Question

why a human can walk on both legs but animal cant

Answer 1

Walking on two legs is way more efficient than walking on four. We use about half of the energy to go a given distance than a chimp does. Walking on two legs also means we can put our hands to work for other tasks, like carrying things and using tools.

Limited and exclusive bipedalism can offer a species several advantages. Bipedalism raises the head; this allows a greater field of vision with improved detection of distant dangers or resources, access to deeper water for wading animals and allows the animals to reach higher food sources with their mouths.

Some animals evolved bipedalism to be correct: Just about all birds are bipedal, not only ostriches, turkeys and penguins. Kangaroos and their relatives are essentially bipedal unless moving very slowly when they use their arms as props. Some lizards use bipedalism when running also.


HUMANS-
Scientists know that bipedalism was one of the first features to develop in early hominids -- early human ancestors that had already diverged from apes. And a variety of theories attempt to explain why, yet some of the theories seem to contradict one another. A study published in the July 2007 issue of Proceedings of the National Academy of Sciences attempts to provide a definitive answer. It claims that human bipedalism boils down to one thing: energy.

The study, performed by three researchers from the University of Arizona, the University of California, Davis, and Washington University in St. Louis, examined differences in upright walking between four adult humans and five adult chimpanzees. Chimpanzees were used because they're the closest modern-day relative to humans. (Four to seven million years ago, humans and chimpanzees diverged from a common ancestor. They then developed independently.)

Answer 2

Evolution is most important thing for Mother Nature. Mother nature believes in Survival of the Fittest. Every organism evolves with time.

Understand with simple example of Smartphone. With time newer Smartphones are being developed which are way more efficient and better than previous ones. Mother nature also do the same. She evolves living organism.

The evolution started from water, why there are still fishes why they are not evolved… they didn't evolved completely only few from their species evolved and became amphibian. There are still amphibians, but 1 part of them became reptile. There are still reptiles but I arm evolved to birds and other one evolved to mammals. Evolution takes very very long time and not everyone gets evolved. Just a small group evolves and keep their journey. Primates like human are the last or latest jump in evolution. We can use our hands to protect us and feed us. also we have big brain to help us survive rather than run. because of we use our brain and hands, we sacrificed the 4 legs velocity in nature. We are the slowest animals and we can never survive without our brain, in nature.

NOW BELOW IS THE ANSWER OF YOUR QUESTION

Animals that walk upright on two legs either some the time or all the time have several advantages over those who do not. This posture raises the head higher up which gives the animals a wider field of view to look for prey and keep an eye out for predators. Being upright also makes it easier to reach up into trees for food (with the hands or the mouth) and it frees up the hands to hold and carry things. Self-defense using the front limbs is also possible with bipedalism. Overall, quadrupeds run faster on four limbs than animals that use bipedal locomotion, but in endurance running, humans can outrun most quadrupeds. Over millions of years, natural selection has repurposed limbs that are not used in bipedalism into structures like wings and hands