Why do animals have four legs

Science in dialogue

Why are there no three-legged creatures?

For developmental reasons, there are almost exclusively living things with an even number of legs. At the same time, biologists are still discussing how the term “leg” should be defined. Currently, it is usually understood to be a body appendage which is used for locomotion and which is movable in itself, i.e. has joint-like structures.

Bilateral symmetry emerged very early in the evolution of multicellular animals, more than 600 million years ago. This means that animals consist of two symmetrical halves of the body. During the embryonic phase, both halves of the body develop largely in the same way. So if a leg forms on one side of a body segment, it does so on the other side as well. Today 98 percent of all multicellular animal species belong to these so-called bilateria. The only exceptions are sponges and hollow animals such as jellyfish.

Apparently odd numbers of legs could now occur due to the fusion of two leg systems in the middle of the body. In springtails, arthropods that live in the ground, the fourth pair of legs is fused in the middle of the body. This structure is called a furka (jumping fork). It is used by the animals to push themselves backwards in case of danger and thus to escape by jumping. So at first glance, these animals have seven legs. An animal would also be conceivable that after the jump does not land on six but on two legs. However, evolution on earth did not produce such a three-legged creature.

A three-legged animal could also arise from a mutation in the course of embryonic development that suppresses the formation of a leg on one side. The single leg would then have to do the work that is shared on the other side by two legs. Especially if there is only a relatively small number of legs anyway, as is the case with most animals, this would bring the animal more disadvantages than advantages in the struggle for survival. Such a mutation - should it have occurred - could therefore probably not prevail. Such creatures do not occur today, nor are they known from the history of the earth.

Paired legs, on the other hand, come in various numbers: one pair in humans, two pairs in terrestrial vertebrates, three pairs in insects, four pairs in arachnids, eight pairs in woodlice (crustaceans) and up to several dozen pairs in centipedes and millipedes.

The question was answered by Prof. Dr. Thomas Bartolomaeus, Institute for Biology at the Free University of Berlin.