The planet we live on is approximately 4.54 billion years old. And for most of that time, it was completely different from the Earth as we know it. Even the most exotic planets outside the solar system may not be as interesting as our own at some points in its history. Let's put on a spacesuit, get into a time machine - and imagine ourselves as alien travelers who visited our system in its early days.
System of two moons
Your feet are buried in gray dust, the surface is dotted with craters to the horizon, and overhead, despite the day, is a black starry sky. It may seem that you are on the moon, but when you try to find the Earth in the sky, you find a giant crescent moon hanging low. Its disk diameter is much larger than that of the sun.
Welcome to the Earth of the toddler age - the eon katarchei. This is the so-called pre-geological era, the very first period of the life of the Earth, and if scientifically, it is a geological eon. It began with the birth of the Blue Planet - about 4.54 billion years ago - and lasted 600 million years. Not even any "normal" rocks available for study were left of it.
Many young lifeless planets in other worlds will look like this. In the katarchea, the planet is still completely devoid of life: it will appear only after - in the archean. But today the planet is still extremely cold. The very thin atmosphere is unable to store the heat received from the young sun. At that time, our star was not as bright as it is now, and its luminosity was 30 percent less.
A day on the planet lasted only six hours, which was equal to the period of the Moon's revolution around the Earth. And her companion itself was at a distance of some 17 thousand kilometers. Now at this height is the outer radiation belt, which protects the planet from the solar wind. But in katarchea there were no radiation belts, no magnetic field around the planet. By the way, now even the orbits of navigation satellites are higher than the Moon at that time - from 19 to 23 thousand kilometers above sea level, and geostationary ones are located twice as far - about 35 thousand.
But this distance gradually increased (at first at a speed of about 10 kilometers per year). By the end of the Katarchean, the rate of the Moon's removal from the Earth dropped to four centimeters per year, and the distance between them at that time was about 150 thousand kilometers. Today the Moon has already flown away from us at a distance of about 384, 3 thousand kilometers.
At that time, the Moon orbited the planet so close that as a result of the action of tidal forces, earthquakes on its surface almost never stopped. Immediately after its formation, the Earth had a fairly homogeneous composition, there was neither a core nor an earth's crust. It was a relatively cold space body, and the temperature in its interior did not exceed the melting point of the substance. But during the catarchy, including thanks to our satellite, the processes of condensation of proto-matter continued on the planet. With the beginning of its melting and heating, a new geological period in the history of the Earth began - the archaea. The moon remained practically pristine - as we know it. Meanwhile, on Earth, it becomes hot and life is being born.
We'd better not land on this Earth. Conditions are cooler here than on modern Venus. The Archean stage in the history of the Blue Planet lasted one and a half billion years. And during this time she changed her appearance several times.
The planet of the early Archean is shrouded in a dense vapor-gas envelope. Impenetrable to sunlight, it plunged the Earth into darkness - darkness reigns on the surface. There is no oxygen here - only carbon dioxide and water vapor. The density and pressure of such an atmosphere is much higher than modern values. The hydrosphere of the planet is just emerging: there is little water on the surface, and a single World Ocean does not yet exist. Individual bodies of water are isolated from each other and are very hot: the temperature in them reaches 90 ° C. And meteorites are falling from the sky every now and then. At the beginning of the Eoarchean - the first geological era of the Archean eon - the frequent fall of asteroids that accompanied the Earth from the very birth of the Earth continued, this was the time of the completion of the so-called late heavy bombardment.
Over time, the vapor-gas envelope will separate into the hydrosphere and atmosphere. By the end of the Paleoarchean, the next era of the Archean eon, the formation of the solid core of the Earth is completed. As a result, the strength of the planet's magnetic field has already become quite high and amounted to at least half of the current level. This is enough to protect the gaseous atmosphere from solar wind.
Mesoarchean, the third era of the Archean, is also not the best time to visit the planet. The Earth is a “planet-ocean”: its entire surface is a global ocean. It is shallow, very salty and hot. Its waters are greenish due to the high concentration of dissolved ferrous iron, and the sky above it is orange due to the high concentration of methane in the atmosphere.
The land is represented only by volcanic islands, which during the Middle Archean grew in number and gradually formed into the first large areas of the surface. The moon is still close to the planet, and its presence this close causes giant tidal waves up to 300 meters high. In an atmosphere practically devoid of oxygen, hurricane winds prevail. The earth gradually cools down and slows down its diurnal rotation.
At the end of the Archean - Neoarchean - oxygen photosynthesis enters the arena of Earth's history: life begins to change the planet. The atmosphere is filled with poisonous gas - oddly enough, it is oxygen. At the very beginning of the next era, the Paleoproterozoic, a global change in the composition of the atmosphere caused an oxygen catastrophe.
Most living organisms of that time were anaerobes: they not only did not need oxygen, but also could not exist at its significant concentrations. In turn, aerobes are organisms that use oxygen for energy synthesis processes, the ancestors of most modern animals, plants and microorganisms. They are limited only by "oxygen pockets", isolated from the rest of the atmosphere by spaces where there was oxygen. With the change in the atmosphere, scientists say, "the biosphere has turned inside out." Organisms for which oxygen was a poison became a minority and hid in places where it was not, and aerobic communities took the dominant position.
Such a planet is completely covered with ice - from horizon to horizon. We know this is Earth. But for what period? One of the oldest and longest-lasting glaciations on the Blue Planet is the Huronian glaciation. It began and ended in the Paleoproterozoic, a geological era that started after the Neoarchean.
The Archean ocean, which covered the Earth, froze over for 300 million years. The cause of this oldest and most prolonged glaciation on Earth was an oxygen catastrophe, during which a large amount of oxygen, produced by photosynthetic organisms, entered the atmosphere. Methane, previously present in large quantities in the atmosphere, was the main contributor to the greenhouse effect - much more so than carbon dioxide. An increase in the concentration of free oxygen removed methane from the atmosphere, oxidizing it to carbon dioxide and water. The sun at that time was still much weaker than it is today. Therefore, it was methane, as a strong greenhouse gas, that protected the planet's surface from freezing. In the absence of the methane greenhouse effect, temperatures dropped, leading to global glaciation.
However, the freezing of the surface could in no way stop the tectonic activity of the earth's mantle. Volcanoes continued to emit carbon dioxide into the atmosphere, and over time, it accumulated enough to restore the greenhouse effect and melt the ice that chained the planet for millions of years.
In fact, the Earth has become a "snowball" at least three times. More than a billion years after the Huronic glaciation, in the Proterozoic, the planet froze twice: first for 60 million years, and another time for 15 million years. Hence the name of this geological period - cryogeny, from the ancient Greek "cold".
Purple plant planet
It is a purple planet. Such, probably, was the Earth at the time of the appearance of the first plants. But this assumption has not yet become generally accepted, and scientists have long been looking for an answer to the question of why modern plants are green. Most of the radiation from sunlight falls on the green part of the visible light spectrum, through which the bulk of the energy is transmitted.
However, plants using chlorophyll ignore this very “nutritious” part of the spectrum. It is illogical. Scientists from the University of Maryland suggest that the first microorganisms - the precursors of plants - still had a purple color.
To absorb solar energy, they did not use the green photosynthetic pigment chlorophyll, but retinol. It absorbs green light and reflects back red and violet, the combination of which appears to be violet. Perhaps, in the conditions of the origin of life, when free oxygen was in short supply, the use of retinol was more acceptable, since organisms synthesized it more easily. Today we can find retinol, for example, in the membrane of photosynthetic microbes called halobacteria.
Purple life dominated the young Earth. She captured the green part of the spectrum, and in order to survive in these conditions, the ancestors of modern plants that appeared after, were forced to adapt to the use of the free part of the spectrum.
The competition took place, most likely, in the ancient ocean. Competitors of violet organisms, primarily chlorophyll ones, developed under a rich layer of microorganisms - retinol ones, which had already withdrawn the green part of the light spectrum and reflected the blue and red ones. But in the end, chlorophyll turned out to be more effective for photosynthesis than retinol. And purple plants were soon driven out of the earth.
Planet of giant insects
Three hundred million years ago: Carboniferous, or Carboniferous period. The air is filled with oxygen. But do not rush to take off your spacesuit - there are too many of them! The proportion of oxygen in the earth's atmosphere and the level of its pressure are higher than today.
Giant insects enter the arena of life. And they do not feed on nectar. Giant dragonflies mega-neuras with a wingspan of up to one meter are carnivorous. Their food - dictyoneurids - "insects" the size of a dove. And here is the largest arthropod in the entire history of the Earth - the giant centipede arthropleura with thirty pairs of legs. The length of her body can reach 2.6 meters. This insect eats up to a ton of vegetation per year.
Someone might think that this is the planet of nightmares. And it seems that a giant spider is about to emerge from the rainforest. In fact, the only thing that you will not see here are these very giant spiders. Arachnids do not breathe with trachea, but with "lung sacs", which are similar to gills screwed into the body. Breathing in insects is different.
Carbonic gigantism is characteristic only for trachea breathing. Hemolymph - the "blood" of both modern and Carboniferous insects - does not tolerate oxygen. Breathing is carried out with the help of trachea - branching tubes that directly connect the cells of internal organs with the air environment. The air inside such a tube is stationary, and there is no forced ventilation, as in various types of pulmonary sacs. The inflow of oxygen into the body, like the outflow of carbon dioxide, occurs due to diffusion at a difference in the partial pressures of these gases at the inner and outer ends of the tube. This oxygen supply mechanism severely limits the length of the tracheal tube and, accordingly, the maximum body size of the insect itself. The more oxygen in the atmosphere and, therefore, its partial pressure (that is, the pressure of a single component of the gas mixture), the greater the possible length of the trachea and the size of the insect itself.
The Carboniferous period is the time of the formation of colossal reserves of coal. The chemical equilibrium in the atmosphere has shifted: huge masses of unoxidized carbon have been removed from the biological cycle. In percentage terms, this raised the level of oxygen in the atmosphere and gave impetus to the evolution of insects.
Several tens of millions of years will pass, and an event, the causes of which are not fully known, will remove excess oxygen from the atmosphere. The catastrophe, called the Great Permian Extinction and which happened about 252 million years ago, will lead to the extinction of most marine and land animals and will become the only mass extinction of insects, especially giant ones.