Why are there no 6 legged mammals
 

Just curious. I wonder why none have evolved (unless I'm forgetting about something.)

They couldnt breed well enough. Maybe there were a few, look it up.

Theres no evolutionary advantage to justify the effort of developing an additional two limbs.

Run faster - flee better or catch food better.

Stronger in a fight - 4 legs for balance + 2 for fighting.

Would require more food than a 4 legged version. Maybe thats got something to do with it?

I just find it odd that there is a macro level lack of genetic diversity. Makes me wonder why the DNA is so constrained.

Regardless of anyone's belief, evolution exists. If anyone wants to argue it.... we can do that later.

With that out of the way:

Six legs would not be practical for running faster, just like six wheels don't help a car go faster. You might have a point in the case of self defense.... anyone else think this is a strange conversation for this forum?

DNA is not necessarily constrained by its structure. It is constrained by time. Evolution takes a long time. Does it occur to anyone that we share the same fundamental DNA structure as Dinosaurs? The fact that four bacic nucleic acids can produce humans and giant reptiles is truly amazing. I don't see any constriction there.

In order for the evolution to take place, there first must be a mutation that provides a benefit. If this mutation enhances the creature's ability to survive and reproduce (and if the mutated gene is not recessive) it's offspring will have this trait. Most animals that may sprout a few extra limbs do not do very well at all in their environments and are usually killed off quickly.

Evolution is like good engineering in that it doesn't solve things aren't problems. If <= 4 legs are enough, there is no pressure to experiment with something new.

Its the same reason we're biochemically/genetically so similar to yeast. The problems involving protien synthesis and dna replication were solved by the time yeast developed. So they haven't been changed since, and every organism descended since then has built on top of that foundation. As with yeast, the cost required to tear down the foundation of 4 legs/arms established over the last few hundred million years isn't justified by the returns.

Call me insane, but when I see an ape… or a duck for that matter, I don’t see one
of those species filing tax returns in a hundred billion years, while the other
follows it on a leash. (That would be the duck I guess?)

I’m also sceptical as to whether evolution could explain Angelina Jolies’ lips. I
guess some humans need more grip then others? Still, works for me :drool:

But I must admit, there are times when I think about the bacteria in my water
loop, and wonder how long it will be before they evolve into all seeing all feeling
beings, and whether they will forgive me for the boring repetitious lifestyle I had
given their ancestors…still... 3.6ghz and rising, I’m sure their future civilisation will
forgive me.

Yes I endorse single cell captivity, and just maybe, they’ll look back and chuckle
about it? :p

Sexual selection my friend. Some traits are selected by pressures such as living successfully while others are selected for the purposes of reproduction. Look at male peacocks, thats an example of extreme sexual selection pressure.

They might also be fake...


I dont think there is any incentive for 6 limbs. After billions of years, I think the 4 limb design is whats right for the mammals of earth. Think how much more complex a 6 limb creature would be: More back structure, more joints, more mucles, more weight, more brain for control...

S much as we call bateria and stuff "unevolved" and basic, they are amazing good at what they do. Every cell funtion is incredibly simple and streamlined. I think of prokaryote cells as race cars while eukaryote cells as SUVs...

Angelina Jolie's lips are I think as much a product of collagen injections as much as any biological factor, evolutionary or otherwise.

Evolving six legs isn't really beneficial to any verterbrate I can think of, given you would virtually have to start from scratch physiologically. I think people are a strange compromise of sorts as far as evolution goes. We are the only biped mammal, and we had to evolve a lot of compromises to do that. There is a big debate out there in the paleoanthropology world about what environmental pressure or advantage was the prime factor in getting our long ago ancestors out of the trees and onto the ground, with two legs no less.

The fossil record records bipeds as long ago as ~3.6MYa, and basically that creature was a chimp that walked on two legs. It is quite entertaining to watch the creationists cook up explanations for all these things that according to them, weren't supposed to be there in the first place.

In order for an arm to grow from a fish (one of the basic steps in the evolutionary theory of man) there would be many thousands and thousands (millions) of years of transition period of complete uselessness for that trait. For example a dog from its legless predecessor. 4 limbs being nubs, each having 1000's of useless mutations to provide for bone size, muscular attachment, claws, and ultimate design enabling efficient running. Each thousand years having its own fossil evidence.

Who can argue evolution? When I see the transitional models in not 1 or 2 species, but in EVERY aspect of life as the THEORY describes with corresponding empirical evidence, I will subscribe to it. I find it amusing that the scientific 'open' mind has to dismiss such illogical gaps, placing their 1 or 2 'missing links' that allow them to explain an entire process across all living species. One, two or even 100 would be a vastly insignificant amount of fossils to explain an origination of all life theory.

If we have fossils of anything that shows conformity from 'millions' of years ago in any species, doesn't this in itself raise a question with the evolutionary mind? Should we statistically NEVER be able to find a common model of a dinosaur or early predecessor that would have all common points or structure if they were constantly in an evolving state? How can we find remotely similar animal fossils on separate continents in differing environments without each taking a different provable fossil record as they evolve differently?

Today an evolutionist to me seems to be the equivalent of yesterdays 'flat world' theorist. In a hypothesis, if the basic concept is proven wrong, you must reject the hypothesis as false. You cannot 'prove' a theory, only disprove it. However, in this field, you have scientists attempting to prove their hypothesis right and ignoring all past failures or rejected 'evidence' without coming to a point of re-writing the basic hypothesis.

I personally cannot grasp the faith that it takes an evolutionist (spoken as classified as some kind of religion) to justify their belief. Without arguing the abstract petty arguments, the 10 thousand pound elephant that is ignored is the loss of the millions of transitory fossils that MUST outnumber the 'final stage' bones.

In a similar argument, how many millions of favorable mutations across the biological board must occur daily in our living ecosystem and be successfully passed on genetically via a mate? Shouldn’t we see these daily to keep the pace of constant evolution is to be maintained?

I have faith in intelligent design as I admit to seeing the HUGE gaps in the evolutionary model. I have studied both. I can not see how one takes more faith than the others personally.

I don't deny that some points of evolution regarding 1, or even two species may be valid. Wouldn't a scientific minded person have to have it proved across the board to subscribe to an all encompassing working model?

On thread topic, I would think that the model would result in all forms of life evolving higher thought patterns, and ending up as the worlds most sucessful and destructive stage of evolution being .. man. And of course, all women would end up having those lips.

But dogs didn't evolve from fish, they were created by humans from earlier mammals which evolved from earlier reptiles (?) which in turn evolved from amphibians, which evolved from some ancient common ancester of the modern fish which is now extinct. The jump wasn't made overnight, rather there are many thousands of intermediate species, each of which left there mark on the shape of a modern dog. In the end the final goal is not to make the dog, the dog is just something humans bred because they thought it would be useful to their ends. Its just one more species that will eventually pass to make way for others.

Therefore there are no useless mutations (at least none that surivive), each was a solution to some problem that existed at the time of its adaptation. The sum of these adaptations becomes new species as populations diverage and the genetic markers on their protiens change enough that sexual reproduction between individuals that had been of the same species is no longer possible. Then simple genetic drift takes over and eventaully you get something that is a dog for a while.

We do see some of these traits actually. However since most species are not visable to humans (molds, yeast, other microorganisms make up most of the life on the planet). Yeast are the area I'm doing my current research in. Theres actually a lot of adaptation going on in yeast in real time. We can trace back the evolution of yeast pretty well for the last few hundred million years, mostly because we still have so many closely relates species of yeast. You can actually work backwards and calculate the specific mutations that creates some species of yeast. However its not very glamerous to see how protiens change over time in the mold that raises your bread, so no one ever notices. If you want to see elephants adapt spots, or some other change in a macroscopic organism, you'll have to wait many thousands of years because the life spans of macroscopic organisms are, well, macroscopic.

ahhh... the evolution of procooling. :rolleyes:

It was as recently as a week ago we were talking about waterblock design, pump types, radiator optimization, and other things pertaining to water cooling. It's amazing how quickly this forum's intellectual horizons have grown. You guys want to talk about space time next? How about we try to find the appropriate math forms for string theory? :dome:

Sarcasim aside, I am impressed by the logic of many of the below responses. One could say this is proof of some of the members' competence.

Damn, and apparently, evolution has even given some of us the ability too look in the future. :p

Sometimes it's just something random, too. For instance, there are evidences of all sorts of bi-laterally symmetrical critters in the Triassic, but not after (and there are traces of a marker for a meteorite hit at the end boundary of that era...iridium?). It might be that, had that meteorite not hit, or hit a million years earlier or later, some of these forms might have survived - possibly including something that might have become, say, a fish with six caudal fins, progressing possibly to some sort of air-breather.
Just a thought, of course...

I read somewhere that, on average, people have 30 mutations each - that is 30 genetic settings that are not inherited from either parent.

I personally have a rather obvious mutation - too obvious to mention and ruin my online anonimity - but it hasn't been passed onto any of my three kids (well, not visibly).

Also read or saw somewhere a theory that, as time goes on, the male chromozone weakens, and fewer and fewer males are born, eventually resulting in the death of the species.

I bet there are a thousand times that many mutations. Each cell can and does have its own mutations. Since most of the human genome is junk (or at least it has no currently known function) it really doesnt matter. To many mutations or a critical mutation and the cell in question may turn cancerous. Really, mutations are a good thing, without them evolution doesnt take place.

You guys should read Genome by Matt Ridley.

http://tolweb.org/tree/phylogeny.html

"just curious" was the start. Hmmm. Define "leg" and "mamal"

Octopuses are mollusks, insects spiders and crabs are arthropods; diverged from deuterostomia precambrian... well, saying cnidaria or ctenophoria might have "legs" is really kind of pushing it for me, but that might be slanted by my subjective fascination with the neuro. Homoplastic but not homologous, yet wider than taxonically apomorphic i.e. largely plesiomorphic, to a tune of 750 million years ago, do I have that right? Well, 8-leggedness anyway, not to take the six of the original post too weightily eh? And I do think coelenterates are beautiful, deeply, holily even, just not leggy, per-se.
But back to the deuterostomia, let alone chordate, lol even vertibrate, it does appear that having a mouth separate from an ass is not backwards-compatible with 2*(n>1) legginess, touche!.

Wholly crap, did you swallow a dictionary or what!!!!

:D

"Ah, your google skills have increased."
... (insert random starwars cliche here).

;)

Actally I couldn't find my Cambridge Dictionary of Biology and it kind of bothered me. I did look up some spelling in something on hand, Comparative Vertebrate Neuroanatomy, Butler & Hodos ... It is very much the only way that I know how to think.

...None of this is going to matter when the sun explodes... :p

I am a 3 legged animal.

TRIPOD!!!!

hmmmm

I wanna engineer a 5 legged dog. =)

Land animals don't benefit much from having more than four legs. It's not faster or more agile. But if an animal developed two arms and four legs, while having limbs to manipulate objects with rather than use it's mouth would be beneficial, it would likely have poor balance and mobility would be hindered, rendering it unable to compete with it's simpler four legged predecessors. Imagine a dog with four legs and two arms. It's torso would have to be lengthened or develop another body segment so that there would be a place for the arms without having too much weight in the front. It's just wouldn't happen naturally. If you were to engineer such an animal, if you took into account the weight balancing and such, you'd probably have a very successful animal.

But evolution doesn't just happen. Even the fastest evolution occurs over more than a million years (like the development of eyes) and it a progression of a trait. Taking the development of eyes for example, this happened some 400+ million years ago. The first 'eyes' were no better than light sensitive spots on the creature giving it a basic sense of direction, which over millions of generations developed into something more and more sophisticated until it reached what we now call an eye. Before this, all life was blind and moved and fed and killed other lifeforms based upon touch. Once the eye was developed, lifeforms with this feature were able to actively search for food and navigate much more efficiently. Within the first few generations, a mutation must be beneficial to be successful or it will be replaced with something that IS useful. You'll notice that nearly all successful life on the planet has eyes, or is at least light sensitive, because it is a particularly useful trait. You may also notice that there are no land animals with gills or flippers (discounting frogs and platypus because they are predominantly water based). These traits aren't useful on land. So they died out. So for an animal to mutate and develop a third set of limbs, there would have to be some benefit to it if it is to become successful. Because there would be a long period of time where the third set of limbs would not be beneficial, or would even be detrimental, it's unlikely a four limbed animal would develop a third set or limbs. It's far more likely that a six limbed creature, like an ant or other insect, would evolve into a more advanced creature, something more along the lines that you described. But insects are mostly genectially stagnant, so this isn't particularly likely.