[eli5] Why do insects have lots of legs while larger animals have 4 or 2? Also, why aren’t there any animals (that I know of) with 3 legs?
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Evolutionarily larger animals were just born with four legs. No really rhyme or reason behind it other than it worked. Then later depending on the animal it would become two legs for weight variation. And insects and spiders are similar as it all stems from evolution to create the best form for speed and weight carrying. There’s no real reason why we have these number of legs other than it works like there’s no real reason why we have two eyes
More redundancy -> good
More total cells -> more room for errors
A simple insect leg adds few cells for a lot of redundancy, an additional human leg adds a lot of cells for a little redundancy.
As with most evolution questions, it’s about what works and is good enough to survive in that specific environment. More legs may be more suitable to carry weight across a certain body size, but too many legs would mean more mass to the creature, which can be undesirable. Insects evolved to be mobile across very odd surfaces, like plants, so more legs would provide more grip. A mammal is suited to walk on flat ground, and four legs provides the most stable platform and balance.
Humans became bipedal because they evolved to adapt to using their hind legs for more purposes, such as observing their surroundings and reaching higher things. The human brain also evolved to use their front limbs to hold things, so the more successful humans passed on the genetic mutations that favoured stronger hind legs, opposable thumbs on the front limbs, and the bone structure to enable better balance on the legs. The benefit of 2 legs is that movement is more efficient – it requires less energy to move on two legs than on four. This benefited human development, as they could outrun their prey over long distance, cementing bipedalism as the superior trait in that environment.
It is unusual for an animal to evolve with an odd number of legs, like three, because paired legs literally provide the best balance of traits. Movement with three legs would be very unbalanced and require additional effort to maintain. Most animals develop to be more or less symmetrical, so the movements are coordinated and balanced.
Insects always have six legs, because the first insect to evolve happened to have six legs, and there was no evolutionary pressure to change that. Animals have four limbs because the first one did, and therefore so did all its descendants. There has occasionally been evolutionary pressure towards bipedal motion in some species, leading to two legged creatures, but there has to be a huge benefit to using two of your limbs as arms or wings for that to happen, because standing upright on two legs requires much more effort than standing on four legs.
Almost all animals are bilaterally symmetrical, so outside of things like starfish and jellyfish you’re pretty much only going to see even numbers of legs.
All land vertebrates evolved from an ancestor that had two pairs of legs. Gaining or losing a leg on one side but not the other would be a very strange leap to make, evolutionarily speaking.
Arthropods, on the other hand, mostly evolved from animals with segmented bodies; gaining or losing a segment may not require as large of a mutation, and if each segment has one or more pairs of legs (as in centipedes and millipedes) you can end up with more variation between species. That said, most arthropod groups have a fixed number of legs, e.g. all insects have six legs, all arachnids have eight, all decapods (crabs and lobsters) have ten, etc.
The closest you’re likely to find to a three-legged animal is the kangaroo, which walks with its tail.
Starfish are radially symmetrical, and depending on the species can have a wide variety of legs, including 3. Most animals are bilaterally symmetrical, and so have even numbers to maintain balance and move efficiently.
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the number of legs an animal has depends on its evolutionary history and adaptations to specific ecological niches. Considering Insects have six legs because they evolved from an ancestral lineage that had three pairs of legs. Over time, these legs became specialized for different functions such as walking, grasping, jumping, and swimming. The presence of six legs allows insects to move efficiently in the complex microenvironments they inhabit, such as dense vegetation, soil, and water.
While larger animals with four legs (quadrupeds) and two legs (bipedal) also evolved from different lineages with different ecological pressures. Quadrupeds evolved from terrestrial ancestors that needed to move on land without sinking in, and four legs provided a stable foundation for this movement. Bipedal animals, such as humans and birds, evolved from quadrupedal ancestors and gained an advantage in running and jumping.
As for the absence of three-legged animals, it may be due to the nature selection theory that support the advantages of bilateral symmetry. Most animals have a left and a right side that are nearly mirror images of each other. Three legs would break this symmetry, potentially leading to imbalanced movements or a disadvantage during “Locomotion” which easily lead them to face the hardship in striving for food and survival as well as escaping from the dangers
Do you want to see 8 legged house cats? Because asking questions like this is how it starts!
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