Evolution gave us a fifth

2009/12/01 Roa Zubia, Guillermo - Elhuyar Zientzia Iturria: Elhuyar aldizkaria

• Eboluzioa

There is no need to go to a museum to find an exciting achievement of evolution. Just analyze any kind of life to see the designs that have triumphed in evolution. The human body, for example, has many. One of the most striking is the hand, the end of the arm that ends in five fingers.

They are five fingers with the ability to grip the objects and the muscles that move the five are in the forearm. In this way, the hand itself does not have large volumes and, however, the fingers have the ability to make a lot of strength. The design of the hand has allowed us to build tools, has helped us communicate and has served to create the basis of the most used numbering system: the decimal system. Only with the brain would man not do all the things he does, but he also needed his hands.

But the design of man's hands does not only appear in the hands and in human beings. Human feet also have a five-finger design with small variations. As we walk upright, hands and feet have to look different, but they are based on the same scheme.

And for other variants, this design also appears in many other life forms. In fact, it appears in all four-legged animals that circulate on land or, at least, has appeared someday in evolution. In fact, the five-finger scheme was born about 380 million years ago, when some sarcopteric fish came out of the water and began to disembark. Amphibians emerged from them.

Thus, all the descendants of amphibians, that is, all tetrapods have inherited the design of the five fingers. It is clear that amphibians left us with a wider heritage than the human hand.

It is even wider if we consider another feature. Beyond the model of the five fingers, they discovered long ago that the inherited design is the skeleton of the entire limbs; in tetrapods animals the whole structure of the man appears from the shoulder to the hand.

One, two, many and five

The human arm has only one bone on the top, a humerus; two bones between the elbow and wrist, the cubit and the radius; a set of small bones on the wrist, the carp and the metacarpus; and finally, the hand has a row of bones on each finger, the phalanx. The human legs have the same structure: femur, tibia and fibula, tartsos and metatarsos, and finally, five-finger phalanges.

This structure is called kiridium and appears in all tetrapods. Starting with amphibians, and although one species is very different from the other, all current amphibians have spiral-shaped legs.

The kiridium was very successful in moving ashore, since in the second part of the limb the presence of two bones gives the next joint a great freedom of rotation. On the forearm of our arms, for example, is the cubit and the radius, and with two bones, it is very easy to turn the wrists. When walking on all fours, the turning capacity allows adjusting the position of the hand to the area that is being treaded, which gives great stability.

It is a perfect design for the skeleton of a limb. And it has had a great success in almost 400 million years. However, evolution does not stop. Successful designs change or adapt over time. That has also happened to the kiridium. There are many examples.

In horses only one finger remains of the initial structure, in cows and camels two fingers and in pigs four. Other mammals have also molded their limbs. Those of the sea form a beautiful example: in the whales and in the dolphins appear small arms, but the rear ones have disappeared completely.

Changes are also observed in reptiles, in the straight lines after amphibians. It is evident that some do not have an operating room structure in the extremities because they do not have limbs: In the cretaceous, all the snakes and several lizards lost their legs and, with them, the entire skeleton of their legs. Other reptiles did not lose the legs, but have the modified skeleton; turtles, for example, lost the structure of the kiridium by joining several bones of the fingers.

Birds have also undergone changes. On the one hand, when the wings were formed several fingers were lost and, on the other, the number of fingers of the legs was reduced. In addition, the radius size has decreased considerably compared to the cube. It appears when eating the chicken thigh with a thick bone and with it a very fine bone.

There are many more examples, evolution is not left. But kiridium has disappeared in many cases, so what success has it had in evolution? A great success, because many of the designs that were created later would not be possible if a kiridium structure had not been created previously. In this sense, there is no inconvenience in the structure of our hand being in a museum of the achievements of evolution.