Why do we have four limbs?



Why do us mammals, and most animals actually, have four limbs? Why not more or less? Octopi have 8 limbs, why can’t I?

In: Biology

This got locked in when some fish which all vertebrates, including mammals, descended from, ended up having that particular amount of fins on the lower side of its body. And mutants having extra limbs now just doesn’t work.

Because this is where chance mutations got us. Evolution doesn’t work with reasons. We coincidentally evolved four limbs, and there was never any pressure to evolve more or fewer. That’s all.

All mammals and probably all the animals you’re thinking it of descended from a common ancestor that had had four limbs (a lobe-finned amphibious fish, to be specific), and all are based on slight modifications of that four-limbed body plan. We call this family of life forms “tetrapods,” which in Latin means “four-footed.”


The real question is why would you need more limbs? Evolution can’t really tell what traits are the best, just what are good enough to survive. Snakes have no limbs so think about that.

This is going to sound simplistic and dismissive but it really is the answer. Humans are that way because they evolved that way through natural selection. That’s the answer to literally every question about why an animal is a certain way or acts a certain way. It’s because over countless generations that trait is beneficial to survival and so those with that trait are more likely to produce offspring.

In 10 million years (assuming we don’t totally kill the planet) will humans, or whatever new species we become, be different? It’s a near certainty the answer is yes. Maybe we will have evolved traits that allow us to better deal with rising temperatures or lower oxygen levels. We are not the end product of evolution, only a step along the way in a process that has been going on since amino acids combined in some little tidal pool billions of years ago.