But I'd look stylish with a tail...

[Rakasha]

I'm all for evolution, but some part of it is a mystery to me. And by some, I mean one : how no acquired trait through evolution should disappear if it does'nt consist in a disadvantage for surviving and mating.

 

Unless the above sentence is false, which is quite possible since I am a neophyte at evolution, then my body seems to hint some disapproval.

 

First, I believe, the ancestors of the homo sapiens, the hominids, were not very far from being primitive apes. Seems to me like they possessed a tail. In our present state, our vermiform appendix hint strongly that we once possessed tails. Or so I've heard.

 

Yet how many time I look back there, I don't see a tail. Of course, me being a guy last time I checked, the front is a different matter . But back there, it's smooth.

 

... I don't see any disadvantages to having a tail, seems like it only help balance the body, and us being biped, that seems handy. Did we ever had a tail, and if it seems that yes, then why is it gone ? >_<

[Mokele]

In our present state, our vermiform appendix hint strongly that we once possessed tails. Or so I've heard.

 

The appendix hints that we used to have a more vegetable-oriented diet. I think what you're referring to is the cocyx, our vestigial tail.

 

I don't see any disadvantages to having a tail, seems like it only help balance the body, and us being biped, that seems handy.

 

Firstly, iirc, a tail would actually probably interfere with our bipedal mode of locomotion. But that's moot since our ancestors lost the tail long before they stood upright.

 

Primarily, the disadvantage of a tail is that it exists. Any part of the body which exists takes calories; calories to maintain it, to use it, and to grow it in the first place. Thus, when our ancestors became terrestrial, they no longer had such a strong reliance on the tail. Because the benefits were now reduced, those who had shorter tails (and thus needed less food) lost very little but gained a reduced caloric consumption (which would allow them to channel the calories they did get into things like muscles and activity and other reproductively or otherwise advantageous traits and actions). As tails got shorter, the benefits lost by further shortening decreased even more, and eventually, the entire tail was lost, but for a small vestige without extenal parts.

 

So basically, it all boils down to the tail not being worth the calories it took to grow and keep and use.

 

Mokele

[Rakasha]

The appendix hints that we used to have a more vegetable-oriented diet. I think what you're referring to is the cocyx' date=' our vestigial tail.

Mokele[/quote']

 

That very well may be the case. Yay !

 

Thanks for the reply, it's quite interesting. But something still seems to be missing. If tails became enough of a evolutionnary disadvantage to disappear when we became terrestial beings, why are tails so common in the animal kingdom ? I'm perplexed.

 

The only thing I see that may have a correlation with our lack of tail is our biped characteristic. Yet it seems to me that tails are a benefit especially for biped creatures and other life forms for whom balancing is quite important.

[Artorius]

The appendix hints that we used to have a more vegetable-oriented diet. I think what you're referring to is the cocyx' date=' our vestigial tail.

 

 

 

Firstly, iirc, a tail would actually probably interfere with our bipedal mode of locomotion. But that's moot since our ancestors lost the tail long before they stood upright.

 

Primarily, the disadvantage of a tail is that it exists. Any part of the body which exists takes calories; calories to maintain it, to use it, and to grow it in the first place. Thus, when our ancestors became terrestrial, they no longer had such a strong reliance on the tail. Because the benefits were now reduced, those who had shorter tails (and thus needed less food) lost very little but gained a reduced caloric consumption (which would allow them to channel the calories they did get into things like muscles and activity and other reproductively or otherwise advantageous traits and actions). As tails got shorter, the benefits lost by further shortening decreased even more, and eventually, the entire tail was lost, but for a small vestige without extenal parts.

 

So basically, it all boils down to the tail not being worth the calories it took to grow and keep and use.

 

Mokele[/quote']

How wrong could one be in a single post.I find it a COMMON reply among people who support some evolutionary literature that suggests the coccyx as a "vestigial organ".To suggest this shows ineptitude of Human biology,the facts are there are no useless or left over parts from evolution in the human body.You can support evolutionary theory all you want,that is your perogative.However to misinform people with this explanation is wrong.Dont assume from your darwinist ignorance of an organs function,that they have no function.

The facts are you very much require your coccyx important muscles levator ani and coccygeus attach to this bone.Without such attachment the pelvic organs would collapse,you could not have a bowel movement,nor walk or sit upright.

They are many other so called useless vestiges as evolutionist refer to them, that are present .Which in reality more up to date scientists accept that the coccyx isnt a remnant of a primate ancestors tail,or a modified one.

[Auburngirl05]

There have been cases reported of people being born with an elongated coccyx: basically, a tail. Not a long fluffy thing, of course, and nothing with any extra function, but I'm pretty sure that it's not unheard of in the medical field. You might try Googling it and see what you come up with.

[Martin]

..

 

Thanks for the reply' date=' it's quite interesting. But something still seems to be missing. If tails became enough of a evolutionnary disadvantage to disappear when we became terrestial beings, why are tails so common in the animal kingdom ? I'm perplexed.

...[/quote']

 

I too am perplexed and always have been so, about the tails of terrestrial animals like

 

lions, wolves, horses (short stubby but with long hair), pigs (little curly tails)

 

all these animals are running around on the ground, not climbing trees

 

their tails are rarely prehensile, and they dont appear to need tails for balance since the mass of the tail is typically too minor to provide balance even if the animal was rearing up on its hind legs.

 

mostly it looks to me like the 4-leg beasts of the world are using their tails either to swat flies or for self-expression

 

like, they communicate with their tails instead of by facial expression

 

or, with cats, it serves as a toy for the children to play with

and keeps them occupied learning how to stalk and pounce

 

the only animals living on the ground that really use their tails are snakes, which ARE tails

 

So I would have expected that tails would have been evolved away in a lot more cases than did actually do

[Martin]

hey rakasha, I had an idea

 

one of the unseen costs of the tail is the brain volume devoted to it

 

all the apes lost their tails-------chimpanzee, bonobo, gorilla, human---and all of them have really skillful HANDS with thumbs.

 

think about those brain maps you have seen that show what part of the brain controls the tongue, and what part the thumb, and what part the fingers, and what part the legs and feet-----a big big volume of your motorcontrol brain is dedicated to controling your hands. the map shows them as very big, so the associated part takes up a lot of room in the brain

 

 

brain volume is really costly----a lot of our food is burned just maintaining our brains, and it interferes with birth too---big heads get in the way

 

So I guess that Apes lost tails when they started having really good hands and needed extra brain volume. So they ditched the tail not because IT was costly in metabolism to maintain but because CONTROL resources were scarce

 

Well, that is a theory anyway. maybe there is some way to test the theory.

[mossoi]

Possibly the tail is still in the process of evolving out of animals although many animals do still use their tails for communication.

[Rakasha]

Woohoo, nice replies. And I was thinking after all that it was a silly subject.

 

Yep, I did'nt consider much that tails were a lot more than simple balancing tools. Now, it seems to me that it's usually quite an unimportant usage of it, although the t-rex might think differently.

 

Also, maybe our developped facial features replaced the tail's communicative fonction, rendering obsolete an important part of it's presence among animals. As for swating flies, well, clapping sure seems natural to us.

 

It's true that an extra appendage such as a tail would require more ressources brain-wise. It's worth taking into account. It seems more important than a ineficient use of calories, unless it is understood why tails would be too costly to maintain for the human's ancestor and not with most of the other animal species that possess this sexy tail that I am unfairly denied

 

P.S. Artorius, notice how Mokele simply corrected my false statement ? It was quick & smooth and that's all there was to it. Wheter it was a good correction is not the point, what is important is that it was not aggressive because it was not needed.

[Artorius]

P.S. Artorius' date=' notice how Mokele simply corrected my false statement ? It was quick & smooth and that's all there was to it. Wheter it was a good correction is not the point, what is important is that it was not aggressive because it was not needed.[/quote']

 

Omg...nobody has said that to me before,at least not without being offensive and attacking me personally.So ive failed to take it in.

But yes your damn right!!,ive looked at my posts they all share a common format.They are intensely aggresive,maybe the reason ive not noticed myself was they was no real malice intended.When im sat here typing im not thinking "idiot i will put him/her down" in truth im thinking "eh thats not right"

Thankyou for your ps i agree totally.

And to all here at SFN if ive gave you that perception of me i apologise to each.Heavens i have no ill feelings towards anybody even if their posts are trash...yep i just read my post back and that last phrase is awfull.I will make the effort in future to convey better what i mean.I have faults but im not a nasty person to anyone.

[Martin]

...

 

Yep' date=' I did'nt consider much that tails were a lot more than simple balancing tools. Now, it seems to me that it's usually quite an unimportant usage of it, although [b'] the t-rex might think differently[/b].

 

....

 

this is a strong point some bipeds have a real need for a substantial tail to cantilever them. those running two-leg carnivores there were a lot of them weren't there? not just T. Rex.

 

I cant picture them being able to run except head way forwards and tail way back and legs churning in the middle, at the fulcrum point. So without his tail for balance he is basically not real mobile!

 

we have several people at SFN who are really savy about biology and paleontology

there is Skye, whose avatar is a gun-toting cartoon individualist (he is really smart about biology) and a fund of information named Coquina for whom it is a hobby, she likes asteroid events and major extinctions, which accelerate evolutionary changes

 

keep trying questions out, if you cant get one of them to respond then you are not really getting all you can out of the board

 

oh, about balance. It does seem that would be the FIRST functional justification, and evolutionary value, at least on land.

 

the problem is why keep them after the balance function isnt there any more?

 

another function is HEAT CONTROL

sometimes you want to lose heat so you can increase your convective and radiative surface area by stretching out your tail

 

sometimes you want to retain heat, so you can curl your tail around you

and reduce your surface area

 

squirrels use them as down jackets or sleeping bags

[Mokele]

the coccyx as a "vestigial organ"

 

I said "vestigial tail" and it clearly is that. But just because something is the vestige of one trait, doesn't mean it can't be used for other purposes. The cocyx *is* the vestige of a tail, but it has been co-opted for other uses by evolution, mostly as a muscle attachment point as you pointed out.

 

A good analogy is the rear limbs of boas and pythons. These are technically vestigial, as the snakes do not have any external limbs anymore (though they do have a small claw on each side of the cloaca, called a "spur"). However, they have be adopted by evolution to a new purpose: the male uses them to stimulate the female during mating. That's why, if you want to sex a boa or python, you can compare the length of the cloacal spurs. They're the vestige of one trait that's been adapted to a new use.

 

Something can be vestigial in one sense, but still be used for something else.

 

To suggest this shows ineptitude of Human biology

 

And I give a crap about human or even mammal biology why?

 

My sole concern in the biology of mammals in their nutrient value for both myself and my reptiles.

 

the facts are there are no useless or left over parts from evolution in the human body.

 

Then what, pray tell, is the appendix? To me, that looks suspiciously like a leftover cecum.

 

more up to date scientists accept that the coccyx isnt a remnant of a primate ancestors tail,or a modified one.

 

Then where did it come from? God? Oh, please.

 

The cocyx *obviously* used to be a tail, and anyone who says otherwise is blind. Now, it *has* been adapted for other uses, but that no more changes it's origin than the human earbones' current use changes their origin as reptile jaw bones.

 

So I would have expected that tails would have been evolved away in a lot more cases than did actually do

 

My guess would be thermoregulation. Mammals have to maintain a particular, set body temperature. The tail has a lot of surface area for it's volume, and thus would be a useful heat-disipation tool in warm climates. Dilate the blood vessels in the tail and redirect more blood there and instand heat loss. Wagging it around would only increase the heat loss. Mammals pay their cooling bills in water, so any means of cooling off without loss of wat would be selected for.

 

I'd also suspect it works similarly in reptiles, but for both heat capture and heat loss.

 

But this is just a guess, after all, though I'd love to actually do experiments to test it.

 

I cant picture them being able to run except head way forwards and tail way back and legs churning in the middle, at the fulcrum point. So without his tail for balance he is basically not real mobile!

 

Fun annecdote: Apparently, a large hadrosaur (duckbill) skeleton was being mounted in a museum. They completed the framework, and had to support it at many points as they placed the bones . Finally, when the bones were in place, they took the supports away and it balanced so perfectly that you could easily rock it back and forth by pushing on one end, in spite of the huge mass. Not terribly scientific, but it does show just how perfectly the tails of bipedal dinosaurs balanced them.

 

Mokele

[Rakasha]

Hey, thanks a lot for the nice greeting to the board. It's awfully social of you .

 

About the experienced member, I agree that having their participation is always interesting, but naah, I don't think that I should press until they deliver. It seems harsh to me to have forum celebrities and consider their posts more important than others. What's important is what consist of the post, regardless of personnal records. But I do concede that Skye's avatar is the best

 

Going back on the thread, I mentionned the tail of Big T just as an example to explain why I initially presumed that tails where sticking out there primary for balancing. With more thoughs, I see that it's not a good example of usual tail usage at all. Plus, discussing the evolutive advantages of dinosaurs seems queer. Since there is'nt any dinosaurs posting in this thread, I suppose that their evolution did'nt work out very well in the end (or that they were really unlucky).

 

Of course, those good old dino's are a nice example of stabilising with a tail, but it seems like a rather unique case which has to do with their uncommon massive body.

 

I would'nt say that tails first were there for balancing. I see it that way : primordial life emerged from the oceans and breeded into land species with generally the same configuration, that is, a mouth/eye/brain forepart and an anus/elongation in the posterior. Basically, well, a tube. The last mentionned item, the elongation, seems to have been present to procur movement in bodies of water. But on land, new usages had to be found or it disappeared from the gene pool.

[Artorius]

definition of vestigial organ(from the evolution of life)

defines a vestigial organ as one ‘which has lost its function in the course of evolution, and is usually much reduced in size’.

(dorlands dictionary)

‘a vestige, trace or relic’, and defines the term as ‘the remnant of a structure which functioned in a previous stage of a species

(churchills dictionary)

defines vestigial as an organ that has ‘no obvious function’, and notes that the word vestigial derives from the Latin vestigium, ‘meaning footprint, imprint, track, trace’.

in fact i have gave up searching for a description which says a previous useless organ thats now co-opted for something else!!!.

 

The appendix is part of the immune system, strategically located at the entrance of the almost sterile ileum from the colon with its normally high bacterial content.Helps establish and maintain bowel-blood barrier.

Where is your evidence that it was any bigger in the hominid species?

 

what about that protubrence we call a nose is that a vestigial remnant of a snout,

[Martin]

That reminds me of the fascinating fossil record of the bones in the ear

which are (as I am accustomed to use the word) vestigeal jawbones.

I remember this great Paleo class with a smart woman teacher and lots of slides

 

the old jawbones were maybe even going back to amphibeans, I dont know, maybe even fish

and she kept rolling the slides and the bones kept getting smaller

and drifting back up to where the ear was developing

 

and new jawbones evolved to the place of the vestigeal jawbones

 

and finally these little remnants were CO-OPTED by another function namely they were becoming RESONATORS to amplify and channel vibration and get it to the nerves of the ear

 

and that was typical of what is ALWAYS HAPPENING, structures are always being taken over and put to new uses, even chemical pathways, that you cannnot see, anything that the genes create can be re-assigned in function and take a new evolutionary direction. it is what it's about

 

This slide-lecture was something you could not freeze-dry in the definitions of a few words of English with their stock dictionary defintions

you cant build science or understanding into a finite static set of words

you have to see the slides and see what's really going on---then describe it however seems to work in whatever terms seem best

 

whoeverr the guy who punned with malleus aforethought was clever

 

 

I am searching for synonyms for when people deny the wonders of nature and hold up half-ass beliefs which demean and diminish nature through their poverty.

what can it be called? a sacrilege. a desecration of the moment that expansion began. an insult to the past 13.7 billion years. a denial of whatever is wonderful that does NOT bear the mark of primitive (often Sumerian) human storytellers. a disrespect of the fundamental constants of nature.

[Rakasha]

I see the debate around the vestigial quality of the coccyx as a moot. It basically comes to a showdown of terminology.

 

One refers to vestige as something that hint the past but may still be of use (which is all the reason why it's still there).

 

The other refers to it as something that lost it's function and thus is on it's way out.

 

Quick glance at the two definitions listed by Artorius (thank him) :The dorlands dictionary define a vestigial organ as something having lost it's fonction; the churchills dictionary define the term as a remnant of a structure ¨which functioned in a previous stage of a species¨. I'm not trying to attempt strawman and yells that everybody win, but it sure does'nt say that the structure is no longer functional, that is, that it has'nt been assimilated by a different structure.

 

I don't see much point in arguing terminology when the supplied definitions does not concur on the subject. Unless it is added more information concerning the definitions that are given that point out one as reasonably more acceptable, then everybody wins and get a milkshake.

[Mokele]

defines a vestigial organ as one ‘which has lost its function in the course of evolution, and is usually much reduced in size’.

(dorlands dictionary)

‘a vestige, trace or relic’, and defines the term as ‘the remnant of a structure which functioned in a previous stage of a species

(churchills dictionary)

 

And since when did dictionaries become legitimate scientific sources? I'm willing to bet both of those define a snake as "a limbless reptile".

 

in fact i have gave up searching for a description which says a previous useless organ thats now co-opted for something else!!!.

 

You'll often find that actually knowing something reduces your reliance on finding something to quote.

 

Where is your evidence that it was any bigger in the hominid species?

 

Given that such things don't fossilize, you know **** well that asking for such is ridiculous. But it does not refute my point.

 

Where, pray tell, do *YOU* think the cocyx and the appendix came from?

 

Ignore what they do currently, and tell me where they came from.

 

The correct answer is that they came from the tail and cecum, respective, and are vestigial in that sense, though they have become used for other functions currently.

 

Just because something has a function in one sense does not mean that is it's original function, and I see nothing incorrect in nothing that these traits are vestigial with respect to and in comparison to their ancestral form.

 

Now, if you wish to continue your idiocy, I suggest you actually try thinking, rather than just vomiting forth what you read in books.

 

Mokele

[Artorius]

I POSTED A POLITE APOLOGY EARLIER TO EVERYONE THAT USES THIS FORUM FOR MY SOMETIMES AGGRESIVE POSTS,and you return the compliment with insult and swearing.You obviouslty live in the world of were mokele knows best.pray tell me were does the appendix come from and also the coccyx,with empirical evidence?

If your correct i will apologise humbly.I must have missed this empirical evidence in my research.yes i remember now didnt they find a hominid fossil with a tail,i think it was your father

[Mokele]

You obviouslty live in the world of were mokele knows best.

 

Yep, it's called the real world.

 

Now bow before me, worm.

 

pray tell me were does the appendix come from and also the coccyx,with empirical evidence?

 

As for the tail, comparative physiology and embryology answer that. Hey, look, our embryos have tails. As development proceeds, those tails get afew bones in them that are identical to vertebrae. In other animals, the tail elongates, and more vertebrae form. In us, the tail remains small as we grow, and the vertebrae fuse.

 

Therefore the tail and the coccyx are homologous structures. It is obvious the tail is the ancestral form.

 

As for the cecum, a simple jaunt down comparative biology shows that many birds, reptiles and mammals have a cecum, and as the degree of carnivory increases, the cecum size decreases. The veriform appendix is homologous to the end of the cecum.

 

For more information, with loads of peer review to back it up, go here.

 

There is a smaller section on the vestigiality of the human coccyx here.

 

Given that both the appendix and coccyx can be surgically removed without any deleterious effect (and I can directly attest to that for the former), any effect they do have is so inconsequentially minor that they can safely be called "vestigial".

 

Mokele

[Artorius]

Embryo development answers nothing,other than when were forming we resemble a chicken embryo.Unfortunately parts develop at different rates and what might appear to resemble a tail later everything starts to fit into its place and hey no tail.

You asked me were the appendix comes from yet you cannot answer it yourself.Everyone knows were it is but you asked were it comes from.I already explained it functions today like it always did so how is is vestigial.

Removing an organ does not qualify as vestigial.you can take many parts from the body without serious ill effects.There are no fossil records that show an hominid with a tail.And no evidence to provide the slightest indication of a phylogeny of the coccyx from a tail

[Mokele]

Embryo development answers nothing,other than when were forming we resemble a chicken embryo.Unfortunately parts develop at different rates and what might appear to resemble a tail later everything starts to fit into its place and hey no tail.

 

Embryology *PROVES* that parts are homologous. In this case, it shows that humans form with a tail, and simply don't develop it fully. The most logical explanation for this is that we had a tail, and lost it. Otherwise, why would we have it at all, during embryogenesis or otherwise?

 

What, exactly do you want as "proof". I've given you more than ample evidence with which to extract your head from an orfice in the proximity of your coccyx, yet you simply dismiss it out of hand.

 

At this level of stupidity, I'm almost tempted to think you're a creationist.

 

You asked me were the appendix comes from yet you cannot answer it yourself.

 

I did answer it. Pull your head out of your rectum and read the links.

 

I already explained it functions today like it always did so how is is vestigial.

 

You *claimed* that, and your claim is false. My links give you all the evidence you need. Read them or shut up.

 

Removing an organ does not qualify as vestigial.you can take many parts from the body without serious ill effects

 

It refutes the claim the the organ is vital. Try removing your lungs and see how well you do. No, seriously, do us all a favor and try.

 

There are no fossil records that show an hominid with a tail.

 

That's because the tail was lost before then, moron.

 

You want evidence of ancestors of ours with tails? They're called *MONKEYS*.

 

Honestly, who taught you science, Bozo the clown?

 

Mokele

[Artorius]

if you insist on this silly line of vistigial organs,which is well outdated and only suggested to strengthen the theories of evolution as it was getting a foothold.You leave us all open to the creationist nutters here who jump all over it.You propose devolution not evolution.Natural selection would not give pigs two toes that dont touch the ground for no reason,nor make a rabbits digestive system so poorly that it eats its own faeces.We too have a cecum you may have ancestors that ate their own shit,i didnt.but old habits die hard and i see you still talk it

[Artorius]

you really are stupid,you would get along fine without your penis and scrotum,are they vestigial,take a kidney you dont need two,or an arm a few legs,you talk and offer nothing based on fact.As an embryo you could give the appearance of resembling a king prawn doesnt mean we once were

[Auburngirl05]

Sorry to sort of change the subject, I'm going to stay out of the discussion on vestigial organs and address the dinosaur connection mentioned earlier in the thread...

 

 

Going back on the thread' date=' I mentionned the tail of Big T just as an example to explain why I initially presumed that tails where sticking out there primary for balancing. With more thoughs, I see that it's not a good example of usual tail usage at all. Plus, discussing the evolutive advantages of dinosaurs seems queer. Since there is'nt any dinosaurs posting in this thread, I suppose that their evolution did'nt work out very well in the end (or that they were really unlucky).[/i']

Actually the dinosaurs were a huge evolutionary success, they not only existed but also dominated their ecosystems for about 200 million years, which is pretty darn successful, especially considering the 1)amount of things humans have managed to screw up during our relatively brief career as a species and 2) the fact that mammals were unable to gain ground as anything more significant than a shrew-type creature until something (still a controversy as to the exact "something") came in and wiped out all of the non-avian dinos for us.

 

Of course' date=' those good old dino's are a nice example of stabilising with a tail, but it seems like a rather unique case which has to do with their uncommon massive body.

 

I would'nt say that tails first were there for balancing. I see it that way : primordial life emerged from the oceans and breeded into land species with generally the same configuration, that is, a mouth/eye/brain forepart and an anus/elongation in the posterior. Basically, well, a tube. The last mentionned item, the elongation, seems to have been present to procur movement in bodies of water. But on land, new usages had to be found or it disappeared from the gene pool.[/quote']

 

Many groups of dinosaurs independently evolved ossified tendons along their caudal vertebra to stiffen the tail and provide a cantilever mechanism, mainly to aid bipedal running. This is the definition of the saurischian group "Tentanurae", but it was a feature of ornithischians as well. They held their backbones horizontal to the ground, as opposed to the kangaroo-type posture that a lot of early dinosaur artwork depicts, and that was made possible by those ossified tendons. You're right that the feature may not have originated as being used for that purpose (kind of the "half a wing" situation), but it was perfected in extremely similar ways in several lineages of bidpedal dinosaurs. (Obviously their bipedalism functioned much differently than ours...I'm not implying a link there). I don't think a stiffened tail would have been very effective for aquatic creatures, though, imagine how hard it would be for a crocodile to swim if its tail were held rigidly behind it.

 

One more point, though, is that the stiff tails had nothing to do with massive bodies, many of the Tetanurans were tiny compared to other dinosaurs, some were smaller than the turkey that probably sat on your table for Thanksgiving.

 

The UCMP page has an overview of that group: http://www.ucmp.berkeley.edu/diapsids/saurischia/tetanurae.html , although keep in mind that the ornithischians (Hadrosaurs, etc) had well developed ossified tendons in their tails also. http://www.ucmp.berkeley.edu/diapsids/ornithischia/hadrosauria.html

[Aardvark]

I'm all for evolution' date=' but some part of it is a mystery to me. And by some, I mean one : how no acquired trait through evolution should disappear if it does'nt consist in a disadvantage for surviving and mating.

 

Unless the above sentence is false, which is quite possible since I am a neophyte at evolution, then my body seems to hint some disapproval.

 

[/quote']

 

Just looking back at the original question and i think the actual answer has been overlooked.

 

A trait will disappear unless it is actively useful. ie it has to provide a consistent advantage resulting in selective pressure to maintain it.If a trait is neutral or disadvantagous then it will be lost.

 

This is the concept of Genetic Drift. Without selective pressure to maintain a feature individuals who have 'faulty' features will not be selectively culled from the gene pool, these 'faulty' genes will spread through the population and the feature will degenerate over the generations.

 

This explains how a feature can disappear even 'if it does'nt consist in a disadvantage for surviving and mating.'

 

I'm not sure if the tail is an example of this as a case could be made for it being both advantagous or disadvantagous, but it explains the principle you were considering.