Today, the vast majority of living things need oxygen to live. But scientists know very well that this precious element was not always present in our atmosphere. In fact, many of the first organisms that populated the Earth did not need oxygen, and this was the case for the first more than a billion years of the history of life on our planet. However, most of those early organisms did make oxygen as a by-product of their metabolic processes, and they emitted it into the atmosphere. In this way, about 2,400 million years ago there was already so much oxygen, a poison for living beings at that time, that a great ecological crisis took place, a ‘Great Oxidation’ that led to a great extinction that ended with most of the living beings that were then.
Later, practically only those who found a way to take advantage of the new resource remained, and since then life has developed and branched into millions of very different species, but which have in common their dependence on oxygen.
Now, a team of researchers from the British University of Newcastle has just discovered that, long before all this, 3.8 billion years ago, another, hitherto unknown, oxygenation event took place. And that that could power some of the most primitive forms of life on Earth.
The ‘blame’ for this early emission of oxygen was the powerful earthquakes that shook the planet around 4,000 million years ago. The earthquakes opened the crust of the then young planet and favored the development of chemical reactions in the depths of the fractured rock. With the help of water and temperatures close to the boiling point, a significant amount of oxygen was released that served to power some of the oldest life forms. The finding has just been published in Nature Communications.
According to the researchers, this oxygen would have come ‘packaged’ in the compound hydrogen peroxide (H2O2), which contains two tightly bound hydrogen atoms and two oxygen atoms. Perhaps best known as a potent antiseptic, hydrogen peroxide is inherently toxic to living organisms, but still, explains Jon Tellinglead author of the study, can be a useful source of oxygen once it breaks down, either due to enzymes or due to reactions that occur at high temperatures.
In a series of laboratory experiments, Telling and his colleagues discovered a way in which large amounts of hydrogen peroxide could have been formed on the early Earth and thus served as a potential source of oxygen for some of the earliest living organisms. Such reactions occur most efficiently at temperatures close to the boiling point of water (100 degrees Celsius), but still produce some hydrogen peroxide at lower temperatures, up to 80 degrees.
Furthermore, it is the case that precisely at these temperatures thermophiles and hyperthermophiles, that is, heat-loving bacteria and archaea, thrive. The common ancestor of all life on Earth is thought to have also evolved to live in scorching hot environments, so in theory this mysterious ancient organism could have been influenced by the presence of hydrogen peroxide forged deep inside. of the earth’s crust.
In their experiments, the researchers pulverized rocks and then exposed them to the action of water. A small-scale imitation of the real stress that rocks endured in tectonically active regions of the early Earth’s crust, where the crust opened up and water was able to seep into it. When Earth was less than a billion years old, the planet did not yet have large blocks of crust sliding over its mantle in the form of tectonic plates moving across the globe, as they do today. However, according to Telling, at the time the crust was still warping and cracking due to volcanic activity and interactions with much smaller pieces of crust.
Although previous experiments showed that this early tectonic activity could produce hydrogen (a component of hydrogen peroxide) and fully formed hydrogen peroxide, those studies only succeeded in generating small amounts of these compounds. In his new work, Telling and his colleagues performed similar experiments, but exposed the crushed rocks to a wider range of temperatures and for longer periods of time, up to a week. And they managed to increase the amount of hydrogen peroxide produced.
In their experiments the team used granite, a rock found in continental crust, and basalt and peridotite, which would have been abundant in early Earth’s oceanic crust. They ground those rocks to a fine powder in oxygen-free containers, carefully transferred the crushed rock to airtight bottles, added water, and then turned up the heat.
As rock powders reached near-boiling temperatures, “defects” within the minerals that make them up became less stable and more likely to react with water. Specifically, these defects included “peroxy bonds,” or places where two oxygen atoms bond within the crystal structure of minerals, where normally oxygen would only bond to the element silicon. Such defects can creep into a crystal if water is inadvertently added to its structure as it forms, Telling said.
“When these rocks that contain these peroxy bonds are subjected to stress -explains the scientist-, these defects can dislocate. They can move through the crystal structure to surfaces where they can then start to interact with water,” and this interaction ultimately produces hydrogen peroxide.
These results suggest that, at least in regions of the early Earth shaken by earthquakes and baked at high temperatures, hydrogen peroxide may have been a common feature of the environment. However, Telling makes clear that experiments cannot capture the exact rate or scale at which these H2O2-producing reactions took place on early Earth.
The researchers believe that it would be very interesting to study further how widespread this process was, and to what extent it could influence the first living organisms. Something that, by the way, could also be happening, or have happened, on other planets.