The human body is a vessel of constant cell-regenetation. It’s how we’re able to live as long as we do… So why do some parts seem to ignore regeneration? Specifically sight, hearing, and brain-function?
Will evolution one day grant us the extreme privilige of eternal life, which, based on the mechanism that allows us to live to the age of ninety is entirely possible, a mere formality of evolution’s process.
Would you really want to live in a world where people were immortal? You’d pretty much have to sterlize the entire population.
No. By prolonging our lives we are going against evolution.
And there is evidence the brain can regenerate. I’m too lazy to find links right now, but all I’d do is google it, and you can do that 
Related questions: how deep (in terms of both layers and length) do you have to get cut before it leaves a scar?
Well, nerve cells are some of the most complicated and sophisticated (relatively) cells in the body, and generally they lose the ability to divide and replicate themselves once they go into ‘active service’, unlike muscle cells, connector cells, bone marrow cells, intestinal lining cells, kidney cells, etcetera.
And, unfortunately, evolution didn’t happen to hit on a design that would have included keeping viable pre-nerve cells around as ‘backups’ in case large numbers of the operational nerves were destroyed.
Just my two cents: I know the OP is talking about skin, but doesn’t the liver regenrate, as well as the tips of fingers?
As I recall, anything beyond that is too complex. 
Actually, there are some nerve cells that do regenerate. Your olfactory nerves, for instance, die and regenerate all the time. There are nerve stem cells up in your nose that hang around for life. A lot of stem cell research has been done with these cells. They’ve been used experimentally, with some limited success, to treat spinal cord injuries.
So it’s not that these cells can’t regenerate, it’s just that we haven’t developed a body plan that includes a provision to make them regenerate for whatever reason.
Depends more on the person and the placement than the size of the cut. I scar very easily - I have one scar from a particularly nasty paper cut. My brother, on the other hand, hardly scars at all. He suffered a three inch gash which required stiches, and there’s no scarring whatsoever. (He also can’t pierce, BTW - any holes close up within moments of taking a piece out to switch it.)
The OP didn’t mention anything about skin.
It’s not about complexity. The liver is pretty damn complex. It’s just that for some reason, evolution left it with an ability to regenerate. And as someone said earlier, nerve cells can regenerate, its not often though.
If you’re looking for an answer in the context of evolution, there’s only one answer:
Some body parts don’t regenerate because our ancestors were able to survive long enough to procreate without those body parts regenerating.
In general, our sight, hearing and brain function start to degenerate many years after our procreative years (at least what our procreative years used to be), so it’s not a “flaw” that would be “corrected” by evolution.
I disagree, and I’m pretty sure others would as well. We have already, in fact, prolonged our lives by previous inventions and discoveries. Evolution merely (!) naturally selects the most fit to survive and reproduce. If at this point in time, we have evolved to the stage that we can use our minds to increase our lifespan even further, then that is the natural state of events and the point to which we have evolved. And so on and so on. Our brains, and what we can do with them are our most significant adaptation to the threats and challenges to our continued existence.
I meant more along the lines of natural selection. People, despite not being the most fit people, will survive because medicine allows them to. But I see that what I said was fundamentally wrong.
I posted late at night. :smack: