Bird are reptiles. Whether or not birds descended from dinosaurs or if they just share a common, non-dinosaur ancestor is still up in the air. The biggest sticking point is the number of toes between the group of dinosaurs thought to be ancestral to modern bird, and the number of toes birds have. Recent genetic analysis of chickens suggests that, for chickens at least, they have genes for as many toes as dinosaurs have, but they're expressed differently. It's in a Hox cluster, I believe. Whether or not this is true for avians other than chickens is unknown.
dinosaurs and birds
- Thread starter Orleander
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Bird are reptiles. Whether or not birds descended from dinosaurs or if they just share a common, non-dinosaur ancestor is still up in the air. The biggest sticking point is the number of toes between the group of dinosaurs thought to be ancestral to modern bird, and the number of toes birds have. Recent genetic analysis of chickens suggests that, for chickens at least, they have genes for as many toes as dinosaurs have, but they're expressed differently. It's in a Hox cluster, I believe. Whether or not this is true for avians other than chickens is unknown.
If I recall correctly there is an other even more serious problem with the birds from dinosaurs theory related to their different types of lungs. Dinosaurs had lungs much like humans (big sacks whose volume could be changed by mussel action to suck in and expel air. Bird lungs work differently, but I forget the details. The problem is that any small change from the dinosaur type lung towards the bird type lung would be fatal for that evolving creature.
To make an analogy: vehicles have two different ways to push on the ground: Wheels and treads (like a tank). A wheeled vehicle that "evolved" part way to treads, would not move or certainly would be at great selective disadvantage.
Perhaps some one will search and explain nhow bird lungs function and do so with very different mechanism. I think they are sponge like, not bag like and evolving from one to the other does not seem to have viable inter mediate possibilities.
Fact that dinosaurs did evolve feathers does not count as much evidence that birds derived for dinosaurs. For example eyes have evolved indepentantly at least 8 differnt times. The octupus has the retina in front of the blood vessels and nerves, not behind as in the poorly designed human eye. One little slow moving creature has only a single photo-sensitive cell in each eye. It is on a stalk, inside the eye and there are mussle that can move that stalk latterially in 2D. I.e. It sequentially "SCANS" the visual field - does not need a complex brain for parallel processing 100,000 or more neural imputs as humans and many others do. Many birds of prey have two separated fovea in each eye. One is used in the last seconds as it catches it prey* and they other, much more sensitive and with much higher resoultion than humans have, is used while bird is high in the sky looking for some prey to swoop down on.
Point is feathers are a form of "generalized fur" and very useful for thermal control. Dinosaurs may have had them for that reason. I do not know if the flying dinosaurs had feathers on their wings or skin flaps, like a bat, but it really does not matter or prove much if they used feathers for flight too.
SUMMARY: Probably birds do come from dinosaures - I am not arguing against that; only pointing out that in addition to the toes problem there is a more serious lung probelm.
I have a bird (calopsita, is the type in portuguese) and they all have two toes going forward and two toes going backwards. Most birds have three forward and only one going backwards so the toe arument against the "birds from dinosaurs" is not very strong with me. I.e. it is easy to get an extra toe (Or lose one). Humans sometimes have other than five fingers. (4 or 6 being the most common alternatives.) This is probably a "development error" and not a genetic one, but there may be a genetic component.
Too bad we do not normally have six fingers. Twelve based math is so much better than base 10 with its only divisors being 2 & 5.
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*Copes with a very high rate of images shift on that retina well,but has poor resolution - prey is just a blur anyway but birds needs to accurately know where it is wrt the bird claws and beak.
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I thought they all had 3. But ostriches don't seem to. Ducks/chickens have 3 with that little thing in the back. And pigeons seem to have 4 ????
Ah, my bad, it's not bird toes, but bird fingers- wing digits. I confused that with BMP/gremlin experiments which can turn a chicken foot into a duck foot.
Ok, so there's a group of dinos called the theropods which are thought to have evolved into the modern bird. In 1969, Ostrom noted 22 similarities between Archeopteryx and Deinonychus, found between no other groups, and linking birds & dinos. In the 1980s, Guathier's cladistics showed 17 similarities between theropods & birds shared by no other groups. A key similarity being the 3 fingers and four toes that theropods & birds have.
However, developmental biologists showed that theropod dinosaurs have wing digits 1-2-3, while embryological evidence shows that modern birds are 2 3 and 4. (1= index finger, 2=middle finger, etc). Development of cartilage also shows the 2-3-4 in modern birds. This means convergent evolution had to happen in 3 different places in supposedly related groups. Birds also have reverese, grasping feet, which no dinosaur found has, and dinos also have a pointed jaw (note that dino here means theropod dino, not an anklyosaurus, etc).
Genetic investigation of these bird digits, however, is interesting. While phenotypically it looks like 2-3-4 development, the Hox gene (major gene organizers that set up the pattern of an organism, Hox homologues have been found in all multicellular life) expression patterns are indeed 1-2-3. At least, in chickens this is the case. In other birds, it's not known.
As for lungs, bird lungs are huge, filling up the bones and stuff. They also breathe in & out continuously, so that air goes one way. Extremely efficient. I haven't heard anything about dinosaur lungs one way or the other.
Ok, so there's a group of dinos called the theropods which are thought to have evolved into the modern bird. In 1969, Ostrom noted 22 similarities between Archeopteryx and Deinonychus, found between no other groups, and linking birds & dinos. In the 1980s, Guathier's cladistics showed 17 similarities between theropods & birds shared by no other groups. A key similarity being the 3 fingers and four toes that theropods & birds have.
However, developmental biologists showed that theropod dinosaurs have wing digits 1-2-3, while embryological evidence shows that modern birds are 2 3 and 4. (1= index finger, 2=middle finger, etc). Development of cartilage also shows the 2-3-4 in modern birds. This means convergent evolution had to happen in 3 different places in supposedly related groups. Birds also have reverese, grasping feet, which no dinosaur found has, and dinos also have a pointed jaw (note that dino here means theropod dino, not an anklyosaurus, etc).
Genetic investigation of these bird digits, however, is interesting. While phenotypically it looks like 2-3-4 development, the Hox gene (major gene organizers that set up the pattern of an organism, Hox homologues have been found in all multicellular life) expression patterns are indeed 1-2-3. At least, in chickens this is the case. In other birds, it's not known.
As for lungs, bird lungs are huge, filling up the bones and stuff. They also breathe in & out continuously, so that air goes one way. Extremely efficient. I haven't heard anything about dinosaur lungs one way or the other.
Perhaps that is what I was trying to remember. It is nearly impossible to imagine how a cyclic breathing system could evolve (by small changes) into a one-way flow system via viable intermediaries.
Presumably it did happen, at some point.
More genetic evidence will likely be forthcoming.
I recall reading that birds simply lack a single hormone, which if applied to their diet, causes them to grow teeth like archaeopteryx. The genetic mechanism for teeth is present, but suppressed. Easy to see how suppression of the protein expression would have been favored.
Also, I recall reading that some species of extant birds have claws on their wings when the bird is hatched, though they subsequently lose the claw as they develop. Again, easy to see how repression of that genotypic expression would be favored.
I recall reading that birds simply lack a single hormone, which if applied to their diet, causes them to grow teeth like archaeopteryx. The genetic mechanism for teeth is present, but suppressed. Easy to see how suppression of the protein expression would have been favored.
Also, I recall reading that some species of extant birds have claws on their wings when the bird is hatched, though they subsequently lose the claw as they develop. Again, easy to see how repression of that genotypic expression would be favored.
Yeah, genes are weird. Us vertebrates are full of genetic material for all sorts of ancestral traits, but its activation is all controlled by genes that control genes that control genes. Activation of inhibitors that prevent tail genes from turning on.
It's like it's easier for evolution to work on downstream elements rather than the core genetic material.
It's like it's easier for evolution to work on downstream elements rather than the core genetic material.
John Connellan
Valued Senior Member
Well, that's pretty much how the blind watchmaker (evolution) works. Evolution never 'thinks' about the most efficient wat to get things one - merely the easiest way at any given time!
I guess what I mean is that in the evolution of any given trait, it's more likely that the basal trait already exists, and changes affect genes that affect that gene, or even genes that affect genes that affect that gene. Homeotic genes, for instance, are conserved, but by moving around where they get expressed you get virtually all the different morphology of animal life. I'm exaggerating here, but not a *whole* lot.
John Connellan
Valued Senior Member
No, evolution thinks.
I like to colour my language a bit
. Read like this then - evolution does not happen in the most efficient way - merely the easiest way at any given time! :bugeye:Fraggle Rocker
Staff member
Yes. Most birds have three pointing forward and one pointing backward. Psittacines (the order of parrot-like birds) are zygodactyl, meaning one of the front three is bent around the other way, so they have two and two. This gives them their amazing dexterity to go with their intelligence, so they can dismantle their cages from the inside. A few other birds like woodpeckers also manifest zygodactyly; it's very handy for climbers.
Heterodactyly is similar to zygodactyly, it's just a different toe that got bent around. You'd never know the difference without a close examination. It's 1-2/3-4 instead of 2-3/1-4 like parrots.
Syndactyly is the fusion of two toes into one, similar to the way the hoof of an ungulate is merely all the toes fused together. Syndactyl birds, for all practical purposes, have three toes, but biologically the four are still evident.
Birds have air sacs which allow their lungs to expel air at the opposite end from the trachea. Be grateful that your bird has never had an infestation of air sac mites. When you kill them they obviously stop moving; their bodies can clog the air passages and kill the bird. It's not hard to picture the air sac evolving incrementally. As the bird exhales normally through the trachea, a small amount of air leaks out through the air sac with its one-way valve, making exhalation incrementally more efficient. Eventually the air sac takes on the majority of the load, and the musculature and interior of the lung are during the same time period gradually reshaped to take advantage of this, until finally air flows through the lung in only one direction.
That's a difficult hypothesis to peer-review since no details are provided. How exactly would two wheels evolve into two rollers with a tread? Is the tread the vestige of a third wheel, or some new growth?
Most bird and reptile embryos develop with an egg tooth, which they use to break out of the shell, but it is not a true tooth. Some snakes and lizards grow a real tooth for this purpose that is shed after use.
Turkeys are closely related to grouse, and they're all galliforms, members of the order of chickens. There are fossils of ancestral turkey species all over North America, going back twenty million years. The California turkey was still in existence when humans first colonized the region several thousand years ago, but it died off due to climate change.
which one is the ostrich?
Fraggle Rocker
Staff member
All ratites have three toes except the ostrich, which has two. Without becoming a member of a couple of ornithological societies so I can read more than the first page of their papers, what I have gleaned seems to indicate that the ratites are syndactyl, with two toes fused into one. Presumably the ostrich just took it one step further and underwent an additional fusion.
Ratites are a separate clade of class Aves, distinct from all other birds. (Although this taxonomy is not universally accepted.) They have no sternum to which wing muscles could be attached, and therefore are incapable of flight. (Duh. As I mentioned elsewhere, a bird as large as an ostrich would require an eighty-foot wingspan to maintain lift.) The ratites include the ostrich, emu, three species of cassowary, two species of rhea and five species of kiwi. They range from ten feet tall and 350 pounds to one foot tall and three pounds.
The largest bird known to humans was a ratite, the Aepyornis of Madagascar, one of the species in the "elephant bird" family. It weighed half a ton and survived until--you guessed it--Africa's European colonial period.
John Connellan
Valued Senior Member
Didactyly?