Evolution is not a straight line. It’s more like a branching tree, a series of experiments as to what gives a life-form an advantage. Some animals mutated in such a way that they could achieve flight (probably by gliding first), and this was so useful that they passed the trait on to their offspring.
Any particular animal can trace back in a line, but the different branches are separate.
Actually, the only unnecessary items eliminated are those that interfere with reproduction. Those that don’t may continue for a long, long time.
There is no “why” - it just happens. And wings happened several times, first in insects (many of which still fly today), then reptiles (the pterodactyls) , then birds, then mammals (bats).
At present, insect wings are believed to have evolved from outgrowths of their carapaces. Pterodactyls, birds, and bats all use modified forelimbs, but details of those limbs indicate that they all evolved wings independently.
I’m not a biologist, but the basic idea of evolution-by-natural-selection is that if a member (or members) of a population of a species develop something a bit different from the other members of that population, and that something gives them a slight advantage (in finding food, finding a mate, or anything else that contributes to survival or procreation), then the next generation will have more members with that something, and the next generation even more, etc.
So imagine a population of land-dwellers. Some of them are born with, say, slightly flattened front limbs, and so they can glide just a bit when they jump. This gives them a little advantage in getting to food and mates. So they contribute just a bit more than their “normal” share to the next generation. So the subsequent generations have a higher and higher percentage of members with flattened front limbs. Eventually, they’re all gliding, just a bit.
Then sometime later, a few are born with even more flattened, and maybe elongated, front limbs. So, again, they’re just a bit better at the survival and procreation stuff than their neighbors.
Continue that over millions and millions of years. The gliding gets longer and higher, at some point some bright member of this species thinks to try flapping and gets some added lift, the flattened limbs start getting lighter, the scales elongate into feathers, and eventually you’ve got birds.
Remember, millions and millions of years. Humans have a hard enough time thinking in terms of multi-hundreds of years; trying to apply your normal “common sense” to millions of years just doesn’t work.
Moreover, it’s meaningless to say that one species is evolved more than another. All species currently alive have evolved for three point something billion years. We happen to come from a line which has been successful at doing the things that humans do, but cockroaches also sit at the end of a long line of things that are very good at being cockroaches, and bacteria are at the end of a long line of things very good at being bacteria.
If anything, you could even argue that cockroaches and bacteria are much more evolved than humans, since the cockroach line has gone through far more generations since the origin of life than humans have.
In regards to birds specifically, it should be pointed out that they are descended from a branch (maybe more than one?) of the dinosaurs. And it now seems to be accepted that some dinosaurs already had feathers. So it’s not like feathered wings just suddenly appeared. Evolution used an already existing structure and slowly adapted it to use for flight.
Flying squirrels and flying fish are examples of animals that have partially evolved flight. In the case of the fish, the fins are the wings. Or, at least, that’s the closest way of expressing it, depending on how you want to define fin and wing and so forth. If these animals kept evolving in a somewhat stable world the future versions might be as capable as birds or insects in flight - but it can be hard to guess and impossible to wait and see.
Smaller dinosaurs evolved feathers (probably down at first) for insulation purposes, for the same reason as small mammals have fur. Some of those feathers developed into prototype pinnate feathers like what you probably envisioned when you read ‘feathers’ above.
From there, two competing hypotheses, both with evidence but not enough to prove it:
Cursorial (running) bipedal mini-dinosaurs discovered that getting a little lift from spreading their ‘arms’ aided in speed and in catching prey they needed to leap to catch. Natural selection favored those with better lift.
Arboreal mini-dinosaurs discovered that their jumps between trees could be stretched into a glide by the same means. It’s only a short hop from an extended glide to self-sustaining flight.
That’s vastly oversimplified but gives the picture. Colibri may have some supplemental data I haven’t read on evidence for the origins of bird flight.
Note: All the other specializations of modern birds are to improve flight, more or less. E.g., a beak is lighter and more easily healed than teeth – teeth are needless weight. For mastication, use a muscular craw, not teeth. Some Mesozoic birds did have teeth; they didn’t survive. Bones and muscles need to be light – except the ones that support the body in flight. And so on.
Actually, a more correct evolutionary timeline is from fish, to fish and amphibians, to fish, amphibians and monkeys, to fish, amphibians, monkeys and humans. Of course, several million species are left out of that timeline, but you get the idea. We still have fish, amphibians, and monkeys.
primates, not monkeys. I believe the average gorilla has more in common, genetically, to a human than to the average monkey. Monkeys and gorillas are very different.
just a nitpick, but I hate the “humans evolved from monkeys” thing cause we didn’t, we’re all seperate branches of the same tree, but we didn’t evolved from them, we (probably) evolved from a common primate ancestor
True flight has evolved independently a total of four times:
Insects, more than 330 million years ago.
Pterosaurs, about 220 million years ago.
Birds, more than 150 million years ago.
Bats, more than 50 million years ago.
The wings of all of these evolved separately; one did not descend from the others. The wings of pterosaurs, birds, and bats all developed from the forelimb and so have some structural similarities; however, they differ entirely in detail.
As far as I am aware, the question hasn’t been resolved definitively yet.
Humans are apes, apes are monkeys, monkeys are primates, primates are mammals, mammals are vertebrates. Apes are a particular group of monkeys in the exact same sense that birds are a particular group of dinosaurs.
Only in the very loose sense that “monkeys” is a common generic for Anthropoidea.
Old World monkeys belong to one family (Cercopithecidae); marmosets and tamarins to another (Callithricidae); and the rest of the New World monkeys to a third (Cebidae). Humans and their extinct relatives are Hominidae, and modern classifications include the great apes in that family as well; older classifications put them in Pongidae. (Gibbons and the siamang are in Hylobatidae, unless that’s regarded as a subfamily (Hylobatinae) of Hominidae or Pongidae.
Although I maintain that since we kept the generalist form of the original plains apes, while the great apes and gibbons specialized, it would be more correct to say that apes descended from early hominids than that we descended from ancestral apes.
Right - which suggests if evolution was rerun, that wings (like eyes, which also evolved independently multiple times) are much more likely to evolve in the new timeline than human-style intelligence (which so far as we know has only evolved once)
Evolution only gets rid of unnecessary qualities if they interfere with reproduction. Along the same lines, evolution only views qualities as beneficial if they help with spreading genes around.
Being able to flap away from predators is helpful. Even chickens do it and they can’t fly for shit. There are lots of animals that can “sort of” fly. Compared to creatures that can’t, they may or may not have an advantage. If they do, they probably spread their flying genes. If flying doesn’t help, having useless wings might hinder them and get them killed before they have X number of kids. Where X is the average number of kids they need to procreate reliably.