Karen McCarthy answered on 20 Nov 2013:
Hey Sarah, I actually just answered a really similar question!
Although they are some different ideas, most scientists agree that this has evolved because of balance! It is easiest to balance on 4 legs vs any other number – think of tables and chairs at home.
Humans evolved to stand on 2 legs because it became more beneficial to them – animals such as dogs, cats etc have stayed on 4 legs because again it provides them with more benefits – such as being able to run faster, be more agile.
Parkour for example, or street running is a classic example of the benefits of 4 limbs – be encouraging humans to use their arms and legs more equally, you can mover faster and more dextrously !
Angela Stevenson answered on 20 Nov 2013:
Great question Sarah 🙂 These mammals need their legs to move and also need them for balance. Can you imagine how hard it would be to move and balance with only three, two or one leg even. A dinner table is a good example of the importance of four legs for balance. Most tables have four legs, so they’re nice and sturdy to eat on. Now imagine the table with only three legs…. it might get a little wobbly, but as long as you don’t push on the wrong part, it would collapse – making a total disaster of your dinner! Imagine the table with only two legs or one leg… now it just tips over. That’s the same idea for any mammal, their four legs provide them with the necessary support that they need to stand and move without falling… and even run very very quickly. They evolved this way in the wild so that they could catch their food – four legs can move faster than just two. Compare our running ability to that of our four legged dog friends (we only use two at a time), also horses, cheetah, etc, who use all fours to run. Hope that helps 🙂
Shane Mc Guinness answered on 20 Nov 2013:
I’m not too sure about the balance thing guys…. sorry.
We evolved from fish with four limbs. This fish, very similar to the coelocanth fish around today, had four fins which were made of joints (just like our legs and arms) instead of spines (like modern day fish). But fish float in the water right? Yes they need to remain straight, but do they need to balance? The evolution of the four limbs that our early fish ancestors had was probably to allow it to move forward in the water. And that’s exactly what the first land animals did as well; they slid along from side to side. They didn’t use the limbs to balance but to slide forward. Then, walking on these limbs was easier than sliding along so that evolved too……
I’m not too sure that four legs are faster than two either. Think about how fast an ostrich can run! All birds evolved away from four limbs to just using two at a time. What does that say? I would say that animals are slowly evolving away from four legs actually.
These are the kinds of things we need to consider when answering scientific questions Sarah. What do we actually know? And do the stories we make up based on these things make sense? Always question!