Don't get too hung up on natural selection. A world of efficient animals optimized for cutthroat survival does not look anything like ours. Such a world doesn't have peacocks or eye-stalk flys. It doesn't have vibrantly colored plumes, shiny scales, maned lions, elaborate and energy-consuming mating rituals. You have to take sexual selection into account. Say the females REALLY dig a powerful third set of otherwise useless limbs, like the peacocks tail. It's a fitness marker. If that's the common ancestor for a major animal group, the 6 limbs configuration could easily have become locked in. You don't NEED a good reason for a feature. From there, evolution could have a hard time getting rid of the extra legs, but of course it would produce a lot of variety for what those extra limbs were used for. Collecting and carrying things would be easier. Look at how leopards drags prey to treetops, squirrels and ravens move food to stashes, you could see a lot more of that. Even tool use - you don't need to develop upright walking to have an extra set of limbs free to develop fragile digits that are useless for running. Heck, the limbs could evolve into "tools" - think of the many specialized beaks and teeth that animals have to allow them to access or crack open food, otters and chimpanzees using rocks to open clams or nuts, getting thermites with sticks or ant-eater tongues. Mating ritual combat use could develop clubs or horned limbs, like antlers and horns. Mating display could develop all sorts of interesting limbs. 4 sets of running legs plus 2 limbs for climbing, gibbon-style swinging, or swimming. Predators could have 4 running legs and a set of limbs optimized for grabbing or killing prey, so they don't have to rely on front legs or teeth for that (canines use their teeth for both grabbing and killing, felines use front leg claws for grabbing and teeth for killing, but with 6 limbs you have a lot more options). Imagine a cheetah running down a gazelle, twisting and turning and dodging, mere inches being the difference between becoming dinner and escaping - a set of long, thin clawed limbs on the cheetah could make all the difference and easily be worth the speed penalty. Prey animals could have limbs evolved to push off a predator trying to grab it.

If I remember correctly "Blue Planet" RPG, with its planet Poseidon, has animals with six limbs as a standard. It's explained as an adaptation to amphibious lifestyle (Poseidon is a mostly-aquatic planet, with most snippets of dry land being swamps and marshes). I highly recommend Blue Planet as a source of inspiration - its flora and fauna are very original, passably realistic, and fun to read about (I mean mostly the species in the core rulebook - there's a separate bestiary too, but it's less inspired).

NewtonPulsifer wrote:

On second thought if you're close to the ground with splayed out limbs (like a crocodile) 6 limbs might work out just fine.

There is the issue of [url=http://en.wikipedia.org/wiki/Terrestrial_locomotion]posture (wiki)[/url]. The sprawling posture of reptiles compresses their lungs when they move, ([url=http://en.wikipedia.org/wiki/Carrier%27s_constraint]carrier's constraint[/url]) and requires them to rest their torso on the ground when they're not moving, which requires heavier skeletal structure to avoid compressing the lungs again. Top land predators with endoskeletons will probably all have the benefit of erect posture with it's more efficient gate. These problems can be avoided by exoskeleton configuration but then you end up with size constraints or aquatic species that can't develop tool use. However, I'm sitting here wondering why lizards never developed a valve system between their lungs that would have made a sideways flexing gate beneficial to respiration.

Insects use 6 legs, spiders use 8. You don't really need different world to support many legged creatures ;) Maybe this is the route to explore - fix the reasons why insects are not ruling the world. As far as I remember from biology lessons, issues are related to breathing (not efficient enough to support huge mass) and carapace (exoskeletons have limitations). Maybe very oxygen-saturated air would help here (in addition for making nice fireballs) ?

I agree that it doesn't mean that all species have to be six-legged - but the question is, could they realistically be six-legged so it fits the story requirement? On Earth, there are huge clades where body composition tends to be extremely stable. Even though we have dolphins and bats, the vast majority of mammals have 4 legs. Almost all land vertebrates have a 4-limb body composition. I've never been fond of statements like "evolution uses whatever is there". In my opinion it makes evolution sound all too powerful and fails to encompass how limited it really is. It is an optimization process that explores the fitness landscape with glacial speed and it is extremely unlikely to make anything but microscopic steps outside of the existing genetic variation and so it gets stuck in local maxima.

Smokeskin wrote:

I agree that it doesn't mean that all species have to be six-legged - but the question is, could they realistically be six-legged so it fits the story requirement? On Earth, there are huge clades where body composition tends to be extremely stable. Even though we have dolphins and bats, the vast majority of mammals have 4 legs. Almost all land vertebrates have a 4-limb body composition.

Body plans evolve rather slowly, because they are controlled by core development genes: mutations there typically make you un-viable. So the vertebrate core body plan has not changed in *half a billion years* (vertebrates appeared 525 million years ago, but the jawed vertebrates appeared around 426 my ago) since our ancestors doubled the hox genes and got a useful set of controls - everything since has been about tweaking what you build on top of the core control program, adding extra complications. The number of legs can change fairly quickly (consider the evolution of seals or whales) if there is an open ecological niche. But this is not a change in the core body plan, just what fins/legs get expressed. So we are unlikely to ever see a six-legged vertebrate ever, since that would require changing the core controls (I think). I think a planet with six-legged ancestors sounds just as reasonable as our four-legged one.

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I've never been fond of statements like "evolution uses whatever is there". In my opinion it makes evolution sound all too powerful and fails to encompass how limited it really is. It is an optimization process that explores the fitness landscape with glacial speed and it is extremely unlikely to make anything but microscopic steps outside of the existing genetic variation and so it gets stuck in local maxima.

I often explain that evolution is smartdumb - it is amazingly powerful because it can search through a ridiculously high dimensional fitness landscape and has some weird tricks we still do not fully understand (modularity, genetic control networks, some self-organised aspects of development), yet it has no forethought: most evolved species will get wiped out for the most trivial reasons. But what I meant was that if you have an organism of a certain body plan evolution will try all sorts of tweaks of that body plan, and if anything thrives it will spread. But some directions are easier than others: our kind of control genes are unlikely to allow viable six- or three-legged mammals but two- and zero-legged mammals are easy. Another program (like the one found in the myriapodia) might be fine in adding or deleting an arbitrary number of segments (think pillbugs and millipedes - closely related). And maybe an alien program would be fine at changing body shape, producing an ecology of far more morphologically diverse animals.