Why most of the creatures in the world have four legs, like human \ lion \ even birds?

Question

Fish is out of discuss. I wonder why?

Answer 1

Actually, the statement is not true. Most creatures in the world don't have four legs. Insects, with 6 legs, are far more abundant than all four legged creatures.

However, the main groups of vertebrates - amphibians, reptiles, mammals and birds all evolved from a common tetrapod (meaning four-legged) ancestor. This is the reason they all have four legs, even if they have subsequently lost them (such as snakes, whales, etc).

Fish are also vertebrates. However, they are an evolutionarily older group, and the tetrapod ancestor of the other vertebrates is believed to have evolved from sarcopterygii, which evolved from lobe finned fishes.

Answer 2

Simply because we evolved that way. Check out some pictures of lungfish. They have fins that are limblike, as they could leave the water for periods of time (as their name suggests). Fish don't just have two fins; they have a few pairs, so these pairs eventually became the four limbs you see on other creatures. Birds evolved from dinosaurs (in fact, birds are dinosaurs), which walked upright because their hips have sockets perpendicular to their femurs, but all things have four limbs.

The creatures that walk on all fours do so because their bodies are structured so that all their legs touch the floor. This is for speed and stability; you don't see humans outracing cheetahs or horses.