodate a predetermined brain size, the opposite is true, brains grow to fit a genetically determined skull size. When you think about it, brains exactly fit skulls. This may seem intuitively obvious, but wait, other body organs cannot follow this growing strategy, because they are not enclosed (and subsequently restrained) in a rigid structure like a boney skull. This is why (as any surgeon will attest) organs can and do vary in size constrained only by an inherited genetic growth limit. Is it possible that brains follow a different growth strategy. The physiology of brain growth may be different from other organs, relying more on the pressure of the container size to restrict growth rather than having a preset genetic size, (the folds in the brain may have something to do with this pressure growth relation). One observation seems to confirm this, which is, have you ever heard of a person with a large skull but a small brain, the answer is of course no, brains always grow to container size. If brain size of an individual is predetermined by inherited genetics (skull size most certainly is) we should expect statistically to see varying sizes of brains to skulls (small brain, large skull, would be one) and of course we don't. If this idea is correct, large Brains would now seem to be a physiological development phenomenon rather than a naturally selected adaptive trait. The traits usually quoted as the adaptive reasons for big brains, speech, mental maps, group hunting, social situation manipulators, heat radiators, tool use, dexterity (opposable thumbs) etc. can now be viewed as exaptations (which are not the reasons for large brains but a consequence of them), which have found utility within the confines of a larger brain. Of course once any or all of the above traits become incipient, a positive feedback loop would set in reinforcing the value of the larger brain. So there it is, 1- Bipedalism evolves for some reason, 2- Childbirth with rigid skulls increases mortality rates, 3- Soft, plastic more malleable skulls are naturally selected for, 4- Neotenist brain growth restricted by the pressure of the container now pushes on a more malleable skull, 5- Large brains. .... Could it be this simple .... Gould would have liked this, he favors more random, contingent processes. Are there any stats to indicate larger [or smaller] brains in c sections? Any correlations with IQ? (I believe there is). A good Grad thesis would be to remove mouse embryos and place radial cuts in skull caps, replace to term and measure brain growth to size. Anyway, this is all off the top of my hard rigid head." Good stuff Elzi. Out of the context of the above argument, what hapens when the skull does not expand as fully as it could. But still is at least normal if not bigger size? Let me explain. I knew a couple people who skull plates did not become rigid in the normal manner. They were told by docs that the plates fused too early while growing. But they have either normal or above normal sized skulls. Both people are very smart. Each has a ridge down the top of the skull. It would seem, out of the context of the above argument, that if these skulls have been allowed to grow larger, their brains would have been bigger. And as such, these individuals may have been even more intelligent. Does this sound reasonable? Yes and no. That things seem is not that they are. Look up phylogeny and ontogeny and then recapitulation. Combine all in a blender and tell us what you get. |
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On Sat, 27 Mar 2004 20:52:56 GMT, "AlphaOmega2004"
wrote: There is more of a correlation )of IQ) with the relative number of glial cells (not Swanns mind you - astrocytes and oligodendrocytes) to neurons than size/volume per se. Do you have a source for this? I wouldn't mind reading the study. We know the various ATP-mediated communications mechanisms among the glia and betwixt the glia and neurons, What do you mean by ATP-mediated communications? I know that nitric oxide mediated, CB-mediated and other receptor mediated signalling is important in neuroglial communication. Although there are receptors that respond to ATP (such as the polymodal TRPV1 for example), extracellular ATP is usually a signal of damage. I realize that there are a large number of gap junctions between astrocytes, but ATP wouldn't typically be an important intracellular signal such as cAMP or Ca2+. and that that evidence points to/supports several functional cognitive enhancments in which glia are involved over and above their normal scenarios (e.g., Einsteinn's brain). Explain please. -- kj |
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