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Recreation of oxygen molecules and erythrocytes floating in a vessel of the bloodstream.

One of the most fascinating aspects of science is that it provides us with a narrative of how and why things are the way they are.

This narrative reveals an enormous number of events that, if they had not happened or had happened otherwise, would have made our existence impossible.

These events are usually of a huge dimension: large volcanic eruptions, asteroid collisions, drastic climate changes.

All of them modulated the evolution of life and the emergence of intelligence.

However, there are small events that have happened in some molecules that are at least as important to our existence. One of those events caused hemoglobin.

If hemoglobin did not exist, it would be impossible for there to be animals larger than a small worm. In the absence of this carrier protein, oxygen could not diffuse from the air more than a few millimeters into the body.

Oxygen is not a very water-soluble gas, so blood plasma alone cannot transport the amount necessary for rapid metabolism to tissues.

Without hemoglobin, a brain of the complexity and size of ours would be unthinkable.

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Hemoglobin carries oxygen to the body.

Why is hemoglobin so suitable for oxygen transport?

The answer to that question lies in its molecular structure. It is made up of four proteins equal to two, the alpha chain and the beta chain. An alpha chain binds to a beta and these two bind to another identical combination.

Each of the four chains has a molecule attached to it that contains an iron atom.

This molecule is called the « heme group, » which gives its name to hemoglobin. The iron atom is in charge of attaching an oxygen atom.

Thus, each hemoglobin molecule is capable of binding four oxygen atoms.

A GPS for hemoglobin

To adequately transport oxygen in the blood, it is not enough to capture it in sufficient quantity. Hemoglobin must also detect where it is in the body.

If it is in the lungs, it must be able to capture oxygen with great force and not release it.

When blood circulation reaches organs and tissues, hemoglobin must know this to decrease the force with which it binds oxygen and to release it.

Letting hemoglobin know where it is is possible because it detects both the amount of available oxygen and the acidity of blood plasma in its environment.

Oxygen abounds in the lungs and carbon dioxide is expelled, a gas that dissolved in water produces carbonic acid.

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Without hemoglobin, oxygen could not diffuse from the air more than a few millimeters into the body.

This expulsion causes decrease the amount of carbonic acid, which also lowers the acidity of the blood.

In contrast, oxygen is consumed in the rest of the organs, which is much less abundant than in the lung. At the same time, carbon dioxide is generated in the metabolism.

This causes the gas to generate carbonic acid as it dissolves in the blood plasma and increases the acidity of the blood.

Not only can each hemoglobin chain detect oxygen level and acidity, but it can also communicate this information to its neighboring chains. Thus they all collaborate and do the same as them at the right time.

In the lungs, the collaboration between the chains allows coordinated oxygen uptake and very efficient. In the tissues, this gas is also released in a coordinated and extremely efficient way by the four chains.

Molecular resurrection

How has this hemoglobin capacity evolved?

The ancestors of this protein were known to initially be made up of a single isolated chain that captured a single oxygen atom.

The lone chain was unable to detect the amount of oxygen or acidity in the environment accurately and, of course, lacked partners to collaborate with.

For hemoglobin to appear, it was necessary that its ancestral gene first duplicated: where there was one, there were now two, the alpha and the beta.

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Without hemoglobin, a brain of the complexity and size of ours would be unthinkable.

Subsequently, it was also necessary for these genes to mutate, each on its own, to generate the combination of the four chains that exists today.

This whole process seems complex, and unlikely.

To try to find out how it could happen, an international group of researchers has managed to resuscitate, through a combination of bioinformatics and molecular biology techniques, the most probable ancestor of hemoglobin.

This molecule arose around 400 million years ago and was formed by the union of only two identical protein chains. It did not possess the properties of current hemoglobin.

The researchers are trying to reconstruct the evolutionary path that, from the second ancestor, could have originated modern hemoglobin.

They expected dozens of different mutations in the two prime genes, occurring over tens of millions of years. However, it was not.

Surprisingly, only two mutations in particular areas of the ancestral genes, which modify the surface of the alpha and beta chains and allow their interaction, are enough to generate a hemoglobin very similar to the current one.

These studies show that, at least at the molecular level, specific changes in certain genes can allow for great evolutionary leaps.

In this case, two small mutations made possible nothing less than the process of respiration by the lungs and gills and the emergence of animals with rapid metabolism, such as mammals.

Thanks to them we are here.

* Jorge Laborda Fernández is Professor of Biochemistry and Molecular Biology at the University of Castilla-La Mancha, Spain.

This article was published in The Conversation. You can read the original version here.

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