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Brief introduction into limbs/appendages. Arm, leg or mobility device?

brachiosteve

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All primates (apart from humans) have hands AND feet for grasping. Most primates also have opposable thumbs (4 exceptions) on hands and feet. Opposable thumbs are not limited to primates or even mammals. So, monkeys can be said to have 4 arms/legs or 2 arms and 2 legs.

So, what defines a leg or an arm, especially if both are, (or can be) used for walking and grasping?

Do kangaroos have 4 legs or two? This slender loris?

The key problem is, that we tend to have an original representation of something, (in our mind), like us, and then extrapolate equivalents to other creatures, many of which have different origins. Think about starfish, octopus, squid, spiders, centipedes, and also adaptive/evolved limbs. Like fins and wings, with some fish even having evolved fins into walking limbs with elbow joints. Arms, legs, tentacles. What do caterpillars have? Slugs and snails? It’s about context and fair comparisons and definitions. If, by definition, a leg is (for example), a form of contact mobility amongst animal life, we then have to decide it that is across all forms of animal and on land, water and air and how we differentiate a foot from a leg. Like sorting through any system of things, one needs to have all possible components known first, to ensure we can categorise them all.

If I ask someone to sort my books or box shapes into an order, one needs to know what we are working with. If all my boxes are just round or square, blue or red, then one can decide. If one the other hand, I have twenty colours and 3 shapes, I can decide on the best system, based on that. And so it is with legs or whatever. In fact creationists started categorising based on limited knowledge, assuming that everything would fit nicely into strict categories. Wings have only evolved 4 times for example, insects, mammals and reptiles (dinosaurs (birds) and pterosaurs. that makes things simple, though bats were first put in with birds (as whales were put with fish, being wholly aquatic). We now know a much vaster range of species and how then are not immutable and are all related and have a common ancestor. So we should expect multiple origins for different functions, be it sensory, mobility, ways to breathe or eat etc.

If you take different kinds of envenomation or venom sources. If centipedes were not known, we wouldn’t know that some animals adapt legs into venom glands/appendages.

So, we could probably have a limb/appendage that can be both an arm and a leg.

Image: Slender loris. Four opposable thumbs and the only venomous primate.



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