Why aren鈥檛 there any big animals (not insects) with six legs, or eight legs or more than this?
鈥� The answer lies with evolution and natural selection. Land animals with four legs evolved from fish, and their legs were originally fins. Fish have developed a variety of fins to enable them to swim, and a typical arrangement is two pairs of them (known as pectoral and pelvic fins) and a tail that actively propels the fish, plus additional fins on the back or underside that provide stability.
Evolution is very good at using features that evolved for one purpose as a basis for something else, and some ancient fish started living at the water鈥檚 edge, crawling on muddy land using their four pectoral and pelvic fins. We can see this today with fish called mudskippers, whose two pairs of fins have become fleshy lobes so they can skip through mud when not in the water.
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Over millions of years, fish like these evolved into amphibians and later into full land animals, and their four fins ended up as proper legs. The other fins were single ones, so were no good for evolving into legs, as they would have just made walking awkward.
And don鈥檛 forget, four legs have proved perfectly adequate for animals. It would have been a waste of energy (requiring more food) to start evolving more.
Richard Swifte, Darmstadt, Germany
鈥� A big animal would not develop six or more legs unless a certain evolutionary variation was needed to increase the success of this animal, and this variation has not been required. Additional limbs could also impair the animal鈥檚 mobility, camouflage, or even its way of life entirely.
Furthermore, it would take time and energy to grow these extra limbs, which could be diverted from growth of more important body parts. This might weaken the animal and, in extreme cases, jeopardise it or drive it to extinction. For instance, why would humans develop a tail or a third leg? This would be a waste of time and impair us.
There may have been some species that we don鈥檛 know about with extra limbs. They may have been driven to extinction or forced to adapt to survive.
Luke Worster, Nottingham, UK
鈥� Well, there are 鈥� giant squid for example, and octopuses. But for land animals, it is a consequence of the history of life on Earth. Big land animals are all 鈥渢etrapods鈥� (originally meaning animals with four legs) because insects, which have more legs, can鈥檛 grow big enough. They have a different type of respiratory system, one that limits their size with the amount of oxygen that is in Earth鈥檚 atmosphere now.
Some tetrapods have lost limbs (like snakes), and some have become animals that stand on just two limbs (birds and humans). But it would be too much of a mutation to suddenly have six limbs, or eight, instead of four.
Eric Kvaalen, Les Essarts-le-Roi, France
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