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Comment by CodeWriter23 | Hacker News Reader

> It's trivially obvious that some kinds of stem cell can become any type of cell, given that we all had our beginnings as a single cell.

It’s not that obvious, as the brain grows, certain kind of cells die off and never come back. For example at 4-5 years of age being able to speak different phonenes is lost due to mass die off of a certain type of brain cell.

Could be the same for the pair of cells that start a human life. Once their purpose is served they may never exist again.

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Do you have a source on the phonemes situation? First time I hear of it, and I'm pretty sure I've seen people learn them past that point. I know they struggle with it, but, as an example, even japanese people can learn the effective difference between l and r given enough effort.

It’s why certain ethnicities cannot pronounce words in their non-native language. Like people from the US not being able to make certain consonant sounds in German or Chinese being unable to pronounce the R consonant. So go look it up yourself.

You are referring to the critical period [1] of (second) language acquisition, which is generally thought to end with the onset of puberty [2]. Neurodevelopmentally, this period coincides with extensive synaptic and dendritic pruning and increased myelination (of axons) [3], which result in the loss of some connections and the strengthening and acceleration of others. Cell loss is not thought to be a major driver of brain maturation, nor is it thought to occur more frequently during this time window.

[1] https://en.wikipedia.org/wiki/Critical_period_hypothesis

[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5857581/

[3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3982854/

> It’s not that obvious, as the brain grows, certain kind of cells die off and never come back.

I'm not sure what difference that would make. Those brain cells (and all the other brain cells that don't die off) were still formed by successive divisions of the single cell that resulted from sperm meeting egg. Therefore, that original cell is capable of producing any cell type found in the body.

By your logic, every cell in the human body is a brain cell.

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