I wrote this about 6 years ago, before the more recent discovery of prehistoric bird finds, but still think it conjures up some provocative questions. You may notice it's not claiming a theory, but acts more as a commentary on the possibilities that both the "Dinosaur-spawned" birds theory and Thecodont-spawned birds theory may BOTH be right.
Dual Ancestry & Avian Evolution
What makes a bird a bird? Just because an animal has feathers does it make it a bird? After all, scales are a common occurrence in both fish and reptiles, so why can’t a feather have other embodiments than birds?
The first thing to keep in mind concerning evolution is that it does not have a clear-cut set of rules.
When I say this, I mean that evolution can manifest itself in a species via different situations and almost certainly due to vastly different genetic and environmental circumstances.
One way to have the public understand this is through comparisons and analogies. For example, there is an old saying: "Lightening does not strike twice in the same place." Well, ironically it does sometimes and as in the case with evolution, certain traits and features do evolve over and over again.
With this in mind, try to accept the concept that a species may actually evolve twice, especially if the circumstances are right. Birds have been said to evolve mainly through two theories.
One is that they evolved from the early ancestors of the Dinosaurs and Archiosaurs called the Thecodonts. Evidence suggest that fossil finds such as Protoavis have clear and indisputable bird-like features…features that are even more bird-like than its more famous counterpart Archaeopteryx.
The other theory is that birds evolved directly from small predatory dinosaurs called Theropods. For years, debates have gone on endlessly in trying to determine the true origin of avian evolution.
The fossil record has been fairly generous in providing some very credible aspects as to where and when birds first came into being.
Both theories have some pretty well established definitions that point to a bevy of fossil evidence to prove their points. However, the idea that birds may have actually evolved TWICE and that both theories may be correct has never been considered seriously. (or possibly even thought of - I can find no articles of a "Dual Evolution" of birds anywhere) This is my personal concept, and one that seems to be a possibility, concerning fossil finds and their hectic, inexplicable age-and-date issues.
The idea of a “repeated evolution” would explain why the fossil evidence seems so correct regarding both theories.
Convergent evolution is mainly stimulated by the similar adaptations to unrelated animals living similar lifestyles in similar environments. But lets, look at the key word that defines convergent evolution…unrelated.
The idea here is to find out what convergent evolution has to do with repeated evolution or dual ancestry. It's quite simply really, the fact that an animal species has developed comparative similar traits proves how evolution can repeat or try the same experiment more than once.
If evolution can produce similar traits in "unrelated" species, how much more can it produce them in a species that are already alike…in fact a species this is related?
Consider the theory at hand…Dinosaurs evolved from Thecodonts, but so did the early birds like protoavis. In time, dinosaurs became more bird-like and due to similar circumstances that had some early thecodonts turn into birds, they too began to evolve their own version of birds.
This scenario, as unlikely as it may seem, may have actually occurred. Archaeopteryx may have been only one species of new bird that evolved from the bird-like dinosaurs and protoavis and kin may have evolved earlier, thus presenting both theories as true.
The concept is, that bird traits including feathers, opposable upper jaws/beaks, and enhanced breastbone for flight may have evolved along different lines.
However, this would not be a case of convergent evolution, since both theories have birds evolving from closely related species…If so modern birds may be heirs to a dual ancestry.
Is there an ancestral line to Species that leads to more than one root?
How could evolution produce so similar a species twice?
(My Mammal Variants Analogy)
Remember the key word in this concept…relation. Evolution has done similar experiments before.
Consider how the therapsids became so very mammal–like. They were no doubt the forebears to all warm-blooded endothermic mammals. However, they would not truly reach huge diversified mammalian status until well after the demise of the dinosaurs.
Yes, there were mammals living well during the reign of the dinosaurs, but why didn’t the therapsids breed a more successful heritage during the Mesozoic? That is still up to conjecture.
However, they did evolve into a comparable evolutionary line. The therapsids have not one line of heirs, but three…The monotremes, the marsupials, and the placental mammals and they all came from those early therapsid ancestors.
All three classes still exist today. However, they did not all evolve at the same time and they probably didn’t evolve from such a directly pinpointed ancestor.
This supports the idea that nature will often repeat its experiments in evolution bearing forth more than one species to compensate for another in a similar niche.
In essence, when convergence is applied to a closely related species, but manifests itself in a different era, it would be safe to call this a dual ancestry.
I believe this may have happened to the birds and thus explain the fossil evidence and confirm both theories -- as much as a theory can be confirmed by a mere concept that is. But, if birds derived from a dual ancestry, which modern birds of today originated from the dinosaur line and which from the thecodont line?
Perhaps studying modern birds via new insights into DNA analysis, and keener exploits into skeletal cladistics can provide an answer to this ongoing riddle.
All in the Feathers - Do Feathers Really Determine Whether an Animal is a Bird?
For years feathers have been the main hallmark in determining an avian species, at least in public view. But could this be wrong? I think that the internal structure and skeletal features are far hardier evidence in determining the ancestry of modern birds. Feathers themselves can be very deceptive.
For example, there is more than one type of feather. Feathers have established themselves among our modern birds in a wide array of forms. Some are combed and lengthy, others are long and wispy.
In today’s flightless birds as in flightless birds in prehistory, feathers did not feature aerodynamic roles and thus became an agent for temperature control and natural advertising. However, feathers must have been formed in a flightless variety before birds took to the air.
When Chaterjee discovered protoavis in the mid-eighties, he was ridiculed for stating his theory that it was the oldest bird fossil ever found.
The main reason why his discovery was not considered a bird was the lack of feather imprints. Despite critical examination by strong experts in the field including John Ostrom, protoavis would go down in history as a misleading link toward bird origins. This seems to be a sad case for a scientific determination of a fossilized species, as some dinosaurs have the unmistakable imprints of feathers in their fossilized plates.
Both Caudipteryx and Sinosauropteryx have feather impressions to prove that the beasts were indeed feathered animals. This fact single handedly proves that feathers do and did evolve on other animals’ besides birds. However, what makes this a strange assessment is the fact that these dinosaurs are not the closest relatives to birds that dinosaurs have produced.
In fact, other species of dinosaurs are much more bird-like in internal structure than the latter two.
I believe this fact supports my concept that feathers are a deceptive trait in the true determination of bird origins. Sinosauropteryx had feathers that may be debatable as untrue feathers. Mainly on the premise of them being more wispy and hair-like.
In fact, they are considered more of a down than actual feathers. However, Caudipteryx did have all the traits of true feathers including the barbed and quill features. It is even thought that some evidence of coloration is provided in the fossil finds. But even if it had true feathers Caudipteryx did not use them to fly, but instead probably benefited from the temperature control qualities feathers can provide.
Protoavis did not present any feather imprints in its fossil record, but did boast a much more bird-like internal structure than the feathered dinosaurs. So with this in mind, Protoavis should have been considered a bird or at least a bird relative, regardless as to whether it had feathers or not.
Still, finds like Protoavis are dated far earlier than the "dinosaur-spawned variations, and as Leonard Marten contends...you can't be older than your ancestor.
It's this simple contention that led me to consider that a "dual" evolution of birds might have occurred.
If the internal structure of the fossil was closer to modern birds, which most paleontologist readily admit, then protoavis must be a strong key to opening the vault on avian evolution. Put simply… feathers don’t make a bird.
Feathers, like scales and hair, are features that are both mutable even among closely related creatures. It may be quite possible that an animal completely unrelated to birds may evolve feathers or feather-like features in the future.
Classifying modern feathers has also had its problems, so it would certainly be logical to make other features that birds generally share as a stronger marking point in determining whether an animal is a bird or not.
Is classifying birds on the premise of feathers like classifying a man because he has hair?
This is not well placed. But the idea that birds (and possibly other groups of animals) may have emerged from a dual evolution, might be worth considering, especially when fossil evidence seems to credit BOTH theores.
For more information of the thecodont evolution of birds - research articles by Leonard Martin.
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If this were the case, it surely would be born out in genetic evidence? We would see two distinct genetic lineages between birds of one line and birds of the other, much like how the Aardvark and Anteater resemble each other, but are genetically distinct.
The lineages could completely blend and there is literally no evidence, genetically, of a "dual" ancestry.
This can happen in just a few thousand years. We discovered this because some "Germanic" people mixed with some existing European people. They brought tools and methods "north" but the genetic makeup of the modern people does not account for them, at all: it is literally indistinguishable.
We know they mixed because the artifacts and influence are there. We just don't see the genetic evidnce because they blended into "oblivion" into the natives.
If such a problem can occur in a few thousand years, imagine what tens of millions of years can do to a blended ancestry.
We would need to find fossils that occurred shortly after the "blending" occurred. It would have to be so specific (less than a few thousand years) that it is far more likely to be impossible to discover (if any fossils remained from that blending, to begin with) than it is to validate the hypothesis. This does not mean we should not look, however. This also does not mean that there is "blind blending" going on: there could still be some evidence for dual ancestry.
However, we would need genetic evidence from both "ancestors" and then look for those markers in the progeny. Is it possible to do that? Sure...but very unlikely to find anything to support or deny the position...as far as nucleotide sequences are concerned. It is far worse than finding a needle in a haystack.
I'll be the first to admit that I have no formal background in science and am little better than the average person in regards to my knowledge therein, but it seems to me like you're using an example of differences between two groups within the same species to forward an argument regarding differences/similarities between separate species or even larger taxonomic groups.
Again, not any kind of expert, but that seems like a logical stretch/imperfect analogy to me.
__________________
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to him who loved them so.
Where the faint murmurs now dwindling
echo o’er tide and shore."
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Not really: it's just an example to show why you may not find two legitimate ancestors. The blending could have occured in such a short period of time as to render the idea that you WILL find two ancestors, almost moot. It's even easier among two ancestors of the same species than it would be for two ancestors that are different species (you'd have to know both the original species...which is not quite as easy as it it with humans because we have all three...the theory of the thread starter is two different species...which ones? We only have "one", at best. Even then, we don't know for sure), that are two different species, from tens of millions of years ago. Far more likely is to find a single ancestor (during a period of stability) rather than an ancestor or two during a period of rapid change. Yes, species go through periods of stability and short periods of rapid change (evolution). This is why when Christian apologists (young earth theory) connecting fossils...they just aren't there: rapid change makes it far harder to find them in "perfect" succession.*
It's not as simple as "definitely two ancestors will be found without exception". There's the problem of blending and rapid change with little hope of finding fossils during that rapid change. This is the bane of geneticists that work with extinct species. Not to mention trying to find usable or actual genetic information from fossils (rare, as well).
*There is speciation that occurs in "circles"...I can't explain that too well. Also, two different species producing fertile offspring...how does that even factor in when that should, by some biological definitions, not work (because they would, by some definitions, be the same species if they can produce fertile offspring). I do not buy the theory of two different ancestors for "birds", but I also cannot dismiss it as completely false and still consider that position "completely scientific". The only way to make a sure statement like that is to have all relevant evidence that allows for a perfect and objective conclusion: we do not have such evidence.
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Last edited by dadudemon on Nov 2nd, 2011 at 01:16 AM
I'm a bit more in line of thought with "Omega Vision" here. Mainly on the premise that Protoavis was found not "thousands" of years before Archeopteryx, but "millions." In fact, it was not identified as a bird due to a lack of feather imprints.
The idea of dual evolution could be applied to several different phenomenon in evolution. I, as a person that believes microbial life evolved on earth more than once (meaning that I think early life started and then went extinct billions of years ago, only to evolve again) am a bit biased and predisposed to the dual evolution idea.
But regarding bird evolution, it's more pressing to suggest it here, because there is such a wealth of credible evidence to support both theories of their evolution.
Because one party declares there finds right, and the other does the same, both pointing to credible fossil evidence - it leads more credence that they BOTH may be corect.
The question is, how could this be?
My suggestion was that they might have been a product of two ancestral lines.
As King Kandy pointed out - this would be a problem best dealt with by genetic research.
In fact, genetic gene-tracing would probably be able to distinguish which modern birds came from an elder ancestral line, and which birds had come from a more recent one.
Still "dadudemon" has a valid argument, at some point in time (probably millions of years ago - it's possible (more likely than not) that these two lines may have merged, being as how they would be somewhat closely related to begin with.
If birds did evolve from dinosaurs (as most people believe) then they still share a very similar genetic make-up of the Thecodonts, since dinosaurs themselves evolved from Thecodonts.
But if birds evolved from Thecodonts, they would either predate dinosaurs or appear at the same time in evolutionary history. Strangely, fossil finds like Protavis point directly at this paradox.
I think the key to differentiating the two is in feathers. The fossil finds that suggest a Thecodont origin are as old as the early dinosaurs. These rarely have feathers, (some have down) but have undeniable anatomical features that only birds share today.
Those feathered dinosaurs discovered, appear a great deal of time afterward - and this is what poses that greatest puzzle...how can you be older than your ancestor?
if there were a dual ancestry of birds, it would be impossible to wash out due to blending
the main crux of the hypothesis would be that birds arose twice in distinct circumstances, meaning, these two streams would be genetically dissimilar to the point where the two lines could not interbreed, one of the definitions of species.
a "dual evolution" situation where the organisms in both evolutionary paths could still interbreed is hardly dual at all, as the organisms they are evolving from are essentially identical. As time goes on, genetic codes tend to become less similar than more, so millions of years with two evolutionary paths is almost certain to produce more dissimilar lines than any sort of convergence on a single genetic code.
In fact, I can't think of any time in evolutionary history something like that could happen... It would be like saying a mouse could evolve to the point that it was genetically similar enough to humans that we could mate... or that chimps and humans may evolve enough that we converge on a point where we could again mate with eachother... idk, but that seems impossible, or so improbable as to be impossible
Gould made essentially the same argument when talking about fish.
essentially, as a category, they don't exist biologically, given the diversity.
this is the same as the eye appearing several times during evolution, and given what we are learning about the ability of epigenetics to "reactivate" dormant genes based on the environment, should not be unexpected.
But you're comparing two lineages of a single species that merged, not the merging of things that were different species to begin with. He is claiming there are lineages from two different types of ancestors, how could they merge when they are unrelated species?
You want it to be apples to oranges, but it's not.
I already asked that question. It's not my theory, ask him.
I was only pointing out that you can't claim that you would see two distinct lineages (in the DNA) when something even easier to see (because it's far younger and from the same species...making it much easier to come up in tests) has blended into oblivion in a far shorter time.
In other words, your criticism is invalid. There' other legit criticisms such as "two different species.....mixed? Then they were two different species: they were subspecies of the same species."
On the premise of species here: there is a bit of confusion regarding the ancestral lines as either subspecies or "unrelated" species, and this falls into conjecture, mainly since the fossils are actually still being debated on concerning their status and classification in science to begin with. (in other words the fossils in question are considered "bird-like" but not fully accepted as birds.)
What is undeniable is that both fossil stocks seem to be transition species - all heading toward bird evolution.
On the idea that two separated species can interbreed:
There are many exceptions to this rule and all have to do with the genetics of the species in question.
If the species are closely related, and in many cases NOT closely related, some times this is possible. My ward against this thought is in the time to which separates the speices.
I was under the impression that if millions of years separated the "diversiance" of species from one another, then they would not be able to interbreed.
Apparently, I was wrong on this notion. As an example, I was told that Neanderthal man and Cromagnon man could have interbred, despite having diverged from a common ancestor over nearly half a million years apart. So I considered them NOT closely related.
It's clear that Neanderthal man was a different species of hominid, then us, and I had figured their genes were too differentiated to be compatible to mix.
Still, even if the birds ancestry did NOT mix, it would not rule out a dual evolution, only that one line went on to become the modern birds we know of today, while the other went extinct.
In fact, some paleontologists hint at this on more than one occasion when talking about the birds "experimental dead-end" species such as the 4-winged Micro-raptor.
Last edited by darkriddle on Nov 3rd, 2011 at 06:05 AM
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