How long is human life
How did life come about?
The origin of life on earth has long been puzzled. It is known that simple bacteria developed as early as 3.8 billion years ago. But how was that possible - can life just come into being?
A student named Stanley Miller had an idea in 1953: He wanted to simulate the environmental conditions on earth in an experiment around 3.8 billion years ago. To do this, he filled a glass flask with water and some gases that were probably components of the primordial atmosphere: ammonia, methane and hydrogen. In this gas mixture he ignited electrical discharges in order to simulate the lightning bolts of the thunderstorms of that time. The water should replicate the natural water cycle. There was also a heater where the water evaporated and a cooling coil where it condensed again.
Miller ran this experiment for several days and then examined the water. In it he found a certain kind of chemical compound: amino acids, an important part of the cells of all living things. Miller had shown that the building blocks of life can be created from simple gases.
This is why scientists today assume that the gases in the primordial atmosphere reacted in a similar way to form organic substances. Rain washed them into the sea, and high concentrations could accumulate, especially in shallow waters. Whether through aggressive sunbeams or lightning - the particles must have reacted with one another over and over again. A random combination of molecules then had a special property for the first time: It was able to reproduce itself - the beginning of life.
The geological ages
The earth has changed a lot since it was formed: mountains, seas and continents have arisen and passed, animal and plant species have spread and become extinct. Most of these changes happened very slowly, over many millions of years. But every now and then there were decisive events: within a few thousand years the environmental conditions changed drastically.
For the scientists studying the history of the earth, these drastic changes are like a new chapter in a book: they divide the earth's history into different sections, the Eons to be named.
At the beginning, 4.5 billion years ago, the earth was completely uninhabitable. It emerged as a hot ball of glowing molten rock, surrounded by hot, caustic and poisonous gases. That sounds like a description of hell - and the name of this time comes from the Greek word "Hades" for hell: Hadaikum. It ended about four billion years ago with the first big change: The earth had cooled down so much that the surface became solid - the earth got a crust.
The earth continued to cool, so that liquid water could collect on the crust: seas were formed. And life began in these seas around 3.8 billion years ago - but initially only in the form of the simplest bacteria. The Greek word for origin or beginning is in the name of this time: Archean. An important climate change about 2.5 billion years ago marked the transition to the next epoch: primitive living things began to influence the environment. They produced oxygen that was previously almost completely absent from the atmosphere.
The early unicellular life forms became more complex over time, they formed cell nuclei. Later, some began to work together on a permanent basis in alliances - this ultimately resulted in the first multicellular organisms. However, they did not yet have solid shells or skeletons, so that hardly any fossils have survived from this period. This epoch owes its name to this time before the fossils were formed: Proterozoic.
The Proterozoic era ended with an explosion of life 550 million years ago: within a short period of time, the primitive forms of life developed into an enormous biodiversity. These species were built much more complex - and some already had hard shells, which were first preserved as fossils. Therefore, the history of life only becomes really visible to scientists from this point in time. And this epoch is named after the Greek term for "visible": Phanerozoic.
This age of life has lasted for 550 million years until today. However, life did not develop evenly either: After the explosive spread of life, there were two devastating mass extinctions. These mark further important turning points in the history of the earth, so that scientists divide the age of life, the Phanerozoic, into three sections, Eras called, divide.
The oldest era of the Phanerozoic began 550 million years ago with the mass emergence of new species. They are called that Antiquity or Paleozoic. At first life only took place in the oceans. Then the plants colonized the land, later the animal world followed suit: first the amphibians developed, which could already feel their way a little on land, and finally also reptiles, which became independent of water and conquered the land. The ancient world ended about 251 million years ago with the greatest mass extinction of all time: Over 90 percent of all animal and plant species died out, especially in the oceans. The reason has not yet been finally clarified. Scientists suspect that an ice age was to blame, possibly as a result of a meteorite impact.
When the surviving animal and plant species had to get used to their new environment, it broke Earth Middle Ages or Mesozoic at. It is primarily the age of the dinosaurs: giant lizards evolved and ruled life for almost 200 million years. But the Middle Ages also ended with a decisive event: about 65 million years ago a large meteorite hit the earth. So much dust and ash was thrown into the air that the sky darkened and the climate changed for a long time. The dinosaurs and many other species became extinct.
Small mammals in particular benefited from this, as they were best able to adapt to climate change. They had already developed in the Mesozoic, but remained in the shadow of the dinosaurs. Now they could spread rapidly, conquer the most varied of habitats and keep developing. Humans also descend from this group. This most recent age continues to this day and therefore becomes the Earth New Age or Cenozoic called.
This rough classification of the earth's history is based on very drastic changes in life: explosive multiplication or mass extinction. In between, however, there were further upheavals due to various other influences - changes in the seas and continents due to continental drift, climate change between ice ages and warm periods, the composition of the air and much more. The new conditions always favored individual species and disadvantaged others. So the three sections of the Phanerozoic (Age of Life) can each be divided into several periods.
The beginnings of the earth
We would not recognize the earth immediately after its formation. It was an extremely uncomfortable planet: there were neither continents nor oceans, but a seething surface of glowing hot, viscous magma. Why couldn't the earth's crust form for a long time?
A good 4.5 billion years ago, comets, asteroids, gas and dust condensed to form our planet. Its own gravity pressed these individual parts together so that they were subjected to strong pressure. This pressure was naturally highest in the core of the earth, on which the weight of the entire outer layers weighed. As a result of the high pressure, the rock was heated up and melted. Outwardly, the pressure and thus also the temperature became less. Even so, the surface of the earth remained very hot for several hundred million years and could not cool down and solidify.
In order to understand the reason for this, the scientists had to look at the moon: Ancient lunar craters from the time the solar system was formed tell us that the moon was hit by numerous meteorites when it was young. It is therefore assumed that the earth was also exposed to a real rock bombardment from space at the same time. The lumps fell to the earth at high speed - and the impacts were correspondingly violent: Even lumps of a few hundred tons could easily cause an explosion the strength of an atomic bomb!
So the earth's surface continued to heat up for a long time, stirred up again and again and remained so fluid. Only when the impacts gradually subsided after a few hundred million years did the temperatures on the earth's surface drop. The rock could slowly solidify and form an earth crust that grew thicker and thicker over the course of millions of years. But to this day it is only a very thin layer that floats on a viscous, hot interior of the earth.
How did the water come to earth?
About two thirds of the earth is covered with water - a unique selling point: the earth is the only planet in the solar system on which there is liquid water. Life originated in water, and water is also vital for us humans. But where does the water on earth actually come from?
Scientists suspect that the water comes from comets. These lumps of ice and dust originally formed on the edge of the solar system. But some also got into the interior of the solar system on orbits and became part of the newly emerging planets.
Initially, the young planets were very hot - so hot that the rock melted and formed a liquid ball. And the ice on the comets not only melted, it even evaporated. Because the water vapor was much lighter than the molten rock, it bubbled up towards the surface. There it escaped into the atmosphere through volcanoes.
As the earth slowly cooled, the steam turned back to liquid water. To put it more clearly: It started to rain. Those first downpours must have been stronger than any thunderstorm we can imagine today. And it must have rained for a very long time - tens of thousands of years. Large parts of the young earth's surface were flooded - in some places up to ten kilometers high. This is how the oceans came into being.
And what happened to the water on the other planets? Why are there no oceans there? Mercury doesn't have enough gravity to hold an atmosphere at all - like all gases, water vapor simply escaped into space. The same thing happened on the moon. The solar radiation on Venus is so strong that the water has also evaporated into space. On the other hand, it is too cold on Mars, but there are suspected large deposits of ice beneath the surface. And the gas planets have no solid surface on which seas could form. One suspects an ocean of water on Jupiter's moon Europa, but the surface is frozen. So the earth remains the only celestial body in the solar system with seas.
The water cycle
The water on earth is always on the move. Huge amounts of it are constantly moving - between sea, air and land - in an eternal cycle in which not a single drop is lost.
The motor of the water cycle is the sun: It heats the water of the seas, lakes and rivers so much that it evaporates. Plants also release water vapor into the atmosphere through tiny openings. The humid air rises, tiny water droplets gather in the air and form clouds. As rain, hail or snow, the water falls back into the sea or onto the earth. If it falls on the ground, it seeps into the ground, supplies plants or flows through the ground, over streams and rivers back into the sea. The eternal cycle of evaporation, precipitation and runoff starts all over again.
The water cycle has been around for almost as long as the earth has existed. He ensures that living beings on our planet are supplied with fresh water. And not only that: Without the water cycle, the weather as we know it would not exist.
How do thunderstorms occur?
A bright lightning flashes in the sky, the clap of thunder can be heard. Rain pelts the earth and in a matter of seconds inundates the land. A thunderstorm is always fascinating. In the past, people feared thunderstorms and believed that angry gods would punish them with it. There was no other way they could explain this natural event. It has long been known how a thunderstorm occurs. Those who stay in the house or in the car and leave the lake or swimming pool in good time need not fear thunder and lightning.
Thunderstorms form when strong sunlight allows water to evaporate and a large amount of warm, humid air rises: a thundercloud is formed. The water droplets in the cloud are swirled far upwards by the buoyancy of the warm air. The drops rub against each other and thereby become electrically charged. Presumably the positive charge collects in the upper part of the cloud, the negative charge in the lower part. This creates ever greater tension, which ultimately discharges in a flash. Electric current flows in the process. The temperature rises to several thousand degrees and makes the flash glow glowingly.
Due to the tremendous heat, the air around the lightning expands with a tremendous bang, similar to an explosion. We call this loud crash thunder. So without lightning there would be no thunder. And because light is faster than sound, the lightning bolt can be seen first and only then the thunder can be heard.
Lightning can shoot back and forth between clouds or from the clouds to the earth. When lightning strikes a building or a tree, the temperature soars to several thousand degrees. The heat can cause severe damage, for example house or forest fires. Before there were lightning rods, thunderstorms were especially feared for this reason.
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