Animals of one species differ from animals of other species in their form and history of evolution, that is, in the sequence of changes in their forms from their ancestors to their modern appearance. The body shape of the animal is determined by the shape of the body of the ancestors and the environment in which the animal developed - climate, nutrition and other factors.
Movement on two and four legs
Some animals walk on two legs, others on four (or more), and some on two and four, in different situations in different ways. For example, chimpanzees, when they carry cubs, hold them in one hand, and move with the help of two legs and one arm, that is, they generally move on three limbs. Animals that move on two legs are called bipedal; those that walk on four are called tetrapods. People are bipedal, although in childhood we move on four limbs. Cats are four-legged, but sometimes they can stand on their hind legs. There are animals that have more than four legs, for example, eight of spiders, thirty of centipedes. And such animals as fish and worms, in general, do fine without legs at all.
Interesting fact: the difference between humans and other primates is that humans have a straight spine adapted to the vertical position of the body.
Movement and life
It sounds corny, but the animal must survive in order to survive. If it lives on the tops of trees, then it should have strong hands, especially hands, that would hang on the branches. If an animal, for its misfortune, suits a lion for lunch, then only tireless legs can save it. Each species has developed its own effective system of movement or, in other words, a strategy. Whether the animal moves on two legs or rides on all four are elements of such a strategy. Consider, for example, walking a person on two legs, this will help us understand what a strategy for effective walking is.
Man is a member of a large group of animals called primates. This group includes monkeys, baboons, gibbons, gorillas and chimpanzees. Other primates, except humans, cannot walk like we do (you probably saw this at the zoo). Most of them combine movement on four limbs with relative upright posture. Many jump on trees, but use their hands. Most primates live on trees, but sometimes they have to go down to the ground, so they must be able to move around both trees and the ground.
For example, gibbons jump on trees from branch to branch, swaying in their long arms. The guarantee of their safety is very strong hands and a strong grip. But, when a gibbon walks along a tree branch, it moves on two legs. When chimpanzees eat fruits on trees, they hang on a branch, clinging to her hands. Moving on the ground, chimpanzees use their legs and knuckles.Baboons run on the ground on four limbs. Gorillas walk like a chimpanzee. Such strategies are justified when living in tropical forests, where most primates live.
Interesting fact: it sounds corny, but animals must move around to survive.
Homo erectus in humans
Although the person belongs to the primates, he is very different in structure from his relatives. The human spine is straight and adapted to stand and walk on two legs. The chimpanzee's spine forms an angle with the femur, this structure is well suited for running on four limbs. A man’s legs are longer than his arms, and, for example, a gibbon’s arms are longer than his legs. Our feet are adapted to walk long distances, and at the monkeys, the feet are adapted to capture tree branches.
A person has many similarities with other representatives of the primates, but of all primates, only humans can walk on two legs all the time. Scientists believe that the main factor that determined the uprightness of a person was convenience when collecting food. In order to survive, our ancestors had to collect and eat food almost all day long. When they left the tropical forests of Africa and went into the savannah, they found a new type of food there - small leaves, nuts, seeds and fruits. In order to collect enough food for food, it was necessary to free both hands for gathering.
Walking on two legs for a long time is a great advantage. Those of our ancestors who were accidentally born with a straight spine had an advantage over those born with a bent ridge.Directly walking gatherers could collect, and therefore eat more food, therefore they were stronger than the others and had an advantage during reproduction: their children inherited direct posture. Gradually, after the change of many generations, all people became straight walking, which freed their hands for hunting, gathering and craft.
