Why is there absolutely no three-legged creature on Earth?

Science answers the questions "Why?" and also "Why not?". Occasionally, scientists will find it extremely valuable to find out why there are things that never exist: A creature with three legs, for example.

Tracy Thomson, a Earth Science research graduate from the University of California, USA, recently published an article in the journal Bioessays to discuss this interesting topic.

Meerkats stand by their tails.

Thomson shared that he started to have research ideas in a lecture of paleontologist Geerat Vermeij. In it, Vermeij challenged his students to find a non-existent model. And he immediately thought of animals with three limbs .

Thomson knew there were some species, such as the Meerkat, that stood on his hind legs and added a rear tail to make it stable. The same thing was observed on woodpeckers, when they used their tails against the trunk.

Having an additional third limb supporting when standing can help the organism save energy. For example, with Meerkat and woodpeckers, they can stand without too much force. Most of the body then can be relaxed.

Unlike us humans, standing on two feet requires a lot of muscle groups that need to be constantly used to balance. If you try to relax as much as possible while standing, you will feel the imbalance and fatigue of the remaining muscle groups keeping you from falling.

This is a fairly common yoga practice, and if you are really curious you can try it for a moment to ask: Why don't I have an extra leg behind it?

Woodpeckers also use their tails to stand.

In fact, a third limb not only supports standing position, but can also be useful while the animal is moving.

Thomson said he observed to see six-legged insects moving in a three-legged pattern: The two legs on the other side and the opposite leg lifted off the ground to move forward, while the other three legs held so that the animal can stand upright.

Many monkeys also use their tails to climb. While parrots also use a healthy beak to grip and pass branches.

The closest real animal model to real life are kangaroos . The extremely strong tail can make it difficult for them to " walk ", but it is extremely well supported when they jump or move like this:

So clearly, having a third leg is sometimes an advantage for the creature. So why doesn't any animal really have three legs? Thomson said the answer to the question probably existed millions of years ago.

" Most animals grow with their bodies facing each other , " he said. Symmetrical " code " may have been embedded in the genomes of all species from the very early evolutionary stage of life - perhaps even before the creation of legs.

And once this evolutionary trait is introduced into the DNA, it's difficult to remove it from the genome of the organism.

Mutations can sometimes cause a third limb creature, such as the case of Frank Lentini , an Italian man who lived in the late 19th and early 20th centuries. But apparently, evolution did not support This mutant protection and natural selection will suppress the genetics of the DNA, preserving the symmetrical "code" on both sides.

Frank Lentini, a three-legged man from Italy who lived in the late 19th and early 20th centuries

Because evolution has favored symmetrical creatures, these three-legged animals will only exist in human imagination. In sci-fi novels or movies, aliens are sometimes built with bizarre shapes and walk on three feet.

They exist on planets like Enceladus, Alpha Centauri or Mars, but absolutely real life, you will not be able to find a creature with three legs.

Thomson said imagination in this condition plays an important role. " If we want to learn about the evolution of life, we need to understand what evolution can do and what it cannot ," he said.

And imagining three-legged creatures and finding the answer to the question why they don't exist is not just a hobby. It deserves a real scientific research.