Oxygen slides have taken life on earth to the next level

(ORDO NEWS) — Life on Earth arose four billion years ago, but for about two billion years it could not actively develop due to an almost oxygen-free atmosphere.

Then oxygen seemed to increase, but even after becoming multicellular, life really did not want to go to land for another billion years, and complex organisms did not appear in the seas either.

It’s a big mystery: why did life seem to be waiting for something for three and a half billion years, before suddenly accelerating and giving birth to a huge variety of forms, including us humans, in the next half a billion?

It seems that scientists have come close to answering this question.

According to the latest data, the earliest traces of life on our planet are at least 4.1 billion years old, and possibly even 4.28 billion years old.

This means that it arose extremely quickly: the age of the Earth itself is only four and a half billion years. However, after this change on the planet went very slowly.

Photosynthetic organisms have accumulated oxygen in appreciable quantities only after more than one and a half billion years of their existence. Then his concentration slowly rose, but despite this, there were very few signs of complex life.

About 2.1 billion years ago, something similar appeared: coiled flu . This is about a centimeter long organism, which is often called the oldest multicellular organism (bacteria of this size are still doubtful).

Many scientists believe that this is also the oldest eukaryote – this is the name of organisms like you and me, descended from archaeal cells that once “captured” a bacterium, which was why they were forced to form complex but useful adaptations such as the cell nucleus.

But flu, once it appeared, practically did not evolve. It is found both in layers 2.1 billion years old and in layers 0.9 billion years old . No noticeable changes: apparently, the first multicellular organism settled down so well that it did not need to change anything.

Not surprisingly, the period up to 0.8 billion years in science is called the “boring billion” or “Earth’s middle ages.” Nothing happened there: the climate is evenly warm, the views are very similar, evolution seems to have stopped.

The question arises: what happened next? Why did organisms such as animals (Animalia), to which we belong, arise in the last 800 million years?

Land plants, without which land animals could drag out only the most miserable existence and never become intelligent? Why did evolution suddenly accelerate from a slow pace to a natural sprint? And why did she almost sleep until then?

Roller coaster

The new paper, published in the journal Science , significantly clarifies the answer to this question.

Analyzing the ratio of carbon isotopes over the past one and a half billion years, its authors showed how the oxygen content in the air changed during this period. And it changed the whole picture.

At first glance, it seems strange how carbon isotopes can be related to oxygen levels at all. The thing is that it is easier for photosynthetic organisms to capture lighter carbon atoms from the environment, that is, the carbon-12 isotope.

Carbon-13 is “garbage” for them, and such organisms “garbage” all their life, leaving an increased concentration of carbon-13 in the sedimentary rocks around them.

Focusing on the amount of “carbon garbage”, you can fairly accurately estimate the total amount of photosynthetic activity in a particular era.

Oxygen slides have taken life on earth to the next level 2
Ediacaran biota that existed 635-542 million years ago, before the Cambrian. Earth’s first truly complex biota. Dickinsonia is visible in the center – these are animals (though very unusual) that lived in that era and are found in sediments from Australia to the coast of the White Sea. Their length reached 55 centimeters. What they ate, how they lived is a mystery. There is no mouth, no reproductive organs, there are big doubts even about the presence of intestines. Even the fact that they are animals is deduced only from the presence of chains of footprints at the bottom next to their relatives. Differences in anatomy become more understandable when you consider that they lived with a negligible concentration of oxygen and therefore must have been enormously different from modern animals

And such an analysis showed: from a billion to 500 million years ago, the level of oxygen in the atmosphere was constantly changing. Moreover, in a very wide range – from one to 60 percent of the current level.

This means that in a matter of hundreds of millions of years, the Earth was either an “oxygenous” planet, with the oxygen content in the air as it is today at five kilometers (you can breathe, but only after adaptation), or actually anoxic, with an oxygen concentration of only 0.2 percent.

The first concentration can support the complex life of the types known to us, but the second cannot.

For example, 750 million years ago, when the complex “Hainan biota” (of worm-like animals) existed on Earth, there was 12 percent oxygen in the air. But over the next three decades, the oxygen content dropped sharply to 0.3 percent (more than fifty times lower than now).

Not surprisingly, the Hainan biota came to an end. It is easy for multicellular organisms to live at a low concentration of oxygen only if they produce it themselves (as, apparently, the helical gyrpania, which was discussed above, did).

Otherwise, providing the cells “in the middle of the body” with respiration becomes far from being so simple: “pumping” oxygen there turns into an energetically difficult task.

But that’s not all. The most mysterious thing is that, judging by the work, with the beginning of cryogeny – a geological period when the Earth was completely covered with ice twice, right up to the equator – a real “oxygen counter-revolution” happened on the planet.

From 12 percent oxygen to cryogeny, there was no trace left. Even when the glaciation ended and the Earth “thawed”, the level of oxygen in the air did not exceed four percent.

Moreover, it remained at this level throughout the entire Cambrian period (539-485 million years ago) and even the Ordovician (485-444 million years ago).

Oxygen slides have taken life on earth to the next level 3
Kimberella, a remote functional analogue of a mollusk (only without a shell) from the Ediacaran time. Length up to 15 centimeters, width up to 5-7 centimeters. Found in Australia and on the coast of the White Sea

Meanwhile, it was in the Cambrian that life experienced a sharp surge in the diversity of complex forms: trilobites and a host of other multicellular living creatures appeared and spread. It turns out that they all existed in conditions of a terrible lack of oxygen, where we could not live for even a few minutes?

And here the surprising conclusions from the new work are just beginning. Another question immediately arises: how did this happen? Until 920 million years ago, there were few sharp fluctuations in oxygen content, and then suddenly this? For what?

Action equals reaction

Chronology is the most likely answer. Two major peaks in oxygen content – 750 and about 660 million years ago – happened right before the two major glaciations that made up the cryogeny. These are the Sturt (715-680 million years ago) and Mariana (650-635 million years ago) global glaciations.

From the glaciations familiar to us, during which our ancestors lived, these two differ as modern dinosaurs from the group of maniraptors (that is, birds) from their ancestors from the Mesozoic.

There is a lot in common, but the ancient analogues were incomparably larger. Land in cryogeny was represented by continents at low latitudes, near the equator. The presence of traces of continuous glaciation there indicates that the entire planet was covered with ice.

Oxygen slides have taken life on earth to the next level 4
Remains of ancient stromatolites, bacterial “mats” that lined the bottom of ancient shallow waters

Ice reflects about 90 percent of the energy of the rays of our star, so such global glaciations did not melt once every several tens of thousands of years ago, like the usual Pleistocene genus Homo, but were depressingly stable: they lasted tens of millions of years.

To complete, they had to wait long and hard for volcanic eruptions to saturate the atmosphere with so much carbon dioxide that it would reduce the heat loss of the planet to very small values.

While such a process was going on, photosynthesis itself was stressful. Algae under a thick layer of ice simply cannot synthesize oxygen: there is no light. On land covered with a kilometer-long ice sheet, you also don’t particularly photosynthesize.

Some microalgae could exist on the surface of glaciers, similar to those that give the red color to modern ice. But due to zero access to the minerals of the earth’s crust, they had to have an eternal deficiency of trace elements. Not surprisingly, the oxygen concentration dropped to 0.3 percent.

These glaciations were, in a certain sense, the product of the same forces that created an excess of oxygen in the atmosphere 750-800 million years ago.

It is obvious that the sharp growth, which is visible on the graph from the work, appeared due to the rapid reproduction of some group of photosynthetic organisms. And what are these organisms?

Oxygen slides have taken life on earth to the next level 5
Bottom graph: The vertical axis shows the level of oxygen as a percentage of present day (black line), the horizontal axis shows millions of years before our time. The peak is clearly visible in the region of ~750 million years, and the subsequent one held almost to zero in the region of the beginning of the global glaciation of Cryogeny

It is difficult to answer this question. The most active oxygen-producing group today is prochlorococci ( Prochlorococcus) , extremely specific cyanobacteria with a diameter of only 0.5-0.7 micrometers (yes, not millimeters).

The body is paramount, because it provides 20 percent of all oxygen in the air. But it was opened only in 1986, less than 40 years ago – simply because it is too small.

Of course, it does not have a durable hard shell, and if it existed 750-800 million years ago and is “responsible” for the outbreak of photosynthesis, which then made the Earth truly oxygen for the first time, then there is no way to know about it.

Prochlorococci are unlikely to have always existed. Because there are too many of them – a billion billion billion (octillion) – and due to this they would inevitably change the composition of the atmosphere.

The important thing to understand here is that oxygen is produced by a lot of organisms, but in many ecosystems, oxygen production is approximately equal to its consumption.

For example, in the terrestrial jungle, plants first produce O 2 during life, and then absorb it after death (due to decay of residues and other processes).

Prochlorococci produce a lot of oxygen, but live in an oligotrophic, that is, nutrient-poor environment. So there is no one there to consume the oxygen they produce. And in the event of death, their remains simply sink, and oxygen is not consumed for the oxidation of the remains.

So it is possible that it was the activity of prochlorococci that “launched” all these oxygen “roller coasters”. However, some other group of organisms could have been a revolutionary photosyntheticist – and the “oxygenation” of the atmosphere really became a biological revolution.

Oxygen slides have taken life on earth to the next level 6
They also meet today. although in a much different form and much less often. These lumps in the photo from Australia are also stromatolites. But in order to survive today, they have to live in hypersaline environments, for example, in a shallow sea lagoon, where an animal that could eat such mats simply does not want to go

But, as you know, action is equal to reaction. By absorbing carbon dioxide and making oxygen out of it, the mysterious revolutionary photosynthetic made the greenhouse effect on Earth weaker and weaker. Until a certain point, it didn’t matter.

Up to the average planetary +17…+18 degrees there is so much water vapor in the air that glaciation does not occur. After all, water vapor also perfectly absorbs infrared radiation and does not allow the surface of the planet to become supercooled.

However, as soon as average temperatures fall below this threshold, water vapor over the poles practically disappears from the air. Indeed, in the cold, its concentration drops ten or more times: the colder the air, the less moisture it contains.

This is where glaciation begins: losing heat through the polar regions, the planet is rapidly covered with ice. Ice reflects the sun’s rays into space, causing glaciation to accelerate … and as a result, in cryogeny, the Earth became a snowball planet.

Further, according to the already mentioned scenario, it again accumulated carbon dioxide for tens of millions of years, and then thawed again.

However, as can be seen in the graph from the new work, after the Sturt glaciation, the mysterious photosynthetic revolutionaries again sharply increased the concentration of oxygen in the air. After that, glaciation began again – the Mariana.

After it, the jumps became noticeably weaker. Neither rises to 10-12 percent of oxygen in the air, nor dips below the percentage are no longer visible.

Characteristically, the Ediacaran explosion of biodiversity began here, and, a little later, the Cambrian. It turns out that something has become a brake on the trailer of earthly life (or someone).

And that brake made the climb and descent of the oxygen rollercoaster not nearly as fast and hard as it used to be.

Apparently, this is why global glaciations have disappeared: where can they come from if there is no one to quickly turn huge masses of CO 2 in the air into O 2 , depriving the Earth of a warming “blanket”?

Difficult life as the result of a series of disasters?

All of this paints an unexpected picture. Earlier ideas that hothouse, stable conditions are needed for life to evolve into something complex do not seem to stand up to the test of facts.

The reverse picture is evident: sharp fluctuations in the biomass of photosynthetics “rocked” the world so much that it was completely (and more than once) enveloped in an ice shell. Undoubtedly, this meant a mass extinction of species: there is no great biodiversity under the ice.

But it is no less certain that life emerged from the crucible of two global glaciations incomparably more developed. Ediacaran, and then the Cambrian.

What could be the trigger for the biodiversity explosion? The rapid reproduction of some photosynthetics has created unique opportunities for organisms that these photosynthetics can eat. According to modern ideas, these could be burrowing organisms that suddenly appeared en masse in the Cambrian.

There are few fossils of them, so exactly what they looked like we can only guess (it is assumed that they were either similar to worms with exoskeletons , or were a type of primitive molluscs). It takes a lot of digging and a lot of luck to know for sure.

Oxygen slides have taken life on earth to the next level 1
The appearance somewhere in the Eidacaran and Cambrian period of burrowing organisms helped to destroy the previously massive stromatolites. Due to this, the layers of the seabed, once saturated with toxic substances, gained access to oxygen and became the site of the development of relatively complex life. Who knows: it may have helped stop the too-rapid conversion of carbon dioxide into oxygen. This process twice for tens of millions of years in a row fettered the Earth with a continuous ice sheet

But it is likely that it was the rapid reproduction of those who ate photosynthetics that prevented them from multiplying again to such an extent as to freeze the Earth.

This is indirectly indicated by the fact that after the beginning of the Cambrian explosion of biodiversity, stromatolites begin to occur much less frequently: stone remains from cyanobacterial mats – films saturated with cyanobacteria.

Scientists believe this is because many stromatolite-eating organisms have emerged that did not exist before.

This is also indicated by another fact: after the Ordovician-Siluyrian and Permian extinctions, stromatolites become frequent again for a short time (after all, those who ate them almost died out). But as the biodiversity of complex life forms recovers, the stromatolites retreat again.

Today, stromatolites can be found mainly in places where there is too salty water or a constant sharp change in salinity: there, complex organisms survive with difficulty, so there is simply no one to eat cyanobacterial mats.

Despite the relative stability brought by eaters of early marine plants, the world of the Cambrian and Ordovician was still, to put it mildly, uncomfortable.

The oxygen content on the graph from the work is such that complex terrestrial organisms were practically excluded there. “Stuffy” is the mildest definition of an atmosphere where oxygen is like today on the top of Everest.

Oxygen slides have taken life on earth to the next level 7
In order for at least the simplest plants to come to land, a serious evolutionary leap was needed. Apparently, it was precisely the constantly changing conditions of the world, where there was either a lot or a little oxygen, that pushed life to this extremely difficult step

To create a modern world, with a very complex biota, one more step was needed. Somewhere around 450 million years ago, it happened: plants came to land. Here everything was very similar.

Soon after the mass colonization of land by green plants, the ice age began again on Earth, although not as severe as in cryogeny. The Ordovician-Silurian mass extinction 443 million years ago occurred after the appearance of traces of mass plant colonization of coastal zones.

It is possible that here, too, the uncontrolled reproduction of photosynthetics, for which there were no enemies on land yet, caused both a rapid increase in the oxygen content in the air and a very vigorous cooling. After all, CO 2 , from which plants build their bodies and make oxygen, clearly had to decline then.

What stabilized the ecosystems of terrestrial plants that completed the oxygen revolution on Earth and raised the concentration of O 2 to modern values?

Probably the same thing that stabilized marine ecosystems before: the massive emergence of vegan organisms that can eat land plants. Ultimately, we also arose from these same organisms.

Lessons for the future

The authors of the new work believe that conclusions follow from it, which are valuable not only in the case of the Earth, but also for the search for life on exoplanets – outside the solar system.

First, they note, the idea that there was only one abrupt transition from an oxygen-free to an oxygen atmosphere on our planet – an oxygen catastrophe 2.45 billion years ago – is now clearly outdated (however, this idea has been criticized a lot before).

It turns out that after it the world received only one or two percent of the oxygen in the air. Further, its level slowly increased until 0.8 billion years ago, sharp jumps in oxygen levels began – the “roller coaster” described above.

It follows from this that exoplanets with a roughly terrestrial climate, but without traces of a significant amount of O 2 in the atmosphere, are not necessarily devoid of complex life.

On the contrary, the authors note, complex forms of life appeared on our planet against the background of just a very low oxygen content in the air. It turns out that only after the appearance of permanent terrestrial vegetation, O 2 in the air can become as much as we have on Earth.

In other words: having established an exoplanet with “our” temperatures, but a different composition of the atmosphere, you need to continue to look for substances on it – markers of life, and not give up, believing that it is dead.

Second, the researchers believe that the roller coaster they have discovered may indicate that complex life requires not stability and resilience, but abrupt changes that destroy old dominant species and allow new ones to advance. Not so much bioclimatic “peace” as “war”.

Oxygen slides have taken life on earth to the next level 8
An artist’s view of a red dwarf seen through clouds on a planet revolving around it. Visually, a star at sunset in such a place should really look very large. More importantly, the radiation from it is mainly in the infrared range, that is, it effectively melts the ice cover

The last conclusion, however, can hardly be called indisputable. As we noted above, the Cryogenian glaciations coincide suspiciously with the epochs immediately after reaching high oxygen levels. On their own, without living organisms, the levels of O 2 and CO 2 on Earth cannot fluctuate sharply.

Consequently, the very drastic changes that the authors of the work discovered may be not so much a condition for the evolution of complex life, but rather the result of the evolution of the life that appeared before this “complex” one.

And then we still do not know what is actually required for the explosive development of complex multicellular organisms: stability or instability.

]It may well be that external stability is needed to give rise over time to a group of actively oxygenating algae. And the subsequent instability is the inevitable result of the activity of such algae.

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