Why can't babies walk?

  • #1
They don't have the muscles or nerves or the connection between them? All three? Animals can walk within days, most of them. I read somewhere that some part of the motor system doesn't mature till the person is in their 20s! That can't be true, but unfortunately I can't find any information on this question. I've done all sorts of google searches, for things like "newborn" "motor neurons" "motor cortex" "muscles" "walking" etc. I had not success. Appreciate any help.
 

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  • #2
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Some people run into car accidents that if injuring their spinal cord will in turn paralyses their legs or whole body either temporarily or permanently. So I think spinal cord is probably the source you're looking for as it controls one's locomotion. I don't know about that "20 years of age to get one's motor system majured" but a baby still needs his fiber tracts in his brain to get fully developed before he can make any functional steps in life, which takes him only some years instead of 20 something.
 
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  • #3
Suraj M
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correct me if i am wrong ..
We humans being one of the few biped species on earth have a well develop system that controls our balance and coordination...
two such organs are the semicircular canals and the vestibular apparatus (inner ear)..these contain fluids(2) that help in 3D orientation ..they relay signals to the cerebellum ...
i feel maybe ...when the baby is born the cerebellum starts getting oriented to the posture to be given ...so that takes time ....
the spinal cord part also makes total sense because just processing the info in the cerebrum is not going to help you walk ..you still have to know what to do with it (learnt response)
 
  • #4
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Why would they need to walk? Clearly it's not needed, babies survive just fine because they have parental care. I imagine a baby that tries to walk would probably just endanger itself. And if humans needed to be capable of walking at birth, that would require some kind of trade-off.

We are born after 9 months not because at that point we are the finished product, ready to survive in the big bad world, but because otherwise our heads would grow too big and both baby and mother would both die. For animals with smaller brains (and without hips designed for bipedal locomotion) this is less of a problem and they can be more fully developed before birth. There might also be other factors favouring quick walking, like the parents being unable to protect or carry their offspring.
 
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  • #5
Ygggdrasil
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Copy/pasting from a previous thread:

In many animals, basic instincts and behaviors are encoded in the organism's DNA. The DNA provides instructions for the animal to build specific neural circuits to perform certain behaviors in response to certain stimuli. For example, flies have an escape response triggered by certain stimuli, such as a shadow passing over them. Researchers have identified a specific nerve cell in the fly that controls this response and this nerve cell is the same in all flies of the same species. Artificial stimulation of this nerve cell triggers the escape response. The nematode worm, C. elegans is probably the animal where the neural circuitry for many innate behaviors, as well as the genetic elements controlling the development of the circuitry, is best understood (for example, see http://www.ncbi.nlm.nih.gov/books/NBK20005/).

In humans and other higher mammals, however, the situation is very different. Humans are born with very few innate behaviors and instincts. For example, whereas many animals (insects, fish, reptiles, amphibians, etc.) are fully capable of walking, feeding themselves and even surviving independently after birth, human babies can do practically nothing after birth and cannot survive without a caretaker. The difference here is that the DNA of humans does not specify a wiring diagram for the brain. Rather this wiring diagram is formed in response to the experiences of the individual. For example, if you were to take a newly born baby and cover its eyes for a critical period in childhood, the child's neural circuitry for interpreting visual stimuli would not develop properly and the child would be blind despite the fact that the child's eyes work perfectly well. Another consequence of this strategy is that everyone will develop different neural circuits to perform the same functions. For example, whereas the same nerve cell will trigger the same escape response in all flies, activating a specific nerve in humans would likely trigger very different responses in different individuals.

While this wiring-on-the-fly strategy has many disadvantages in the younger phases of life (babies and children are very much dependent on others for survival), this plasticity of the brain associated with the wiring strategy gives humans an unparalleled ability to learn. This neural plasticity is likely one reason why humans can learn complex tasks like reading and writing while other organisms cannot.
 
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  • #6
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They don't have the muscles or nerves or the connection between them? All three? Animals can walk within days, most of them. I read somewhere that some part of the motor system doesn't mature till the person is in their 20s! That can't be true, but unfortunately I can't find any information on this question. I've done all sorts of google searches, for things like "newborn" "motor neurons" "motor cortex" "muscles" "walking" etc. I had not success. Appreciate any help.
Typically, infants will begin to stand up and walk on their own anywhere between a year to a year and a half old. Why does it take so long you ask? Well, we can ask the same question as to why does it take humans the same amount of time or longer to talk or to add two numbers together. It takes even longer for humans to subtract two numbers, a cognitive operation referred to by Piagetian scholars as "reversibility," and which marks a significant milestone in the cognitive development of children.

There is considerable evidence that, in the development of the nervous system of humans (and all mammals in general), ontogeny recapitulates phylogeny. This is in a rough sense, of course, and models of heterochrony work to iron out the timing of these developments, but the answer to your question is likely related to the fact that primates only became bipedal roughly 8 million years ago. Therefore, the neural machinery that allows for bipedalism is essentially the same that allowed for human cognition, and the bulk of the evidence shows that this phylogeny is recapitulated in human ontogeny at about the 12-18 month range. What we see at this age is a conspicuous and explosive burst in the synaptogenesis of critical rostro and ventro lateral regions of the prefrontal cortex (PFC) as well as the maturation of major myelenated fiber tracks in the telencephalon, most notably between the lateral PFC and the parvocellular portion of the mediodorsal nucleus of the thalamus. This region is critical for the driving of action of top down motor networks working from the PFC, through to the supplementary, premotor, and finally pre-Rolandic primary motor cortices which are critical in maintaining bipedal gait in human infants.
 
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  • #7
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I'm not too keen on developmental biology, but as far as I know, there's a gross anatomical factor as well as well as the neural that's already been discussed - spine curvature - which isn't developed until walking age. An infant's spine starts out convex and eventually develops the lumbar and cervical concavities, which are important for balance.
 
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  • #8
...Humans are born with very few innate behaviors and instincts. For example, whereas many animals (insects, fish, reptiles, amphibians, etc.) are fully capable of walking, feeding themselves and even surviving independently after birth, human babies can do practically nothing after birth and cannot survive without a caretaker. The difference here is that the DNA of humans does not specify a wiring diagram for the brain. Rather this wiring diagram is formed in response to the experiences of the individual. For example, if you were to take a newly born baby and cover its eyes for a critical period in childhood, the child's neural circuitry for interpreting visual stimuli would not develop properly and the child would be blind despite the fact that the child's eyes work perfectly well. Another consequence of this strategy is that everyone will develop different neural circuits to perform the same functions. For example, whereas the same nerve cell will trigger the same escape response in all flies, activating a specific nerve in humans would likely trigger very different responses in different individuals.

While this wiring-on-the-fly strategy has many disadvantages in the younger phases of life (babies and children are very much dependent on others for survival), this plasticity of the brain associated with the wiring strategy gives humans an unparalleled ability to learn. This neural plasticity is likely one reason why humans can learn complex tasks like reading and writing while other organisms cannot.
Thanks for the replies everyone.

As far as this response, where can I learn more about this, about various ways our development lags behind other animals, and also in what particular way.
 
  • #9
Ygggdrasil
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Thanks for the replies everyone.

As far as this response, where can I learn more about this, about various ways our development lags behind other animals, and also in what particular way.
I don't know of any great references for these topics, but here are a few to get you started:
A news piece from Nature, discussing recent research on plasticity and neural development in humans.

For a nice demonstration of the consequences of the "wiring-on-the-fly" strategy, here's a short article from PLOS Biology , summarizing a nice research paper on the subject.

Here's a relevant quote from the research paper, discussing the differing "wiring" strategies in mice versus invertebrates:
[In mice], the axonal branching structure of each motor neuron was unique. We compared each axon with its functional counterparts, as defined by the size principle, in other muscles, and found substantial topological differences. Left-right pairs of corresponding neurons in the same animal showed no less variation than ipsi- or contralateral pairs from different animals. Such intra-animal variance is surprising, as each pair of neurons had identical genetic background and presumably experienced an identical environment. This result suggests that the branching pattern of these neurons was not predetermined, which contrasts strongly with the situation in invertebrates. For instance, the C. elegans connectome revealed remarkable stereotypy in the structure of the neural circuit. Worm neurons that are ontogenetic counterparts share almost identical branching patterns and connectivity both within an individual and across different animals, even though they may not be exact replicas of each other [18,19]. In annelids [55,56], insects [5762], and crustaceans [63,64] individual neurons can also be identified, and their axonal branching patterns are stereotyped. In particular, this mammalian result contrasts with the stereotypy of neuromuscular innervation in invertebrates. For example, although there are fine structural differences in the terminal branching of axons at NMJs of any particular muscle fiber in insects, even these branches seem to have morphological regularities that are recognizable between different animals [65,66]. In mammals not only is the preterminal branching highly variable (as shown in this paper), but our experience suggests that no two NMJs look the same. Thus axonal branching in this mammalian system seems fundamentally different from that found in invertebrates.
Unfortunately, I don't know of any good, accessible articles comparing how the development of our brains lags behind other animals. Perhaps someone more well versed in neuroscience like @Pythagorean may know more?
 
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  • #10
Pythagorean
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I've only just pulled up these papers based on some loose evo-devo I recall from my year doing wetwork, so I can't attest to their quality. I needed my university creds to access some of them, but mostly I'm just using them for their background.

There's a useful subdivision of traits in this context: altricial vs. precocial. The terms refer to the developmental phase of an animal at birth. Here are some interesting snippets (which are generally not the point of the paper, but provide some background). [1] gives the definitions as most relevant to this thread (locomotion) but also talks a bit about mating strategy. My sense from the literature is that there's distinct mating strategies associated with atlricial vs. percocial animals.

[2] demonstrates that precocial animals tend to have more developed brains at the time of birth.

[3] is a bit confusing, since humans are generally considered altricial. I guess the point is that humans have high indices for both their embryonic and post-embryonic phases. [3] basically spends time discussing the relationship between mother's metabolic rate (and whether it's a significant relationship at all). The ape family (including humans) seems to challenge most of the hypotheses.

"Here, we examine the evolution of bird mating systems in relation to precociality and altriciality, defined as the capability or incapability of young to leave the nest depending on their locomotive development."[1]

"Transitions to polygamy in females are significantly more frequent in birds with precocial young compared to birds with altricial young. In males, however, there is no significant difference in the frequency of transitions to polygamy between birds with precocial young and birds with altricial young." [1]

"individuals of precocial species have much larger neonatal brain sizes and are gestated longer for a given maternal body size than individuals of altricial species" [2]

"Precocial birds and mammals have high embryonic brain growth indices which are compensated for by low post-embryonic indices (with the exception of Homo sapiens). In contrast, altricial birds and mammals have low embryonic and high post-embryonic indices. Altricial birds have relatively small brains at hatching and develop relatively large brains as adults, but among mammals there is no equivalent correlation between variation in adult relative brain sizes and state of neonatal development." [3]

[1] http://beheco.oxfordjournals.org/content/6/3/296.short
[2] www.jstor.org/stable/2408910
[3] http://onlinelibrary.wiley.com/doi/10.1111/j.1469-7998.1985.tb04946.x/abstract
 
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  • #11
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Don't miss the mechanical perspective. A standing person is a good example of an inverted pendulum, which get easier to balance with increased length/height.

Overall, though, my money is on lack of strength. Newborns can't lift their own heads. They've got no business walking.
 
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  • #12
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Don't miss the mechanical perspective. A standing person is a good example of an inverted pendulum, which get easier to balance with increased length/height.

Overall, though, my money is on lack of strength. Newborns can't lift their own heads. They've got no business walking.
Here are simple explanations of basic skills and abilities human babies need to acquire before they can walk (the more complex explanations of how these occur neurologically are explained above).

Babies can't see clearly when they are born, they cannot focus, they have no hand-eye coordination, no balance. There are physical limitations as well as brain development

Up to about 3 months of age, babies' eyes do not focus on objects more than 8 to 10 inches from their faces.

Depth perception, which is the ability to judge if objects are nearer or farther away than other objects, is not present at birth. It is not until around the fifth month that the eyes are capable of working together to form a three-dimensional view of the world and begin to see in depth.

Most babies start crawling at about 8 months old, which helps further develop eye-hand-foot-body coordination. Early walkers who did minimal crawling may not learn to use their eyes together as well as babies who crawl a lot.

By the age of nine to twelve months, babies should be using their eyes and hands together.

At around 9 months of age, babies begin to pull themselves up to a standing position.
http://www.aoa.org/patients-and-pub...fant-vision-birth-to-24-months-of-age?sso=y#1

Then there is the fact that babies must develop muscle strength and coordination.

It has been observed by scientists that motor skills generally develop from the center to the body outward and head to tail. Babies need to practice their skills; therefore they will grow and strengthen better. They need space and time to explore in their environment and use their muscles. “Tummy-time” is a good example of this. At first they are only able to lay their belly on the floor but by around two months they start to gain muscle to raise their head and chest off the ground. Some are also able to go on their elbows. They will also start to kick and bend their legs while lying there, this helps to prepare for crawling. By four months they are able to start to control their head and hold it steady while sitting up. Rolling from belly to back movements is started. At about five months the baby will start to wiggle their limbs to strengthen crawling muscles. Infants can start to sit up by themselves and put some weight on their legs as they hold onto something for support by six months. Around ten months they should be able to stand on their own.
http://en.wikipedia.org/wiki/Gross_motor_skill#Infancy_Development
 

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