Nails are a much older "invention of evolution" than humans: so we have to investigate there...
> how do nails and hairs manage to grow only in one direction
Like all things evolution: nails that grow backwards to not have an advantage (prolly a disadvantage), where nails that grow forwards have an advantage (climbing, clawing, scratching).
"nails that grow backwards to not have an advantage (prolly a disadvantage), where nails that grow forwards have an advantage (climbing, clawing, scratching)."
That still doesn't describe the underlying mechanism of the growth itself.
Looking at the work of Dr. Michael Levin regarding electric communication of cells, I tend to believe him that the main factor in actually creating tissues in their intended, correct shape, is incessant electric chatter among individual cells.
An interesting corollary would be that cancer = cells that don't cooperate/communicate with their neighboring cells anymore.
Another interesting question is how development distinguishes left and right. As I understand it, there's a small object that develops that has cilia in a tilted configuration. The rotation of the cilia causes a flow of fluid to one side that is determined by the sense (clockwise or counterclockwise) of the cilia's rotation. That flow is sensed and sets off signals that drive development.
Where does the rotational sense of cilia come from? From the stereochemistry of proteins, and therefore from amino acids. The left-vs-right handedness of the base chemistry of life is exploited to get a macroscopic signal.
I can't speak about nails and hairs specifically, but directional cell growth is common in nature.
In plants, for example, cells replicate primarily at the tip of a bud, which allows branches to lengthen directionally rather than grow out in all directions. The plant produces growth hormones, which are transported upward throughout all branches until they reach a dead end. When they reach a dead end, they stop moving and just sit there, which causes the cells at the dead end to have a greater exposure to these growth hormones. These cells bathe in growth hormones for so long that they pass the hormone-exposure threshold that triggers cell replication.
I don't read "it exists because it has to exist" in the parent's statement. They're saying that there's an advantage one way and a disadvantage another, and evolution favors advantages. I wouldn't characterize a statement like that as a tautology, and I don't think the author deserves your dig for it.
In my opinion there’s a general fundamental misunderstanding on the purpose of theories. I see it all the time — attempts to explain why something is useful simply because it exists (re: popular science evolution). There are loads of suboptimal traits that are counterbalanced by something else.
Nails are a much older "invention of evolution" than humans: so we have to investigate there...
> how do nails and hairs manage to grow only in one direction
Like all things evolution: nails that grow backwards to not have an advantage (prolly a disadvantage), where nails that grow forwards have an advantage (climbing, clawing, scratching).