[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?
In: 381
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.
Latest Answers