We survived oxygen.

Tuesday race, March 4

Friday race, March 7

Today I had a long run, the longest of 2025, 16.24 km, geting ready for the Guaymas Half Marathon, at the end of April. So taking a breath, to have enough oxygen to develop physical activity.

That's where I started to think, almost no one thinks about it, life survived oxygen.

When the first living beings emerged on Earth, the atmosphere was totally different from what it is today. Composed of hydrogen, methane, carbon dioxide, and ammonium, among the main gases, and no oxygen. The abundance of methane and carbon dioxide caused, due to the greenhouse effect, temperatures to remain high and, in addition, the oceans to be acidic, since when carbon dioxide dissolves in water it forms carbonic acid.

About 2.8 billion years ago, some bugs found how to produce the energy they required from sunlight and carbon dioxide, using a pigment that gave them a blue-green color, producing carbohydrates, rich in energy and free oxygen. The first photosynthesizers, the blue-green bacteria.

Free oxygen is very reactive and for millions of years it did not accumulate in the oceans or the atmosphere, but instead oxidized, living up to its name, everything it could oxidize, which was a lot. It took about a billion years for oxygen to begin accumulating in the oceans. This caused the first mass mortality in the history of the Earth, as the first living beings developed in the absence of oxygen, they were anaerobes and oxygen was very toxic to them.

When most of the blue-green bacteria died, they stopped producing oxygen and it decreased fast, being so reactive. But as the oxygen decreased, the surviving blue-green bacteria once again produced oxygen, which again began to react with what it found and then accumulated, causing a new mortality. This cycle must have been repeated several times, but in each cycle, some bugs found a way to coexist with oxygen, which is why they survived.

In this way, it was possible to reach a time when oxygen not only accumulated in the oceans, but also began to escape from the oceans, finding a lot to react with, especially minerals with iron, so its accumulation in the atmosphere was very slow. But in the atmosphere, it found something else to react with, methane. But this gas has a very strong greenhouse effect, more than 34 times greater than that of carbon dioxide, so the atmosphere lost its ability to store heat and the coldest phase in the history of the earth arrived, that of “THE GREAT SNOWBALL”.

Again a great extinction.

One type of living beings survived all of these extinctions. The main characteristic of these bugs is their information storage system, required to make copies of themselves, to reproduce, which is based on ribonucleic acids. These are deoxyribonucleic acid, DNA, and ribonucleic acid, RNA. They are very similar, but DNA forms double helices and RNA does not. Now, both have a fundamental role in the functioning of bugs. One, DNA, has the instructions, and the other, RNA, passes them on to the bugs' operating machinery.

There are other features, but first, something needs to be explained. Let's say that molecules are formed by joining atoms of different elements into three-dimensional structures. In some compounds, you can have configurations that are mirror images, like our hands. We have left (levo) and right (dextro) hands, which are mirror images of each other. The same thing happens with many compounds: they have levo form and dextro form. Chemically, and physically there is no reason for dextros to have an advantage over levos.

The bugs that survived only have dextro nucleic acids and levo amino acids, no one knows why. 

All current living beings come from them.

So, fortunately, living beings survived oxygen.