Wow, what an interesting article, not all about feathers. There’s so many genetic mysteries about skin appendages still to be uncovered e.g. in humans, how do nails and hairs manage to grow only in one direction (and perhaps even more remarkable, always so).
I was drawn to this side point though: the microraptor has four wings. Not like a dragon, of course, which has to be an insect, but an ordinary quadruped that used all four limbs to fly (compare that to mammals with a membrane between the forelimbs and hind limbs on each side). I imagine it must have looked like an F-35 when flying.
Seems like it turned out to be optimal to stick to two, not just for terrestrial mobility, but due to the (bidirectional!) optimization of the wishbone and the chest musculature. It’s probably hard to get enough power into the dual-mode hind limbs. Sadly the Wikipedia article on the microraptor doesn’t explore this.
You can't conclude that. Evolution is noisy and random.
Besides, birds are not unambiguously the most optimized flying vertebrates around.
You're correct that you can't conclude that evolution is perfection/optimized, but it's also not correct to say it is random. The genetic variation is random, but natural selection is very much not random[1][2].
[1]: https://evolution.berkeley.edu/misconceptions-about-natural-...
[2]: See Number 7. https://www.pbs.org/wgbh/evolution/library/faq/cat01.html
Regardless, it doesn't have agency and isn't clever. Personally I belive in a Designer, but since middle school I've been bewildered by the way evolution is almost always presented, outside of rigorous scientific literature, as if there is agency, intelligence and intent behind it.
I don't have a problem with that, but materialists don't have that luxury and use language in bad faith when do it.
I agree with the sentiment. I also heard of some books that say stupid things like humans have not yet reached the maximum of human evolution.
Of course we have not reached the end of our evolution. That will exist only when we are all extinct.
As for the direction... that is something else. Maybe we will evolve into higher intelligence, but as the Dick's story "The Golden man" or "Idiocracy" show, is the intelligence really the driving force of today?
Yes, certainly. More than anything intelligence differentiates us as a species.
I don't agree. Women, in general, don't find men attractive because of intelligence. It's other factors more like, whether their life is in order, they are fit, or the classic, if they are symmetrical.
So, we are visually, materiallistically oriented. Not all, of course, but evolution works on population size, not exceptions. Exceptions only come into an overbearing effect on cataclysmic events.
It's not that kind of intelligence that matters. Don't think in terms of being good at chess, poetry, or multiplying numbers in your head. Think wheel, writing, domestication, agriculture, petrochemical engineering, nuclear weapons, computers, genetic engineering.
Our intelligence is what allows us to acquire and improve new adaptations without having to change our own genetics. We're able to change living environments and adapt to them multiple times in a single life span, and we went from basic language to walking on the Moon much faster than evolution is able to make meaningful changes through natural selection. It takes however many million years for a species to grow fur to adapt to cold climate; it took a spear for us to adapt by stealing other animals' fur, and couple hundred years to figure out how to make synthetic ones at scale. In that time, we adapted to almost every environment on the planet.
Intelligence very much is the driving force behind humanity.
EDIT: and we're also beating natural selection from the other end - modern medicine allows many people to live and reproduce, who without it would've died from genetic diseases. We're very good at denying the "fitness" criteria nature uses.
While I agree with your general slant, it's not correct to say that evolution can't act quickly. Evolution can act, and does act, slightly faster than a single generation. Populations change rapidly, and the success, failure, life and death of individuals and their guiding behaviors, change with it. The introduction of online dating, for example, has already caused evolutionary changes in our species. Only time can tell if these changes are going to last. Just because we haven't yet generally grown thumbs adapted for interacting with iPhones doesn't mean that the more subtle changes haven't happened.
While I agree in the sense that what you say improves human race survival vs whatever-else comes.
But evolutionary pressure also occurs within the species and for the human race $$$ has largely influenced demonstration of procreational traits.
Intelligence doesn't matter if you are smart but don't have any opportunities. The other social cues, fashion, appearance, physical fitness, health, being "funny". Are all a lot easier if you have $$$.
So many conversations around modern population fertility rates have a $ component in them.
Maybe being intelligent matters more than simple strength and coordination used to. But what kind of intelligence?
Maybe being a sociopath such that you can bully your way to CEO - or to a high enough level you meet some "darwinian fitness" threshold.
But in and of itself, being able to engineer the feat of walking on the moon didn't make all the nasa employees inherently more desirable to partners to procreate. Those employees maybe had attributes that matched with desirable procreation partners and maybe that feat brought $$$ too - but the engineering smarts alone isn't it.
Consider talking to more women.
Diminishing returns. People vastly prefer a partner who can communicate using language over those who don’t, that’s a huge preference for intelligence just not an unlimited one.
Similarly most people want some level of success be that artistic, financial, athletic, etc and success is highly correlated with above average intelligence. 101+ IQ’s might not seem that impressive but over a long time scale that’s an endless treadmill.
It's hard to explain an animistic force of nature to another human without using words that imply agency. It feels like our language and our ways of thinking are hard-wired to see everything through that lens because we have agency. It's like a fish trying to imagine what it's like to be outside of water.
So you believe in a "force" that creates life?
Pastor says that the four fundamental forces are necessary and sufficient to create life.
Didn't Darwin write a book that did just that?
Bruce Lee's injunction "be like water" probably means don't overcomplicate with agency and opposed consciousnesses, just evolve the lagrangian (towards victory).
(Speak not of the relevant XKCD)
As humans we do things for teleological reasons. Meaning we can say we did X in order to accomplish Y.
Ascribing teleological explanations to evolution is technically wrong, since it doesn’t look ahead.
However, it does something very similar. Our brains process competing options, from plausible to nonsensical, before selecting an action, partly in sequence (ideation), but also in parallel (competition processing).
Evolution tries many options in parallel and sequence too. Just by actually doing them and then selecting which of those choices to keep repeating (better survival), and those to forget (extinction of genes, clusters of genes, or whole species).
So over longer time periods, it acts very teleologically. A kind of reverse teleologic by hindsight.
The same is true for the “brilliance” of this teleology. Evolution tries so many things, that it can solve very difficult problems in very novel ways.
Is that “intelligence”? Our casual usage of intelligence isn’t defined precisely enough to say one way or another.
One person would say evolution is blind, and in the short run it is. But another person might point out that evolution is anything but blind. It is an epic version of Edison’s lab, where millions or billions of false solutions are continually tried and ruled out, to find each new fitness enhancement.
It relentlessly experiments and follows the “data”.
On longer timescales, evolution is effectively teleological, highly creative and very intelligent.
And all three aspects compound over time, just like human learning and research, because evolution doesn’t just find new features, but new abstractions and modularity. Such as flexible reusable gene systems for encoding body parts, epigenetic reuse of features in different kinds of cells for different purposes or triggered and “run” by different conditions, nervous systems, etc.
Thus evolution “learned” to speed itself up over time, letting it more rapidly optimize larger more complex solutions. I.e. orders of magnitude faster creation of new novel animals, than it originally took to optimize the first cellular life, colonies of cells, etc.
Watching evolutions first billion years would not have suggested that the plethora of different intelligent animals, from octopus, parrot to human, would have been remotely possible in the time it took. Evolution’s compounding meta learning created brains, our “true” teleology, and its expansion into technological and economic expressions of the pursuit of survival. All meta extensions of evolution, found by evolution.
There are single proteins that need to "evolve" somehow, but they need to arrive at a very particular shape, and extremely near misses offer no feedback information and are just as good as dead.
But the combinations of amino acid sequences they would have to search through in order to find the correct shape is so large that it is greater by several orders of magnitude than the sum total number of all the organisms that have ever lived on Earth since the beginning of time (which is about 10^40, I think).
So your claim that "evolution just tries so many billions of options, man" just doesn't hold water.
There was a time, back during the 1960s, I'm told, when mathematicians in the academy would openly mock evolutionary biologists for their lack of understanding of statistics. But then political correctness took over or something.
Evolution isn't looking for that sequence.
It is looking for any change in sequence with a positive payoff, and in the meantime constantly diversifying sequences with similar outcomes, creating more opportunities for serendipity.
Every large animal is born with mutations. So we are also quite robust to spreading the search, running multiple experiments at a time, taking small risks with genes not quite as good, which will get weeded out quickly when combined with other weaker genes, but in the meantime cast a wider net for meshing with another gene that complements it.
So yes, in any given species with a nontrivial population, millions or billions of genetic variations are being explored at any point in time. We are nothing like carbon copies of each other, differing by just a couple checkmarks.
This is a radical speed up. Just as sexual recombinatory reproduction is. Evolution today operates with vastly more efficient genetic environment, structures and systems than what early life did.
Tractable statistics do no justice to how biology works and all the paths it searches. I am not knocking formal statistics at all, just noting that past one or two step events, the layered statistics of chemistry, genes, gene clusters, epigenetics, populatoin dynamics of complex creatures in their complex environments, etc. are not going to be tractably modelled.
Measurable sometimes for sure, but not symbolically characterizable or calculatable.
That's amazing. Did you just type that out or did you spend a couple of years preparing it just in case?
That's funny. :) I just typed it out in one splat with a few quick edits. But I spend a lot of time trying to get clear and distilled perspectives of everything interesting.
Evolution in humans, from an atheist perspective, is consistent with the idea of a designer: humans are the designer.
Sexual selection in humans is a psychological phenomenon, and it’s subject to all of our most hifalutin ideas.
You have an opinion about what “god” wants and express it through sexual selection, thus influencing our collective evolution in that direction.
Also ironically, by the time we became able to conceive of and communicate about such ideas, natural evolution has long stopped being the driving force behind how humans and human groups look and grow and evolve.
Evolution is still very much a driving force in humanity. Every time a couple struggles to get pregnant, every time birth control fails and results in an unplanned pregnancy, every time someone decides to be child-free, every time someone dies young, etc., etc., humanity evolves toward one genotype over another.
If anything, I would argue that human evolution has accelerated in the last few decades (at least in wealthy nations where people have a lot of control over their reproduction and enormous choice in who they marry, if they marry at all).
But why do they want what they want, and how much agency is there really anyway ?
You might enjoy Dennett’s “The Intentional Stance” for some enlightening exploration of this metaphor, e.g. “The thermostat tries to keep the temperature between 67 and 69” not only makes sense but is a useful way to think of it even when we don’t believe the thermostat has agency.
The thermostat was ultimately designed by a mind that has objectives. That is why it tries to control the temperature.
What about the ocean? The ocean also "tries" to keep the temperature steady.
That one doesn’t quite work for me. A thermostat has a purpose: keep a temperature. An ocean doesn’t really have a purpose.
Dude, that's just the language. "Water wants to flow downhill". It's a model, man. Everyone learns this pretty quickly. I don't get how this position is so popular on HN/Reddit. The language is giving you tools to model the world.
"The water doesn't want anything. It's just the laws of gravity."
Intelligence comes from being able to efficiently compress highly predictive models. Any computational mechanism that is unable to do this is a low-grade intelligence. If you need the whole thing spelled out carefully for you, you're NGMI.
You can say "water flows downhill" instead.
Why not say things in the more accurate way.
Humans anthropomorphise everything, I’m pretty sure it is how we run general intelligence software on small pack hunting tribal creature hardware: we model evolution as a clever sentient trickster and speculate about how does things.
I've observed this as well. People who believe in evolution can't seem to stop themselves from using "intelligent design" language to anthropomorphize evolution.
Except where punctuated by (subjective) catastrophe.
But then it is not the mechanism of evolution itself that is random.
Well, it's never random, is it? It's only random when all is equal, otherwise it's biased. That's why it works.
Consider birds. There was a good article a few days back, on why only non-toothed birds survived. Until the meteor-strike, 65M years ago, all was equal and they survived along-side. Until they were the only survivors.
The asteroid that created the Chicxulub impact came from a rather random location but had a deterministic effect given where it struck.
And here we are, with beaked birds. ^_^
https://en.wikipedia.org/wiki/Chicxulub_crater
Random doesn’t mean all outcomes are equally likely, a coin rarely ends up on its edge.
Thus evolution is often random between local optima. People’s organs don’t represent perfect left/right symmetry but there’s no particular benefit for which of the two options were chosen overall. Ie swap just which lung is smaller and you get lots of problems, but swap everything and it all works.
Local optima are also the reason you get things like the recurrent laryngeal nerve, which runs from the brain to the larynx going under the heart. bad enough in a human, a huge deviation in a Giraffe or a Brontosaurus.
Natural Selection is not the interesting part, though. Natural Selection is the boring part. Obviously a working system will be selected over one that doesn't work.
The interesting question is where the working system that Natural Selection was able to select came from in the first place.
What is a flying vertebrate that is not a bird? Bats are all I can think of.
A bat?
Yea, sorry. I was editing that in before I saw your comment.
There's also flying squirrels, though they don't have true flight. They have enhanced gliding to leap further.
Edit:
Oh, look: flying lizards and flying fish:
https://wildlifeinformer.com/flying-animals-that-are-not-bir...
Maybe optimized for their niche.
I remember reading something about bombers maybe during ww2. I think if fighters were chasing them, the bombers could escape because with their large wings, they could fly high and slow and turn inside them.
the analogy being - some birds might be set up for ground operations and chasing prey, others living in a different ecosystem with high altitude cruising over long distances.
Birds are more optimized than bats in some niches (like long-distance flight), but vertebrate evolution is very stuck into path dependency, and they are way far from the optimum on the things that matter for most animals like maneuverability and acceleration.
Four-winged birds dying up can easily be a complete accident, even more because two-winged birds were almost completely killed once too.
No, but the pterosaurs might have been, because their (likely) quadrupedal launch implies they could optimize away all unnecessary muscle (read: weight) from the legs, so they inherently could optimize their body plan better.
And they also had two wings.
[0] https://en.wikipedia.org/wiki/Pterosaur
[1] https://www.youtube.com/watch?v=scAp-fncp64&t=150s
[2] https://www.youtube.com/watch?v=9TKgupAZVzE
The hair on your head grows indefinitely. The hair on your arms and legs grows to a certain length and then stops. You can shave it off, but it grows again, but only to that same length.
How does the hair on arms and legs 'know' how long it is?
https://www.washingtonpost.com/news/speaking-of-science/wp/2...
Different anagen phase durations.
Still doesn’t answer - How does the follicle know that I cut my hair and needs to grow again to that length?
It doesn't. Hair grows for a certain amount of time - time, not length. Then waits for a while unchanging, then falls out.
growth (3 to 6 years) -> static (2 to 3 months) -> fall out -> rest -> start over
The maximum length of your hair is determined by how long it stays in the growing phase, in general this phase is longer in females than in males. And some people are naturally able to grow longer hair than others.
In pregnant females the static time is increased, i.e. the hair stays there without falling out. This makes their thick and full. After delivery this changes and the hair falls out in clumps, which is unpleasant, but it's the same hair that would have anyway fallen out earlier, it's not extra hair falling out.
If you cut the hair nothing changes, it still grows for as long as it was going to originally.
That myth about cutting hair (or shaving) to make it grow faster or whatever is not real.
This doesn't seem right. If I use clippers to trim hair on my body, it grows back to its 'natural' length. I don't end up with hair of various lengths. Likewise, if I shave it, I don't end up with some stubble and some regular hairs.
I've never experienced a time where I've trimmed body hair and had it remain the same length for a couple of months.
Are you sure? I have fairly hairy body so when I use moisturizers it tends to stick to the hair instead of the skin, and since I'm trying to take care of my forearm tattoos, I trim my forearm hair on a regular basis, after a few weeks, if I look closely, I can see some hairs being around, say, 2cm long, whereas others are just barely growing out.
Look closer, the hairs are random length. You have to wait of course for them to reach natural max length, and then you'll see a randomized mix of lengths.
If you are always clipping hair they will almost all be at one length because > 90% of them are still growing and then getting trimmed. The rest (a minority) have recently fallen out and are just getting started and will be shorter.
Does that make sense?
Interesting. I've occasionally wondered why my hair (from scalp) can reach barely half way down my back, even after leaving it for decades uncut, while other people (typically women?) can grow it to the floor.
Yup, that's exactly it.
I've always wondered if the almost universal preference for women to have long hair is linked with their ability to have long hair, while for males people (not you :) typically want it short, which matches what males are able to naturally do.
i.e. which came first? The ability for hair of a certain length, so that's what people like, or if people like hair a certain way so natural selection helps out.
Perhaps men couldn't win so decided not to play.
IIUC, it doesn't. Hair grows for a certain time period, falls out, and regrows. The follicle has no knowledge of the length of the hair it is producing, it is simply on a grow-stop-shed cycle with a certain period.
So hairs are actually excretions, like sweat and shit.
What’s the opposite of a fun fact
Teeth
There are possible physical answers; it could be lack of inertia on the hair triggers growth. It could be that when you rub your arm, an action as innocent as a yawn, you are also informing the growth of hair. Could it be as simple as looking at your hair or knowledge of shaving causes a subconscious trigger? We already know that these high level shortcuts into low level processes exist (Pavlov's dog anyone?)
It's none of those. Read the sibling replies to your comment.
It doesn't know. Hair cycles are constant. If you didn't cut it, it would eventually fall out as it is replaced by another hair. It grows to the point where the epithelial column contracts and starts forcing it out. This is based on environment, nutrition and genetics and varies from person to person and is affected by everything from stress to blood flow.
You can observe this on many people by looking at hair miniaturization of people who have MBP or similar -- the follicle constricts in size and the resulting hair that comes out is progressively shorter in length and diameter until the follicle is so constricted that it no longer produces hair.
What happens when arm hair get implanted on head? Do they grow like arm hair?
A friend had a skin transplant — inside thigh to foot, IIRC? it was a long time ago — but I do recall that they told me that the hairs come with the skin, and continue to grow as if they were in-situ.
Does it drop off after a certain time period (= length)?
that would be my guess.
No, both grows indefinitely, it's just that head hair takes more time to fall off
This was just asked and answered in /r/askscience
https://www.reddit.com/r/askscience/comments/1c6zztu/why_doe...
and then other parts of skin know not to grow hair at all!
Nails are a much older "invention of evolution" than humans: so we have to investigate there...
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).
I hear this kind of “it exists because it has to exist” thing from non-bioscience types a lot. Essentially, this is just a tautological statement.
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.
The question was how, not why.
Your answer is like saying "how does the eye focus light" and answering "so that you can see".
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.
Tautologies may be unsatisfying, but there's nothing specifically wrong with them.
It is what it is.
"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.
Wow, you amazed me. That is a journey of several orders of magnitude.
That journey of orders of magnitude is the journey of all life on Earth from its genesis to today.
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.
Yeah that’s not how it works. Fitness is not determined for every single attribute.
For example, observation A might be maladaptive, but it is caused by gene B which also causes observation C, which does provide an advantage.
Not always.
On two different occasions, I've had ingrown toenails that had to have a procedure done because, once they started ingrowing, they wouldn't stop doing so. (It's still outward growth but still.)
For others that have had this issue too: Cut your toe nails in a straight line. I used to cut them in a curved line, a 'C' kind of shape. Don't do that. Cut them in a '|' kind of shape. Yes, you'll have a big overhang on the edges, but give your toe time to adapt.
I've had a few of these procedures too and it never really stuck for me. Ingrown toenails kept being a problem. Then I started cutting my toenails in the flat / straight '|' sort of way and I've not have a problem since.
I figure it's worth a shout out to the few of you out there that need the help. But, again, it may not work for you too.
Wow, amazing. Why does this work? I would think that the nail bed has no knowledge of what the end of the nail is shaped like.
Ingrown toenails basically don't happen in cultures where people don't wear shoes.
I suspect the cause is socks or shoes pushing against the skin at the end of the toe, causing it to grow incorrectly over many months.
An untrimmed nail pushes any sock/shoe away, solving the issue.
The nail doesn't change, rather the skin by the end of your toe does.
It keeps the side from curling in and causing the irritation/cut that results in what we call ingrown toenail.
My friend told me this like 15+ years ago. I was skeptical but it really works. I then just use a file to take a bit of the edge off until its a curve
i’ve had those procedures— super painful. i’ve ended up doing surgeries to cauterize the roots to prevent the ingrown growing direction.
Wasn't my experience FWIW. (Mostly commenting so others won't necessarily avoid.)
Just an in-office procedure using some local anesthesia and acid I think. The first time the whole cycle went on for months while I was regularly going to the podiatrist because of a fairly severe foot fracture of the same foot which may or may not have been connected.
This last time--a good 15 years later symmetrically on the other foot--I was pretty much just "Let's do this" after a couple times trying to just cut the toenail.
Despite the terminology, "ingrown" doesn't mean the nail grows in the wrong direction. The lunula continuously emits keratinocytes in a single direction; these form both the nail bed and the nail itself.
The "ingrown" phenomenon occurs well after the nail has formed (it's getting pushed out from the lunula end) and is due to a combination of your toe's (hallux I assume) ideosyncratic geometry and environmental conditions, likely, as another commenter pointed out, how you innocently cut your nail.
Sorry for the pedantry but when I worked in drug development I used to research the nail unit, which, it turns out, few people do.
The most interesting thing I've learned about nails is that they're now thought to be part of an organ — the enthesis organ [1], which is the tissue structures around the site where the tendon attaches to the bone. This is relevant to spondylarthropathies, some of which show up as nail changes many years before enthesitis occurs.
[1] https://www.enthesis.info/anatomy/enthesis_organ.html
It's all designed. Top down, one mind. Designed. That's why it all works.
Think about all the cells, and the thousands/millions of jobs they do, in perfect coordination just in your fingers or hands.
It's all designed.
never thought I'd see a creationist on hn. Giraffe's have a nerve that goes all the way down the neck and back up, merely because it couldn't unwrap from a vein as giraffes evolved longer necks. No designer would include such a needless waste of resources. If there's a watchmaker, he's a blind idiot.
https://timpanogos.blog/2011/10/08/evidence-of-evolution-gir...
exactly and clearly. Stray chance, dump nature ,blind force, unconscious casuality and the elements that without restriction are scattered in every direction -none of these can have any part in the most balanced,wise perspicacious, life giving ,orderly and firm deeds of the Creator. They are used ,rather by the command will, and power of the Glorious Doer as an apparent wil to conceal of His power.
According to the meaning of the verse: Who has created everything in the best way,(Quran)
everything is cut out according to its innate abilities with perfect measure and order, and put together with the finest art, in the shortest way, the best form, the lightest manner, and most practicable shape. Look at the clothes of birds, for example, and the easy way they ruffle up their feathers and continuously use them. Also, things are given bodies and dressed in forms in a wise manner with no waste and nothing in vain; they testify to their number to the necessary existence of an All-Wise Maker and point to that Possessor of Absolute Power and Knowledge
It’s a shitty designer that includes cancer et. al. in the spec.
There is not some guy coordinating all our cells to grow and work in lockstep. It's actually just a few very simple algorithms (e.g., cells differentiation is triggered by strength of a signal, a la HOX genes) that have been very slowly refined over millions of generations of evolution.
Just because the output of a system looks complicated does not mean that the input into a system is complicated.
Dragonflies are pretty much the best flyers among the insects, and they have four wings. Maybe not directly comparable, but still.
insects are so different from vertebrates (e.g. multi compartment bodies thanks to how they express the HOX gene) that the flying strategies are also quite different.
At the current level of O2 in the air I don't believe that flying insects can grow as large as even a hummingbird, much less an eagle.
"At the current level of O2 in the air I don't believe that flying insects can grow as large as even a hummingbird"
This got me curious. At a quick search, I found out that there in fact is at least a species of flying insect larger than (the smallest) hummingbird:
https://en.wikipedia.org/wiki/Gauromydas_heros
https://en.wikipedia.org/wiki/Bee_hummingbird
The insect grows up to 7 cm in length whereas the bird can stay less than 6 cm at adulthood. No information about the insect weight though (for a proper comparison).
Hummingirds are the best flyers among vertebrates, with only two wings. They can hover very precisely, and they can also fly backwards.
Generally, all insects have four wings, but they are not always used for flying.
Flies in particular are also among the best flyers, and yet, they use only two of their wings for flying, the other two shrank to became halteres, a sensory organ acting like a gyroscope.
Dragonflies may be the best fliers in the insect world thanks to their four independently controlled wings, but flies may be the second best, and they achieved that by losing two of their four wings. Evolution is interesting.
Because nails and hair are produced at a fixed site. The nail itself is dead cells and do not grow.
That isn't the question (in a prior job I studied the physiology of the nail unit). Most cells don't normally have an orientation, so you'd think that thefollicle would push out a hair in some random direction, sometimes towards the outside world and sometimes in the direction of your bones.
Obviously they don't (!) but the question is how?
The nail is the same: the lunula emits these keratinocytes in only one direction; even more weirdly it's a planar structure.
> Obviously they don't (!) but the question is how?
Don't cells "just" orient themselves using mechanotransduction [1] or am I missing something? That's a bit hand wavey but since cells don't form 3d structures in tissue themselves, they orient against the extracellular matrix using mechanotransduction and other growth factors.
The development of multicellular life was essentially cells learning how to orient themselves into a digestive tract.
[1] https://en.wikipedia.org/wiki/Mechanotransduction
According to simulations of plate tectonics, these two locations would have been somewhat closer... 100m years ago.
If they fly through the same air as airplanes, you also lose efficiency wherever you have wingtips (pressure below leaks above, basically), and the rear wings can get messed up by turbulence/vortices from the front wings if you're not careful.